pcreapi(3)
PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
NAME
PCRE - Perl-compatible regular expressions
#include <pcre.h>
PCRE NATIVE API BASIC FUNCTIONS
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
void pcre_free_study(pcre_extra *extra);
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
PCRE NATIVE API STRING EXTRACTION FUNCTIONS
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
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int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
void pcre_free_substring(const char *stringptr);
void pcre_free_substring_list(const char **stringptr);
PCRE NATIVE API AUXILIARY FUNCTIONS
int pcre_jit_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
pcre_jit_stack *jstack);
pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
void pcre_jit_stack_free(pcre_jit_stack *stack);
void pcre_assign_jit_stack(pcre_extra *extra,
pcre_jit_callback callback, void *data);
const unsigned char *pcre_maketables(void);
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
int pcre_refcount(pcre *code, int adjust);
int pcre_config(int what, void *where);
const char *pcre_version(void);
int pcre_pattern_to_host_byte_order(pcre *code,
pcre_extra *extra, const unsigned char *tables);
PCRE NATIVE API INDIRECTED FUNCTIONS
void *(*pcre_malloc)(size_t);
void (*pcre_free)(void *);
void *(*pcre_stack_malloc)(size_t);
void (*pcre_stack_free)(void *);
int (*pcre_callout)(pcre_callout_block *);
int (*pcre_stack_guard)(void);
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PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES
As well as support for 8-bit character strings, PCRE also
supports 16-bit strings (from release 8.30) and 32-bit
strings (from release 8.32), by means of two additional
libraries. They can be built as well as, or instead of, the
8-bit library. To avoid too much complication, this document
describes the 8-bit versions of the functions, with only
occasional references to the 16-bit and 32-bit libraries.
The 16-bit and 32-bit functions operate in the same way as
their 8-bit counterparts; they just use different data types
for their arguments and results, and their names start with
pcre16_ or pcre32_ instead of pcre_. For every option that
has UTF8 in its name (for example, PCRE_UTF8), there are
corresponding 16-bit and 32-bit names with UTF8 replaced by
UTF16 or UTF32, respectively. This facility is in fact just
cosmetic; the 16-bit and 32-bit option names define the same
bit values. References to bytes and UTF-8 in this document
should be read as references to 16-bit data units and UTF-16
when using the 16-bit library, or 32-bit data units and
UTF-32 when using the 32-bit library, unless specified oth-
erwise. More details of the specific differences for the
16-bit and 32-bit libraries are given in the pcre16 and
pcre32 pages.
PCRE API OVERVIEW
PCRE has its own native API, which is described in this
document. There are also some wrapper functions (for the 8-
bit library only) that correspond to the POSIX regular
expression API, but they do not give access to all the func-
tionality. They are described in the pcreposix documenta-
tion. Both of these APIs define a set of C function calls. A
C++ wrapper (again for the 8-bit library only) is also dis-
tributed with PCRE. It is documented in the pcrecpp page.
The native API C function prototypes are defined in the
header file pcre.h, and on Unix-like systems the (8-bit)
library itself is called libpcre. It can normally be
accessed by adding -lpcre to the command for linking an
application that uses PCRE. The header file defines the mac-
ros PCRE_MAJOR and PCRE_MINOR to contain the major and minor
release numbers for the library. Applications can use these
to include support for different releases of PCRE. In a
Windows environment, if you want to statically link an
application program against a non-dll pcre.a file, you must
define PCRE_STATIC before including pcre.h or pcrecpp.h,
because otherwise the pcre_malloc() and pcre_free() exported
functions will be declared __declspec(dllimport), with
unwanted results. The functions pcre_compile(),
pcre_compile2(), pcre_study(), and pcre_exec() are used for
compiling and matching regular expressions in a Perl-
compatible manner. A sample program that demonstrates the
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simplest way of using them is provided in the file called
pcredemo.c in the PCRE source distribution. A listing of
this program is given in the pcredemo documentation, and the
pcresample documentation describes how to compile and run
it. Just-in-time compiler support is an optional feature of
PCRE that can be built in appropriate hardware environments.
It greatly speeds up the matching performance of many pat-
terns. Simple programs can easily request that it be used if
available, by setting an option that is ignored when it is
not relevant. More complicated programs might need to make
use of the functions pcre_jit_stack_alloc(),
pcre_jit_stack_free(), and pcre_assign_jit_stack() in order
to control the JIT code's memory usage. From release 8.32
there is also a direct interface for JIT execution, which
gives improved performance. The JIT-specific functions are
discussed in the pcrejit documentation. A second matching
function, pcre_dfa_exec(), which is not Perl-compatible, is
also provided. This uses a different algorithm for the
matching. The alternative algorithm finds all possible
matches (at a given point in the subject), and scans the
subject just once (unless there are lookbehind assertions).
However, this algorithm does not return captured substrings.
A description of the two matching algorithms and their
advantages and disadvantages is given in the pcrematching
documentation. In addition to the main compiling and match-
ing functions, there are convenience functions for extract-
ing captured substrings from a subject string that is
matched by pcre_exec(). They are:
pcre_copy_substring()
pcre_copy_named_substring()
pcre_get_substring()
pcre_get_named_substring()
pcre_get_substring_list()
pcre_get_stringnumber()
pcre_get_stringtable_entries()
pcre_free_substring() and pcre_free_substring_list() are
also provided, to free the memory used for extracted
strings. The function pcre_maketables() is used to build a
set of character tables in the current locale for passing to
pcre_compile(), pcre_exec(), or pcre_dfa_exec(). This is an
optional facility that is provided for specialist use. Most
commonly, no special tables are passed, in which case inter-
nal tables that are generated when PCRE is built are used.
The function pcre_fullinfo() is used to find out information
about a compiled pattern. The function pcre_version()
returns a pointer to a string containing the version of PCRE
and its date of release. The function pcre_refcount() main-
tains a reference count in a data block containing a com-
piled pattern. This is provided for the benefit of object-
oriented applications. The global variables pcre_malloc and
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pcre_free initially contain the entry points of the standard
malloc() and free() functions, respectively. PCRE calls the
memory management functions via these variables, so a cal-
ling program can replace them if it wishes to intercept the
calls. This should be done before calling any PCRE func-
tions. The global variables pcre_stack_malloc and
pcre_stack_free are also indirections to memory management
functions. These special functions are used only when PCRE
is compiled to use the heap for remembering data, instead of
recursive function calls, when running the pcre_exec() func-
tion. See the pcrebuild documentation for details of how to
do this. It is a non-standard way of building PCRE, for use
in environments that have limited stacks. Because of the
greater use of memory management, it runs more slowly.
Separate functions are provided so that special-purpose
external code can be used for this case. When used, these
functions always allocate memory blocks of the same size.
There is a discussion about PCRE's stack usage in the pcres-
tack documentation. The global variable pcre_callout ini-
tially contains NULL. It can be set by the caller to a "cal-
lout" function, which PCRE will then call at specified
points during a matching operation. Details are given in the
pcrecallout documentation. The global variable
pcre_stack_guard initially contains NULL. It can be set by
the caller to a function that is called by PCRE whenever it
starts to compile a parenthesized part of a pattern. When
parentheses are nested, PCRE uses recursive function calls,
which use up the system stack. This function is provided so
that applications with restricted stacks can force a compi-
lation error if the stack runs out. The function should
return zero if all is well, or non-zero to force an error.
NEWLINES
PCRE supports five different conventions for indicating line
breaks in strings: a single CR (carriage return) character,
a single LF (linefeed) character, the two-character sequence
CRLF, any of the three preceding, or any Unicode newline
sequence. The Unicode newline sequences are the three just
mentioned, plus the single characters VT (vertical tab,
U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS
(line separator, U+2028), and PS (paragraph separator,
U+2029). Each of the first three conventions is used by at
least one operating system as its standard newline sequence.
When PCRE is built, a default can be specified. The default
default is LF, which is the Unix standard. When PCRE is run,
the default can be overridden, either when a pattern is com-
piled, or when it is matched. At compile time, the newline
convention can be specified by the options argument of
pcre_compile(), or it can be specified by special text at
the start of the pattern itself; this overrides any other
settings. See the pcrepattern page for details of the
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special character sequences. In the PCRE documentation the
word "newline" is used to mean "the character or pair of
characters that indicate a line break". The choice of new-
line convention affects the handling of the dot, circumflex,
and dollar metacharacters, the handling of #-comments in /x
mode, and, when CRLF is a recognized line ending sequence,
the match position advancement for a non-anchored pattern.
There is more detail about this in the section on
pcre_exec() options below. The choice of newline convention
does not affect the interpretation of the \n or \r escape
sequences, nor does it affect what \R matches, which is con-
trolled in a similar way, but by separate options.
MULTITHREADING
The PCRE functions can be used in multi-threading applica-
tions, with the proviso that the memory management functions
pointed to by pcre_malloc, pcre_free, pcre_stack_malloc, and
pcre_stack_free, and the callout and stack-checking func-
tions pointed to by pcre_callout and pcre_stack_guard, are
shared by all threads. The compiled form of a regular
expression is not altered during matching, so the same com-
piled pattern can safely be used by several threads at once.
If the just-in-time optimization feature is being used, it
needs separate memory stack areas for each thread. See the
pcrejit documentation for more details.
SAVING PRECOMPILED PATTERNS FOR LATER USE
The compiled form of a regular expression can be saved and
re-used at a later time, possibly by a different program,
and even on a host other than the one on which it was com-
piled. Details are given in the pcreprecompile documenta-
tion, which includes a description of the
pcre_pattern_to_host_byte_order() function. However, compil-
ing a regular expression with one version of PCRE for use
with a different version is not guaranteed to work and may
cause crashes.
CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);
The function pcre_config() makes it possible for a PCRE
client to discover which optional features have been com-
piled into the PCRE library. The pcrebuild documentation has
more details about these optional features. The first argu-
ment for pcre_config() is an integer, specifying which
information is required; the second argument is a pointer to
a variable into which the information is placed. The
returned value is zero on success, or the negative error
code PCRE_ERROR_BADOPTION if the value in the first argument
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is not recognized. The following information is available:
PCRE_CONFIG_UTF8
The output is an integer that is set to one if UTF-8 support
is available; otherwise it is set to zero. This value should
normally be given to the 8-bit version of this function,
pcre_config(). If it is given to the 16-bit or 32-bit ver-
sion of this function, the result is PCRE_ERROR_BADOPTION.
PCRE_CONFIG_UTF16
The output is an integer that is set to one if UTF-16 sup-
port is available; otherwise it is set to zero. This value
should normally be given to the 16-bit version of this func-
tion, pcre16_config(). If it is given to the 8-bit or 32-bit
version of this function, the result is
PCRE_ERROR_BADOPTION.
PCRE_CONFIG_UTF32
The output is an integer that is set to one if UTF-32 sup-
port is available; otherwise it is set to zero. This value
should normally be given to the 32-bit version of this func-
tion, pcre32_config(). If it is given to the 8-bit or 16-bit
version of this function, the result is
PCRE_ERROR_BADOPTION.
PCRE_CONFIG_UNICODE_PROPERTIES
The output is an integer that is set to one if support for
Unicode character properties is available; otherwise it is
set to zero.
PCRE_CONFIG_JIT
The output is an integer that is set to one if support for
just-in-time compiling is available; otherwise it is set to
zero.
PCRE_CONFIG_JITTARGET
The output is a pointer to a zero-terminated "const char *"
string. If JIT support is available, the string contains the
name of the architecture for which the JIT compiler is con-
figured, for example "x86 32bit (little endian +
unaligned)". If JIT support is not available, the result is
NULL.
PCRE_CONFIG_NEWLINE
The output is an integer whose value specifies the default
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character sequence that is recognized as meaning "newline".
The values that are supported in ASCII/Unicode environments
are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
and -1 for ANY. In EBCDIC environments, CR, ANYCRLF, and ANY
yield the same values. However, the value for LF is normally
21, though some EBCDIC environments use 37. The correspond-
ing values for CRLF are 3349 and 3365. The default should
normally correspond to the standard sequence for your
operating system.
PCRE_CONFIG_BSR
The output is an integer whose value indicates what charac-
ter sequences the \R escape sequence matches by default. A
value of 0 means that \R matches any Unicode line ending
sequence; a value of 1 means that \R matches only CR, LF, or
CRLF. The default can be overridden when a pattern is com-
piled or matched.
PCRE_CONFIG_LINK_SIZE
The output is an integer that contains the number of bytes
used for internal linkage in compiled regular expressions.
For the 8-bit library, the value can be 2, 3, or 4. For the
16-bit library, the value is either 2 or 4 and is still a
number of bytes. For the 32-bit library, the value is either
2 or 4 and is still a number of bytes. The default value of
2 is sufficient for all but the most massive patterns, since
it allows the compiled pattern to be up to 64K in size.
Larger values allow larger regular expressions to be com-
piled, at the expense of slower matching.
PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
The output is an integer that contains the threshold above
which the POSIX interface uses malloc() for output vectors.
Further details are given in the pcreposix documentation.
PCRE_CONFIG_PARENS_LIMIT
The output is a long integer that gives the maximum depth of
nesting of parentheses (of any kind) in a pattern. This
limit is imposed to cap the amount of system stack used when
a pattern is compiled. It is specified when PCRE is built;
the default is 250. This limit does not take into account
the stack that may already be used by the calling applica-
tion. For finer control over compilation stack usage, you
can set a pointer to an external checking function in
pcre_stack_guard.
PCRE_CONFIG_MATCH_LIMIT
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The output is a long integer that gives the default limit
for the number of internal matching function calls in a
pcre_exec() execution. Further details are given with
pcre_exec() below.
PCRE_CONFIG_MATCH_LIMIT_RECURSION
The output is a long integer that gives the default limit
for the depth of recursion when calling the internal match-
ing function in a pcre_exec() execution. Further details are
given with pcre_exec() below.
PCRE_CONFIG_STACKRECURSE
The output is an integer that is set to one if internal
recursion when running pcre_exec() is implemented by recur-
sive function calls that use the stack to remember their
state. This is the usual way that PCRE is compiled. The out-
put is zero if PCRE was compiled to use blocks of data on
the heap instead of recursive function calls. In this case,
pcre_stack_malloc and pcre_stack_free are called to manage
memory blocks on the heap, thus avoiding the use of the
stack.
COMPILING A PATTERN
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
Either of the functions pcre_compile() or pcre_compile2()
can be called to compile a pattern into an internal form.
The only difference between the two interfaces is that
pcre_compile2() has an additional argument, errorcodeptr,
via which a numerical error code can be returned. To avoid
too much repetition, we refer just to pcre_compile() below,
but the information applies equally to pcre_compile2(). The
pattern is a C string terminated by a binary zero, and is
passed in the pattern argument. A pointer to a single block
of memory that is obtained via pcre_malloc is returned. This
contains the compiled code and related data. The pcre type
is defined for the returned block; this is a typedef for a
structure whose contents are not externally defined. It is
up to the caller to free the memory (via pcre_free) when it
is no longer required. Although the compiled code of a PCRE
regex is relocatable, that is, it does not depend on memory
location, the complete pcre data block is not fully relocat-
able, because it may contain a copy of the tableptr
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argument, which is an address (see below). The options
argument contains various bit settings that affect the com-
pilation. It should be zero if no options are required. The
available options are described below. Some of them (in par-
ticular, those that are compatible with Perl, but some oth-
ers as well) can also be set and unset from within the pat-
tern (see the detailed description in the pcrepattern docu-
mentation). For those options that can be different in dif-
ferent parts of the pattern, the contents of the options
argument specifies their settings at the start of compila-
tion and execution. The PCRE_ANCHORED, PCRE_BSR_xxx,
PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
PCRE_NO_START_OPTIMIZE options can be set at the time of
matching as well as at compile time. If errptr is NULL,
pcre_compile() returns NULL immediately. Otherwise, if com-
pilation of a pattern fails, pcre_compile() returns NULL,
and sets the variable pointed to by errptr to point to a
textual error message. This is a static string that is part
of the library. You must not try to free it. Normally, the
offset from the start of the pattern to the data unit that
was being processed when the error was discovered is placed
in the variable pointed to by erroffset, which must not be
NULL (if it is, an immediate error is given). However, for
an invalid UTF-8 or UTF-16 string, the offset is that of the
first data unit of the failing character. Some errors are
not detected until the whole pattern has been scanned; in
these cases, the offset passed back is the length of the
pattern. Note that the offset is in data units, not charac-
ters, even in a UTF mode. It may sometimes point into the
middle of a UTF-8 or UTF-16 character. If pcre_compile2()
is used instead of pcre_compile(), and the errorcodeptr
argument is not NULL, a non-zero error code number is
returned via this argument in the event of an error. This is
in addition to the textual error message. Error codes and
messages are listed below. If the final argument, tableptr,
is NULL, PCRE uses a default set of character tables that
are built when PCRE is compiled, using the default C locale.
Otherwise, tableptr must be an address that is the result of
a call to pcre_maketables(). This value is stored with the
compiled pattern, and used again by pcre_exec() and
pcre_dfa_exec() when the pattern is matched. For more dis-
cussion, see the section on locale support below. This code
fragment shows a typical straightforward call to
pcre_compile():
pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
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&erroffset, /* for error offset */
NULL); /* use default character tables */
The following names for option bits are defined in the
pcre.h header file:
PCRE_ANCHORED
If this bit is set, the pattern is forced to be "anchored",
that is, it is constrained to match only at the first match-
ing point in the string that is being searched (the "subject
string"). This effect can also be achieved by appropriate
constructs in the pattern itself, which is the only way to
do it in Perl.
PCRE_AUTO_CALLOUT
If this bit is set, pcre_compile() automatically inserts
callout items, all with number 255, before each pattern
item. For discussion of the callout facility, see the pcre-
callout documentation.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what
the \R escape sequence matches. The choice is either to
match only CR, LF, or CRLF, or to match any Unicode newline
sequence. The default is specified when PCRE is built. It
can be overridden from within the pattern, or by setting an
option when a compiled pattern is matched.
PCRE_CASELESS
If this bit is set, letters in the pattern match both upper
and lower case letters. It is equivalent to Perl's /i
option, and it can be changed within a pattern by a (?i)
option setting. In UTF-8 mode, PCRE always understands the
concept of case for characters whose values are less than
128, so caseless matching is always possible. For characters
with higher values, the concept of case is supported if PCRE
is compiled with Unicode property support, but not other-
wise. If you want to use caseless matching for characters
128 and above, you must ensure that PCRE is compiled with
Unicode property support as well as with UTF-8 support.
PCRE_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern
matches only at the end of the subject string. Without this
option, a dollar also matches immediately before a newline
at the end of the string (but not before any other
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newlines). The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set. There is no equivalent to this
option in Perl, and no way to set it within a pattern.
PCRE_DOTALL
If this bit is set, a dot metacharacter in the pattern
matches a character of any value, including one that indi-
cates a newline. However, it only ever matches one charac-
ter, even if newlines are coded as CRLF. Without this
option, a dot does not match when the current position is at
a newline. This option is equivalent to Perl's /s option,
and it can be changed within a pattern by a (?s) option set-
ting. A negative class such as [^a] always matches newline
characters, independent of the setting of this option.
PCRE_DUPNAMES
If this bit is set, names used to identify capturing subpat-
terns need not be unique. This can be helpful for certain
types of pattern when it is known that only one instance of
the named subpattern can ever be matched. There are more
details of named subpatterns below; see also the pcrepattern
documentation.
PCRE_EXTENDED
If this bit is set, most white space characters in the pat-
tern are totally ignored except when escaped or inside a
character class. However, white space is not allowed within
sequences such as (?> that introduce various parenthesized
subpatterns, nor within a numerical quantifier such as
{1,3}. However, ignorable white space is permitted between
an item and a following quantifier and between a quantifier
and a following + that indicates possessiveness. White
space did not used to include the VT character (code 11),
because Perl did not treat this character as white space.
However, Perl changed at release 5.18, so PCRE followed at
release 8.34, and VT is now treated as white space.
PCRE_EXTENDED also causes characters between an unescaped #
outside a character class and the next newline, inclusive,
to be ignored. PCRE_EXTENDED is equivalent to Perl's /x
option, and it can be changed within a pattern by a (?x)
option setting. Which characters are interpreted as new-
lines is controlled by the options passed to pcre_compile()
or by a special sequence at the start of the pattern, as
described in the section entitled "Newline conventions" in
the pcrepattern documentation. Note that the end of this
type of comment is a literal newline sequence in the pat-
tern; escape sequences that happen to represent a newline do
not count. This option makes it possible to include com-
ments inside complicated patterns. Note, however, that this
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applies only to data characters. White space characters may
never appear within special character sequences in a pat-
tern, for example within the sequence (?( that introduces a
conditional subpattern.
PCRE_EXTRA
This option was invented in order to turn on additional
functionality of PCRE that is incompatible with Perl, but it
is currently of very little use. When set, any backslash in
a pattern that is followed by a letter that has no special
meaning causes an error, thus reserving these combinations
for future expansion. By default, as in Perl, a backslash
followed by a letter with no special meaning is treated as a
literal. (Perl can, however, be persuaded to give an error
for this, by running it with the -w option.) There are at
present no other features controlled by this option. It can
also be set by a (?X) option setting within a pattern.
PCRE_FIRSTLINE
If this option is set, an unanchored pattern is required to
match before or at the first newline in the subject string,
though the matched text may continue over the newline.
PCRE_JAVASCRIPT_COMPAT
If this option is set, PCRE's behaviour is changed in some
ways so that it is compatible with JavaScript rather than
Perl. The changes are as follows: (1) A lone closing square
bracket in a pattern causes a compile-time error, because
this is illegal in JavaScript (by default it is treated as a
data character). Thus, the pattern AB]CD becomes illegal
when this option is set. (2) At run time, a back reference
to an unset subpattern group matches an empty string (by
default this causes the current matching alternative to
fail). A pattern such as (\1)(a) succeeds when this option
is set (assuming it can find an "a" in the subject), whereas
it fails by default, for Perl compatibility. (3) \U matches
an upper case "U" character; by default \U causes a compile
time error (Perl uses \U to upper case subsequent charac-
ters). (4) \u matches a lower case "u" character unless it
is followed by four hexadecimal digits, in which case the
hexadecimal number defines the code point to match. By
default, \u causes a compile time error (Perl uses it to
upper case the following character). (5) \x matches a lower
case "x" character unless it is followed by two hexadecimal
digits, in which case the hexadecimal number defines the
code point to match. By default, as in Perl, a hexadecimal
number is always expected after \x, but it may have zero,
one, or two digits (so, for example, \xz matches a binary
zero character followed by z).
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PCRE_MULTILINE
By default, for the purposes of matching "start of line" and
"end of line", PCRE treats the subject string as consisting
of a single line of characters, even if it actually contains
newlines. The "start of line" metacharacter (^) matches only
at the start of the string, and the "end of line" metachar-
acter ($) matches only at the end of the string, or before a
terminating newline (except when PCRE_DOLLAR_ENDONLY is
set). Note, however, that unless PCRE_DOTALL is set, the
"any character" metacharacter (.) does not match at a new-
line. This behaviour (for ^, $, and dot) is the same as
Perl. When PCRE_MULTILINE it is set, the "start of line"
and "end of line" constructs match immediately following or
immediately before internal newlines in the subject string,
respectively, as well as at the very start and end. This is
equivalent to Perl's /m option, and it can be changed within
a pattern by a (?m) option setting. If there are no newlines
in a subject string, or no occurrences of ^ or $ in a pat-
tern, setting PCRE_MULTILINE has no effect.
PCRE_NEVER_UTF
This option locks out interpretation of the pattern as UTF-8
(or UTF-16 or UTF-32 in the 16-bit and 32-bit libraries). In
particular, it prevents the creator of the pattern from
switching to UTF interpretation by starting the pattern with
(*UTF). This may be useful in applications that process pat-
terns from external sources. The combination of PCRE_UTF8
and PCRE_NEVER_UTF also causes an error.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the default newline definition that
was chosen when PCRE was built. Setting the first or the
second specifies that a newline is indicated by a single
character (CR or LF, respectively). Setting
PCRE_NEWLINE_CRLF specifies that a newline is indicated by
the two-character CRLF sequence. Setting
PCRE_NEWLINE_ANYCRLF specifies that any of the three preced-
ing sequences should be recognized. Setting PCRE_NEWLINE_ANY
specifies that any Unicode newline sequence should be recog-
nized. In an ASCII/Unicode environment, the Unicode newline
sequences are the three just mentioned, plus the single
characters VT (vertical tab, U+000B), FF (form feed,
U+000C), NEL (next line, U+0085), LS (line separator,
U+2028), and PS (paragraph separator, U+2029). For the 8-bit
library, the last two are recognized only in UTF-8 mode.
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When PCRE is compiled to run in an EBCDIC (mainframe)
environment, the code for CR is 0x0d, the same as ASCII.
However, the character code for LF is normally 0x15, though
in some EBCDIC environments 0x25 is used. Whichever of these
is not LF is made to correspond to Unicode's NEL character.
EBCDIC codes are all less than 256. For more details, see
the pcrebuild documentation. The newline setting in the
options word uses three bits that are treated as a number,
giving eight possibilities. Currently only six are used
(default plus the five values above). This means that if you
set more than one newline option, the combination may or may
not be sensible. For example, PCRE_NEWLINE_CR with
PCRE_NEWLINE_LF is equivalent to PCRE_NEWLINE_CRLF, but
other combinations may yield unused numbers and cause an
error. The only time that a line break in a pattern is spe-
cially recognized when compiling is when PCRE_EXTENDED is
set. CR and LF are white space characters, and so are
ignored in this mode. Also, an unescaped # outside a charac-
ter class indicates a comment that lasts until after the
next line break sequence. In other circumstances, line break
sequences in patterns are treated as literal data. The new-
line option that is set at compile time becomes the default
that is used for pcre_exec() and pcre_dfa_exec(), but it can
be overridden.
PCRE_NO_AUTO_CAPTURE
If this option is set, it disables the use of numbered cap-
turing parentheses in the pattern. Any opening parenthesis
that is not followed by ? behaves as if it were followed by
?: but named parentheses can still be used for capturing
(and they acquire numbers in the usual way). There is no
equivalent of this option in Perl.
PCRE_NO_AUTO_POSSESS
If this option is set, it disables "auto-possessification".
This is an optimization that, for example, turns a+b into
a++b in order to avoid backtracks into a+ that can never be
successful. However, if callouts are in use, auto-
possessification means that some of them are never taken.
You can set this option if you want the matching functions
to do a full unoptimized search and run all the callouts,
but it is mainly provided for testing purposes.
PCRE_NO_START_OPTIMIZE
This is an option that acts at matching time; that is, it is
really an option for pcre_exec() or pcre_dfa_exec(). If it
is set at compile time, it is remembered with the compiled
pattern and assumed at matching time. This is necessary if
you want to use JIT execution, because the JIT compiler
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needs to know whether or not this option is set. For details
see the discussion of PCRE_NO_START_OPTIMIZE below.
PCRE_UCP
This option changes the way PCRE processes \B, \b, \D, \d,
\S, \s, \W, \w, and some of the POSIX character classes. By
default, only ASCII characters are recognized, but if
PCRE_UCP is set, Unicode properties are used instead to
classify characters. More details are given in the section
on generic character types in the pcrepattern page. If you
set PCRE_UCP, matching one of the items it affects takes
much longer. The option is available only if PCRE has been
compiled with Unicode property support.
PCRE_UNGREEDY
This option inverts the "greediness" of the quantifiers so
that they are not greedy by default, but become greedy if
followed by "?". It is not compatible with Perl. It can also
be set by a (?U) option setting within the pattern.
PCRE_UTF8
This option causes PCRE to regard both the pattern and the
subject as strings of UTF-8 characters instead of single-
byte strings. However, it is available only when PCRE is
built to include UTF support. If not, the use of this option
provokes an error. Details of how this option changes the
behaviour of PCRE are given in the pcreunicode page.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set, the validity of the pattern as a
UTF-8 string is automatically checked. There is a discussion
about the validity of UTF-8 strings in the pcreunicode page.
If an invalid UTF-8 sequence is found, pcre_compile()
returns an error. If you already know that your pattern is
valid, and you want to skip this check for performance rea-
sons, you can set the PCRE_NO_UTF8_CHECK option. When it is
set, the effect of passing an invalid UTF-8 string as a pat-
tern is undefined. It may cause your program to crash or
loop. Note that this option can also be passed to
pcre_exec() and pcre_dfa_exec(), to suppress the validity
checking of subject strings only. If the same string is
being matched many times, the option can be safely set for
the second and subsequent matchings to improve performance.
COMPILATION ERROR CODES
The following table lists the error codes than may be
returned by pcre_compile2(), along with the error messages
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
that may be returned by both compiling functions. Note that
error messages are always 8-bit ASCII strings, even in 16-
bit or 32-bit mode. As PCRE has developed, some error codes
have fallen out of use. To avoid confusion, they have not
been re-used.
0 no error
1 \ at end of pattern
2 \c at end of pattern
3 unrecognized character follows \
4 numbers out of order in {} quantifier
5 number too big in {} quantifier
6 missing terminating ] for character class
7 invalid escape sequence in character class
8 range out of order in character class
9 nothing to repeat
10 [this code is not in use]
11 internal error: unexpected repeat
12 unrecognized character after (? or (?-
13 POSIX named classes are supported only within a class
14 missing )
15 reference to non-existent subpattern
16 erroffset passed as NULL
17 unknown option bit(s) set
18 missing ) after comment
19 [this code is not in use]
20 regular expression is too large
21 failed to get memory
22 unmatched parentheses
23 internal error: code overflow
24 unrecognized character after (?<
25 lookbehind assertion is not fixed length
26 malformed number or name after (?(
27 conditional group contains more than two branches
28 assertion expected after (?(
29 (?R or (?[+-]digits must be followed by )
30 unknown POSIX class name
31 POSIX collating elements are not supported
32 this version of PCRE is compiled without UTF support
33 [this code is not in use]
34 character value in \x{} or \o{} is too large
35 invalid condition (?(0)
36 \C not allowed in lookbehind assertion
37 PCRE does not support \L, \l, \N{name}, \U, or \u
38 number after (?C is > 255
39 closing ) for (?C expected
40 recursive call could loop indefinitely
41 unrecognized character after (?P
42 syntax error in subpattern name (missing terminator)
43 two named subpatterns have the same name
44 invalid UTF-8 string (specifically UTF-8)
45 support for \P, \p, and \X has not been compiled
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46 malformed \P or \p sequence
47 unknown property name after \P or \p
48 subpattern name is too long (maximum 32 characters)
49 too many named subpatterns (maximum 10000)
50 [this code is not in use]
51 octal value is greater than \377 in 8-bit non-UTF-8
mode
52 internal error: overran compiling workspace
53 internal error: previously-checked referenced subpat-
tern
not found
54 DEFINE group contains more than one branch
55 repeating a DEFINE group is not allowed
56 inconsistent NEWLINE options
57 \g is not followed by a braced, angle-bracketed, or
quoted
name/number or by a plain number
58 a numbered reference must not be zero
59 an argument is not allowed for (*ACCEPT), (*FAIL), or
(*COMMIT)
60 (*VERB) not recognized or malformed
61 number is too big
62 subpattern name expected
63 digit expected after (?+
64 ] is an invalid data character in JavaScript compati-
bility mode
65 different names for subpatterns of the same number are
not allowed
66 (*MARK) must have an argument
67 this version of PCRE is not compiled with Unicode pro-
perty
support
68 \c must be followed by an ASCII character
69 \k is not followed by a braced, angle-bracketed, or
quoted name
70 internal error: unknown opcode in find_fixedlength()
71 \N is not supported in a class
72 too many forward references
73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
74 invalid UTF-16 string (specifically UTF-16)
75 name is too long in (*MARK), (*PRUNE), (*SKIP), or
(*THEN)
76 character value in \u.... sequence is too large
77 invalid UTF-32 string (specifically UTF-32)
78 setting UTF is disabled by the application
79 non-hex character in \x{} (closing brace missing?)
80 non-octal character in \o{} (closing brace missing?)
81 missing opening brace after \o
82 parentheses are too deeply nested
83 invalid range in character class
84 group name must start with a non-digit
85 parentheses are too deeply nested (stack check)
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The numbers 32 and 10000 in errors 48 and 49 are defaults;
different values may be used if the limits were changed when
PCRE was built.
STUDYING A PATTERN
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
If a compiled pattern is going to be used several times, it
is worth spending more time analyzing it in order to speed
up the time taken for matching. The function pcre_study()
takes a pointer to a compiled pattern as its first argument.
If studying the pattern produces additional information that
will help speed up matching, pcre_study() returns a pointer
to a pcre_extra block, in which the study_data field points
to the results of the study. The returned value from
pcre_study() can be passed directly to pcre_exec() or
pcre_dfa_exec(). However, a pcre_extra block also contains
other fields that can be set by the caller before the block
is passed; these are described below in the section on
matching a pattern. If studying the pattern does not pro-
duce any useful information, pcre_study() returns NULL by
default. In that circumstance, if the calling program wants
to pass any of the other fields to pcre_exec() or
pcre_dfa_exec(), it must set up its own pcre_extra block.
However, if pcre_study() is called with the
PCRE_STUDY_EXTRA_NEEDED option, it returns a pcre_extra
block even if studying did not find any additional informa-
tion. It may still return NULL, however, if an error occurs
in pcre_study(). The second argument of pcre_study() con-
tains option bits. There are three further options in addi-
tion to PCRE_STUDY_EXTRA_NEEDED:
PCRE_STUDY_JIT_COMPILE
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
If any of these are set, and the just-in-time compiler is
available, the pattern is further compiled into machine code
that executes much faster than the pcre_exec() interpretive
matching function. If the just-in-time compiler is not
available, these options are ignored. All undefined bits in
the options argument must be zero. JIT compilation is a
heavyweight optimization. It can take some time for patterns
to be analyzed, and for one-off matches and simple patterns
the benefit of faster execution might be offset by a much
slower study time. Not all patterns can be optimized by the
JIT compiler. For those that cannot be handled, matching
automatically falls back to the pcre_exec() interpreter. For
more details, see the pcrejit documentation. The third
argument for pcre_study() is a pointer for an error message.
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If studying succeeds (even if no data is returned), the
variable it points to is set to NULL. Otherwise it is set to
point to a textual error message. This is a static string
that is part of the library. You must not try to free it.
You should test the error pointer for NULL after calling
pcre_study(), to be sure that it has run successfully. When
you are finished with a pattern, you can free the memory
used for the study data by calling pcre_free_study(). This
function was added to the API for release 8.20. For earlier
versions, the memory could be freed with pcre_free(), just
like the pattern itself. This will still work in cases where
JIT optimization is not used, but it is advisable to change
to the new function when convenient. This is a typical way
in which pcre_study() is used (except that in a real appli-
cation there should be tests for errors):
int rc;
pcre *re;
pcre_extra *sd;
re = pcre_compile("pattern", 0, &error, &erroroffset,
NULL);
sd = pcre_study(
re, /* result of pcre_compile() */
0, /* no options */
&error); /* set to NULL or points to a message */
rc = pcre_exec( /* see below for details of pcre_exec()
options */
re, sd, "subject", 7, 0, 0, ovector, 30);
...
pcre_free_study(sd);
pcre_free(re);
Studying a pattern does two things: first, a lower bound for
the length of subject string that is needed to match the
pattern is computed. This does not mean that there are any
strings of that length that match, but it does guarantee
that no shorter strings match. The value is used to avoid
wasting time by trying to match strings that are shorter
than the lower bound. You can find out the value in a cal-
ling program via the pcre_fullinfo() function. Studying a
pattern is also useful for non-anchored patterns that do not
have a single fixed starting character. A bitmap of possible
starting bytes is created. This speeds up finding a position
in the subject at which to start matching. (In 16-bit mode,
the bitmap is used for 16-bit values less than 256. In 32-
bit mode, the bitmap is used for 32-bit values less than
256.) These two optimizations apply to both pcre_exec() and
pcre_dfa_exec(), and the information is also used by the JIT
compiler. The optimizations can be disabled by setting the
PCRE_NO_START_OPTIMIZE option. You might want to do this if
your pattern contains callouts or (*MARK) and you want to
make use of these facilities in cases where matching fails.
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
PCRE_NO_START_OPTIMIZE can be specified at either compile
time or execution time. However, if PCRE_NO_START_OPTIMIZE
is passed to pcre_exec(), (that is, after any JIT compila-
tion has happened) JIT execution is disabled. For JIT execu-
tion to work with PCRE_NO_START_OPTIMIZE, the option must be
set at compile time. There is a longer discussion of
PCRE_NO_START_OPTIMIZE below.
LOCALE SUPPORT
PCRE handles caseless matching, and determines whether char-
acters are letters, digits, or whatever, by reference to a
set of tables, indexed by character code point. When running
in UTF-8 mode, or in the 16- or 32-bit libraries, this
applies only to characters with code points less than 256.
By default, higher-valued code points never match escapes
such as \w or \d. However, if PCRE is built with Unicode
property support, all characters can be tested with \p and
\P, or, alternatively, the PCRE_UCP option can be set when a
pattern is compiled; this causes \w and friends to use
Unicode property support instead of the built-in tables.
The use of locales with Unicode is discouraged. If you are
handling characters with code points greater than 128, you
should either use Unicode support, or use locales, but not
try to mix the two. PCRE contains an internal set of tables
that are used when the final argument of pcre_compile() is
NULL. These are sufficient for many applications. Normally,
the internal tables recognize only ASCII characters. How-
ever, when PCRE is built, it is possible to cause the inter-
nal tables to be rebuilt in the default "C" locale of the
local system, which may cause them to be different. The
internal tables can always be overridden by tables supplied
by the application that calls PCRE. These may be created in
a different locale from the default. As more and more appli-
cations change to using Unicode, the need for this locale
support is expected to die away. External tables are built
by calling the pcre_maketables() function, which has no
arguments, in the relevant locale. The result can then be
passed to pcre_compile() as often as necessary. For example,
to build and use tables that are appropriate for the French
locale (where accented characters with values greater than
128 are treated as letters), the following code could be
used:
setlocale(LC_CTYPE, "fr_FR");
tables = pcre_maketables();
re = pcre_compile(..., tables);
The locale name "fr_FR" is used on Linux and other Unix-like
systems; if you are using Windows, the name for the French
locale is "french". When pcre_maketables() runs, the tables
are built in memory that is obtained via pcre_malloc. It is
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the caller's responsibility to ensure that the memory con-
taining the tables remains available for as long as it is
needed. The pointer that is passed to pcre_compile() is
saved with the compiled pattern, and the same tables are
used via this pointer by pcre_study() and also by
pcre_exec() and pcre_dfa_exec(). Thus, for any single pat-
tern, compilation, studying and matching all happen in the
same locale, but different patterns can be processed in dif-
ferent locales. It is possible to pass a table pointer or
NULL (indicating the use of the internal tables) to
pcre_exec() or pcre_dfa_exec() (see the discussion below in
the section on matching a pattern). This facility is pro-
vided for use with pre-compiled patterns that have been
saved and reloaded. Character tables are not saved with
patterns, so if a non-standard table was used at compile
time, it must be provided again when the reloaded pattern is
matched. Attempting to use this facility to match a pattern
in a different locale from the one in which it was compiled
is likely to lead to anomalous (usually incorrect) results.
INFORMATION ABOUT A PATTERN
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
The pcre_fullinfo() function returns information about a
compiled pattern. It replaces the pcre_info() function,
which was removed from the library at version 8.30, after
more than 10 years of obsolescence. The first argument for
pcre_fullinfo() is a pointer to the compiled pattern. The
second argument is the result of pcre_study(), or NULL if
the pattern was not studied. The third argument specifies
which piece of information is required, and the fourth argu-
ment is a pointer to a variable to receive the data. The
yield of the function is zero for success, or one of the
following negative numbers:
PCRE_ERROR_NULL the argument code was NULL
the argument where was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
PCRE_ERROR_BADENDIANNESS the pattern was compiled with
different
endianness
PCRE_ERROR_BADOPTION the value of what was invalid
PCRE_ERROR_UNSET the requested field is not set
The "magic number" is placed at the start of each compiled
pattern as an simple check against passing an arbitrary
memory pointer. The endianness error can occur if a compiled
pattern is saved and reloaded on a different host. Here is a
typical call of pcre_fullinfo(), to obtain the length of the
compiled pattern:
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
int rc;
size_t length;
rc = pcre_fullinfo(
re, /* result of pcre_compile() */
sd, /* result of pcre_study(), or NULL */
PCRE_INFO_SIZE, /* what is required */
&length); /* where to put the data */
The possible values for the third argument are defined in
pcre.h, and are as follows:
PCRE_INFO_BACKREFMAX
Return the number of the highest back reference in the pat-
tern. The fourth argument should point to an int variable.
Zero is returned if there are no back references.
PCRE_INFO_CAPTURECOUNT
Return the number of capturing subpatterns in the pattern.
The fourth argument should point to an int variable.
PCRE_INFO_DEFAULT_TABLES
Return a pointer to the internal default character tables
within PCRE. The fourth argument should point to an unsigned
char * variable. This information call is provided for
internal use by the pcre_study() function. External callers
can cause PCRE to use its internal tables by passing a NULL
table pointer.
PCRE_INFO_FIRSTBYTE (deprecated)
Return information about the first data unit of any matched
string, for a non-anchored pattern. The name of this option
refers to the 8-bit library, where data units are bytes. The
fourth argument should point to an int variable. Negative
values are used for special cases. However, this means that
when the 32-bit library is in non-UTF-32 mode, the full 32-
bit range of characters cannot be returned. For this reason,
this value is deprecated; use PCRE_INFO_FIRSTCHARACTERFLAGS
and PCRE_INFO_FIRSTCHARACTER instead. If there is a fixed
first value, for example, the letter "c" from a pattern such
as (cat|cow|coyote), its value is returned. In the 8-bit
library, the value is always less than 256. In the 16-bit
library the value can be up to 0xffff. In the 32-bit library
the value can be up to 0x10ffff. If there is no fixed first
value, and if either
(a) the pattern was compiled with the PCRE_MULTILINE option,
and every branch starts with "^", or
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
(b) every branch of the pattern starts with ".*" and
PCRE_DOTALL is not set (if it were set, the pattern would be
anchored),
-1 is returned, indicating that the pattern matches only at
the start of a subject string or after any newline within
the string. Otherwise -2 is returned. For anchored patterns,
-2 is returned.
PCRE_INFO_FIRSTCHARACTER
Return the value of the first data unit (non-UTF character)
of any matched string in the situation where
PCRE_INFO_FIRSTCHARACTERFLAGS returns 1; otherwise return 0.
The fourth argument should point to an uint_t variable. In
the 8-bit library, the value is always less than 256. In the
16-bit library the value can be up to 0xffff. In the 32-bit
library in UTF-32 mode the value can be up to 0x10ffff, and
up to 0xffffffff when not using UTF-32 mode.
PCRE_INFO_FIRSTCHARACTERFLAGS
Return information about the first data unit of any matched
string, for a non-anchored pattern. The fourth argument
should point to an int variable. If there is a fixed first
value, for example, the letter "c" from a pattern such as
(cat|cow|coyote), 1 is returned, and the character value can
be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no
fixed first value, and if either
(a) the pattern was compiled with the PCRE_MULTILINE option,
and every branch starts with "^", or
(b) every branch of the pattern starts with ".*" and
PCRE_DOTALL is not set (if it were set, the pattern would be
anchored),
2 is returned, indicating that the pattern matches only at
the start of a subject string or after any newline within
the string. Otherwise 0 is returned. For anchored patterns,
0 is returned.
PCRE_INFO_FIRSTTABLE
If the pattern was studied, and this resulted in the con-
struction of a 256-bit table indicating a fixed set of
values for the first data unit in any matching string, a
pointer to the table is returned. Otherwise NULL is
returned. The fourth argument should point to an unsigned
char * variable.
PCRE_INFO_HASCRORLF
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
Return 1 if the pattern contains any explicit matches for CR
or LF characters, otherwise 0. The fourth argument should
point to an int variable. An explicit match is either a
literal CR or LF character, or \r or \n.
PCRE_INFO_JCHANGED
Return 1 if the (?J) or (?-J) option setting is used in the
pattern, otherwise 0. The fourth argument should point to an
int variable. (?J) and (?-J) set and unset the local
PCRE_DUPNAMES option, respectively.
PCRE_INFO_JIT
Return 1 if the pattern was studied with one of the JIT
options, and just-in-time compiling was successful. The
fourth argument should point to an int variable. A return
value of 0 means that JIT support is not available in this
version of PCRE, or that the pattern was not studied with a
JIT option, or that the JIT compiler could not handle this
particular pattern. See the pcrejit documentation for
details of what can and cannot be handled.
PCRE_INFO_JITSIZE
If the pattern was successfully studied with a JIT option,
return the size of the JIT compiled code, otherwise return
zero. The fourth argument should point to a size_t variable.
PCRE_INFO_LASTLITERAL
Return the value of the rightmost literal data unit that
must exist in any matched string, other than at its start,
if such a value has been recorded. The fourth argument
should point to an int variable. If there is no such value,
-1 is returned. For anchored patterns, a last literal value
is recorded only if it follows something of variable length.
For example, for the pattern /^a\d+z\d+/ the returned value
is "z", but for /^a\dz\d/ the returned value is -1. Since
for the 32-bit library using the non-UTF-32 mode, this func-
tion is unable to return the full 32-bit range of charac-
ters, this value is deprecated; instead the
PCRE_INFO_REQUIREDCHARFLAGS and PCRE_INFO_REQUIREDCHAR
values should be used.
PCRE_INFO_MATCH_EMPTY
Return 1 if the pattern can match an empty string, otherwise
0. The fourth argument should point to an int variable.
PCRE_INFO_MATCHLIMIT
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
If the pattern set a match limit by including an item of the
form (*LIMIT_MATCH=nnnn) at the start, the value is
returned. The fourth argument should point to an unsigned
32-bit integer. If no such value has been set, the call to
pcre_fullinfo() returns the error PCRE_ERROR_UNSET.
PCRE_INFO_MAXLOOKBEHIND
Return the number of characters (NB not data units) in the
longest lookbehind assertion in the pattern. This informa-
tion is useful when doing multi-segment matching using the
partial matching facilities. Note that the simple assertions
\b and \B require a one-character lookbehind. \A also regis-
ters a one-character lookbehind, though it does not actually
inspect the previous character. This is to ensure that at
least one character from the old segment is retained when a
new segment is processed. Otherwise, if there are no look-
behinds in the pattern, \A might match incorrectly at the
start of a new segment.
PCRE_INFO_MINLENGTH
If the pattern was studied and a minimum length for matching
subject strings was computed, its value is returned. Other-
wise the returned value is -1. The value is a number of
characters, which in UTF mode may be different from the
number of data units. The fourth argument should point to an
int variable. A non-negative value is a lower bound to the
length of any matching string. There may not be any strings
of that length that do actually match, but every string that
does match is at least that long.
PCRE_INFO_NAMECOUNT
PCRE_INFO_NAMEENTRYSIZE
PCRE_INFO_NAMETABLE
PCRE supports the use of named as well as numbered capturing
parentheses. The names are just an additional way of identi-
fying the parentheses, which still acquire numbers. Several
convenience functions such as pcre_get_named_substring() are
provided for extracting captured substrings by name. It is
also possible to extract the data directly, by first con-
verting the name to a number in order to access the correct
pointers in the output vector (described with pcre_exec()
below). To do the conversion, you need to use the name-to-
number map, which is described by these three values. The
map consists of a number of fixed-size entries.
PCRE_INFO_NAMECOUNT gives the number of entries, and
PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both
of these return an int value. The entry size depends on the
length of the longest name. PCRE_INFO_NAMETABLE returns a
pointer to the first entry of the table. This is a pointer
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
to char in the 8-bit library, where the first two bytes of
each entry are the number of the capturing parenthesis, most
significant byte first. In the 16-bit library, the pointer
points to 16-bit data units, the first of which contains the
parenthesis number. In the 32-bit library, the pointer
points to 32-bit data units, the first of which contains the
parenthesis number. The rest of the entry is the correspond-
ing name, zero terminated. The names are in alphabetical
order. If (?| is used to create multiple groups with the
same number, as described in the section on duplicate sub-
pattern numbers in the pcrepattern page, the groups may be
given the same name, but there is only one entry in the
table. Different names for groups of the same number are not
permitted. Duplicate names for subpatterns with different
numbers are permitted, but only if PCRE_DUPNAMES is set.
They appear in the table in the order in which they were
found in the pattern. In the absence of (?| this is the
order of increasing number; when (?| is used this is not
necessarily the case because later subpatterns may have
lower numbers. As a simple example of the name/number
table, consider the following pattern after compilation by
the 8-bit library (assume PCRE_EXTENDED is set, so white
space - including newlines - is ignored):
(?<date> (?<year>(\d\d)?\d\d) -
(?<month>\d\d) - (?<day>\d\d) )
There are four named subpatterns, so the table has four
entries, and each entry in the table is eight bytes long.
The table is as follows, with non-printing bytes shows in
hexadecimal, and undefined bytes shown as ??:
00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??
When writing code to extract data from named subpatterns
using the name-to-number map, remember that the length of
the entries is likely to be different for each compiled pat-
tern.
PCRE_INFO_OKPARTIAL
Return 1 if the pattern can be used for partial matching
with pcre_exec(), otherwise 0. The fourth argument should
point to an int variable. From release 8.00, this always
returns 1, because the restrictions that previously applied
to partial matching have been lifted. The pcrepartial docu-
mentation gives details of partial matching.
PCRE_INFO_OPTIONS
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
Return a copy of the options with which the pattern was com-
piled. The fourth argument should point to an unsigned long
int variable. These option bits are those specified in the
call to pcre_compile(), modified by any top-level option
settings at the start of the pattern itself. In other words,
they are the options that will be in force when matching
starts. For example, if the pattern /(?im)abc(?-i)d/ is com-
piled with the PCRE_EXTENDED option, the result is
PCRE_CASELESS, PCRE_MULTILINE, and PCRE_EXTENDED. A pattern
is automatically anchored by PCRE if all of its top-level
alternatives begin with one of the following:
^ unless PCRE_MULTILINE is set
\A always
\G always
.* if PCRE_DOTALL is set and there are no back
references to the subpattern in which .* appears
For such patterns, the PCRE_ANCHORED bit is set in the
options returned by pcre_fullinfo().
PCRE_INFO_RECURSIONLIMIT
If the pattern set a recursion limit by including an item of
the form (*LIMIT_RECURSION=nnnn) at the start, the value is
returned. The fourth argument should point to an unsigned
32-bit integer. If no such value has been set, the call to
pcre_fullinfo() returns the error PCRE_ERROR_UNSET.
PCRE_INFO_SIZE
Return the size of the compiled pattern in bytes (for all
three libraries). The fourth argument should point to a
size_t variable. This value does not include the size of the
pcre structure that is returned by pcre_compile(). The value
that is passed as the argument to pcre_malloc() when
pcre_compile() is getting memory in which to place the com-
piled data is the value returned by this option plus the
size of the pcre structure. Studying a compiled pattern,
with or without JIT, does not alter the value returned by
this option.
PCRE_INFO_STUDYSIZE
Return the size in bytes (for all three libraries) of the
data block pointed to by the study_data field in a
pcre_extra block. If pcre_extra is NULL, or there is no
study data, zero is returned. The fourth argument should
point to a size_t variable. The study_data field is set by
pcre_study() to record information that will speed up match-
ing (see the section entitled "Studying a pattern" above).
The format of the study_data block is private, but its
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
length is made available via this option so that it can be
saved and restored (see the pcreprecompile documentation for
details).
PCRE_INFO_REQUIREDCHARFLAGS
Returns 1 if there is a rightmost literal data unit that
must exist in any matched string, other than at its start.
The fourth argument should point to an int variable. If
there is no such value, 0 is returned. If returning 1, the
character value itself can be retrieved using
PCRE_INFO_REQUIREDCHAR. For anchored patterns, a last
literal value is recorded only if it follows something of
variable length. For example, for the pattern /^a\d+z\d+/
the returned value 1 (with "z" returned from
PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned
value is 0.
PCRE_INFO_REQUIREDCHAR
Return the value of the rightmost literal data unit that
must exist in any matched string, other than at its start,
if such a value has been recorded. The fourth argument
should point to an uint32_t variable. If there is no such
value, 0 is returned.
REFERENCE COUNTS
int pcre_refcount(pcre *code, int adjust);
The pcre_refcount() function is used to maintain a reference
count in the data block that contains a compiled pattern. It
is provided for the benefit of applications that operate in
an object-oriented manner, where different parts of the
application may be using the same compiled pattern, but you
want to free the block when they are all done. When a pat-
tern is compiled, the reference count field is initialized
to zero. It is changed only by calling this function, whose
action is to add the adjust value (which may be positive or
negative) to it. The yield of the function is the new value.
However, the value of the count is constrained to lie
between 0 and 65535, inclusive. If the new value is outside
these limits, it is forced to the appropriate limit value.
Except when it is zero, the reference count is not correctly
preserved if a pattern is compiled on one host and then
transferred to a host whose byte-order is different. (This
seems a highly unlikely scenario.)
MATCHING A PATTERN: THE TRADITIONAL FUNCTION
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
int options, int *ovector, int ovecsize);
The function pcre_exec() is called to match a subject string
against a compiled pattern, which is passed in the code
argument. If the pattern was studied, the result of the
study should be passed in the extra argument. You can call
pcre_exec() with the same code and extra arguments as many
times as you like, in order to match different subject
strings with the same pattern. This function is the main
matching facility of the library, and it operates in a
Perl-like manner. For specialist use there is also an alter-
native matching function, which is described below in the
section about the pcre_dfa_exec() function. In most appli-
cations, the pattern will have been compiled (and optionally
studied) in the same process that calls pcre_exec(). How-
ever, it is possible to save compiled patterns and study
data, and then use them later in different processes, possi-
bly even on different hosts. For a discussion about this,
see the pcreprecompile documentation. Here is an example of
a simple call to pcre_exec():
int rc;
int ovector[30];
rc = pcre_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring
information */
30); /* number of elements (NOT size in
bytes) */
Extra data for pcre_exec()
If the extra argument is not NULL, it must point to a
pcre_extra data block. The pcre_study() function returns
such a block (when it doesn't return NULL), but you can also
create one for yourself, and pass additional information in
it. The pcre_extra block contains the following fields (not
necessarily in this order):
unsigned long int flags;
void *study_data;
void *executable_jit;
unsigned long int match_limit;
unsigned long int match_limit_recursion;
void *callout_data;
const unsigned char *tables;
unsigned char **mark;
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In the 16-bit version of this structure, the mark field has
type "PCRE_UCHAR16 **".
In the 32-bit version of this structure, the mark field has
type "PCRE_UCHAR32 **". The flags field is used to specify
which of the other fields are set. The flag bits are:
PCRE_EXTRA_CALLOUT_DATA
PCRE_EXTRA_EXECUTABLE_JIT
PCRE_EXTRA_MARK
PCRE_EXTRA_MATCH_LIMIT
PCRE_EXTRA_MATCH_LIMIT_RECURSION
PCRE_EXTRA_STUDY_DATA
PCRE_EXTRA_TABLES
Other flag bits should be set to zero. The study_data field
and sometimes the executable_jit field are set in the
pcre_extra block that is returned by pcre_study(), together
with the appropriate flag bits. You should not set these
yourself, but you may add to the block by setting other
fields and their corresponding flag bits. The match_limit
field provides a means of preventing PCRE from using up a
vast amount of resources when running patterns that are not
going to match, but which have a very large number of possi-
bilities in their search trees. The classic example is a
pattern that uses nested unlimited repeats. Internally,
pcre_exec() uses a function called match(), which it calls
repeatedly (sometimes recursively). The limit set by
match_limit is imposed on the number of times this function
is called during a match, which has the effect of limiting
the amount of backtracking that can take place. For patterns
that are not anchored, the count restarts from zero for each
position in the subject string. When pcre_exec() is called
with a pattern that was successfully studied with a JIT
option, the way that the matching is executed is entirely
different. However, there is still the possibility of runa-
way matching that goes on for a very long time, and so the
match_limit value is also used in this case (but in a dif-
ferent way) to limit how long the matching can continue.
The default value for the limit can be set when PCRE is
built; the default default is 10 million, which handles all
but the most extreme cases. You can override the default by
suppling pcre_exec() with a pcre_extra block in which
match_limit is set, and PCRE_EXTRA_MATCH_LIMIT is set in the
flags field. If the limit is exceeded, pcre_exec() returns
PCRE_ERROR_MATCHLIMIT. A value for the match limit may also
be supplied by an item at the start of a pattern of the form
(*LIMIT_MATCH=d)
where d is a decimal number. However, such a setting is
ignored unless d is less than the limit set by the caller of
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
pcre_exec() or, if no such limit is set, less than the
default. The match_limit_recursion field is similar to
match_limit, but instead of limiting the total number of
times that match() is called, it limits the depth of recur-
sion. The recursion depth is a smaller number than the total
number of calls, because not all calls to match() are recur-
sive. This limit is of use only if it is set smaller than
match_limit. Limiting the recursion depth limits the amount
of machine stack that can be used, or, when PCRE has been
compiled to use memory on the heap instead of the stack, the
amount of heap memory that can be used. This limit is not
relevant, and is ignored, when matching is done using JIT
compiled code. The default value for match_limit_recursion
can be set when PCRE is built; the default default is the
same value as the default for match_limit. You can override
the default by suppling pcre_exec() with a pcre_extra block
in which match_limit_recursion is set, and
PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field.
If the limit is exceeded, pcre_exec() returns
PCRE_ERROR_RECURSIONLIMIT. A value for the recursion limit
may also be supplied by an item at the start of a pattern of
the form
(*LIMIT_RECURSION=d)
where d is a decimal number. However, such a setting is
ignored unless d is less than the limit set by the caller of
pcre_exec() or, if no such limit is set, less than the
default. The callout_data field is used in conjunction with
the "callout" feature, and is described in the pcrecallout
documentation. The tables field is provided for use with
patterns that have been pre-compiled using custom character
tables, saved to disc or elsewhere, and then reloaded,
because the tables that were used to compile a pattern are
not saved with it. See the pcreprecompile documentation for
a discussion of saving compiled patterns for later use. If
NULL is passed using this mechanism, it forces PCRE's inter-
nal tables to be used. Warning: The tables that pcre_exec()
uses must be the same as those that were used when the pat-
tern was compiled. If this is not the case, the behaviour of
pcre_exec() is undefined. Therefore, when a pattern is com-
piled and matched in the same process, this field should
never be set. In this (the most common) case, the correct
table pointer is automatically passed with the compiled pat-
tern from pcre_compile() to pcre_exec(). If PCRE_EXTRA_MARK
is set in the flags field, the mark field must be set to
point to a suitable variable. If the pattern contains any
backtracking control verbs such as (*MARK:NAME), and the
execution ends up with a name to pass back, a pointer to the
name string (zero terminated) is placed in the variable
pointed to by the mark field. The names are within the com-
piled pattern; if you wish to retain such a name you must
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
copy it before freeing the memory of a compiled pattern. If
there is no name to pass back, the variable pointed to by
the mark field is set to NULL. For details of the backtrack-
ing control verbs, see the section entitled "Backtracking
control" in the pcrepattern documentation.
Option bits for pcre_exec()
The unused bits of the options argument for pcre_exec() must
be zero. The only bits that may be set are PCRE_ANCHORED,
PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
PCRE_NOTEMPTY_ATSTART, PCRE_NO_START_OPTIMIZE,
PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and
PCRE_PARTIAL_SOFT. If the pattern was successfully studied
with one of the just-in-time (JIT) compile options, the only
supported options for JIT execution are PCRE_NO_UTF8_CHECK,
PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and
PCRE_PARTIAL_SOFT. If an unsupported option is used, JIT
execution is disabled and the normal interpretive code in
pcre_exec() is run.
PCRE_ANCHORED
The PCRE_ANCHORED option limits pcre_exec() to matching at
the first matching position. If a pattern was compiled with
PCRE_ANCHORED, or turned out to be anchored by virtue of its
contents, it cannot be made unachored at matching time.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what
the \R escape sequence matches. The choice is either to
match only CR, LF, or CRLF, or to match any Unicode newline
sequence. These options override the choice that was made or
defaulted when the pattern was compiled.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the newline definition that was
chosen or defaulted when the pattern was compiled. For
details, see the description of pcre_compile() above. During
matching, the newline choice affects the behaviour of the
dot, circumflex, and dollar metacharacters. It may also
alter the way the match position is advanced after a match
failure for an unanchored pattern. When PCRE_NEWLINE_CRLF,
PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
match attempt for an unanchored pattern fails when the
current position is at a CRLF sequence, and the pattern con-
tains no explicit matches for CR or LF characters, the match
position is advanced by two characters instead of one, in
other words, to after the CRLF. The above rule is a
compromise that makes the most common cases work as
expected. For example, if the pattern is .+A (and the
PCRE_DOTALL option is not set), it does not match the string
"\r\nA" because, after failing at the start, it skips both
the CR and the LF before retrying. However, the pattern
[\r\n]A does match that string, because it contains an
explicit CR or LF reference, and so advances only by one
character after the first failure. An explicit match for CR
of LF is either a literal appearance of one of those charac-
ters, or one of the \r or \n escape sequences. Implicit
matches such as [^X] do not count, nor does \s (which
includes CR and LF in the characters that it matches). Not-
withstanding the above, anomalous effects may still occur
when CRLF is a valid newline sequence and explicit \r or \n
escapes appear in the pattern.
PCRE_NOTBOL
This option specifies that first character of the subject
string is not the beginning of a line, so the circumflex
metacharacter should not match before it. Setting this
without PCRE_MULTILINE (at compile time) causes circumflex
never to match. This option affects only the behaviour of
the circumflex metacharacter. It does not affect \A.
PCRE_NOTEOL
This option specifies that the end of the subject string is
not the end of a line, so the dollar metacharacter should
not match it nor (except in multiline mode) a newline
immediately before it. Setting this without PCRE_MULTILINE
(at compile time) causes dollar never to match. This option
affects only the behaviour of the dollar metacharacter. It
does not affect \Z or \z.
PCRE_NOTEMPTY
An empty string is not considered to be a valid match if
this option is set. If there are alternatives in the pat-
tern, they are tried. If all the alternatives match the
empty string, the entire match fails. For example, if the
pattern
a?b?
is applied to a string not beginning with "a" or "b", it
matches an empty string at the start of the subject. With
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
PCRE_NOTEMPTY set, this match is not valid, so PCRE searches
further into the string for occurrences of "a" or "b".
PCRE_NOTEMPTY_ATSTART
This is like PCRE_NOTEMPTY, except that an empty string
match that is not at the start of the subject is permitted.
If the pattern is anchored, such a match can occur only if
the pattern contains \K. Perl has no direct equivalent of
PCRE_NOTEMPTY or PCRE_NOTEMPTY_ATSTART, but it does make a
special case of a pattern match of the empty string within
its split() function, and when using the /g modifier. It is
possible to emulate Perl's behaviour after matching a null
string by first trying the match again at the same offset
with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if
that fails, by advancing the starting offset (see below) and
trying an ordinary match again. There is some code that
demonstrates how to do this in the pcredemo sample program.
In the most general case, you have to check to see if the
newline convention recognizes CRLF as a newline, and if so,
and the current character is CR followed by LF, advance the
starting offset by two characters instead of one.
PCRE_NO_START_OPTIMIZE
There are a number of optimizations that pcre_exec() uses at
the start of a match, in order to speed up the process. For
example, if it is known that an unanchored match must start
with a specific character, it searches the subject for that
character, and fails immediately if it cannot find it,
without actually running the main matching function. This
means that a special item such as (*COMMIT) at the start of
a pattern is not considered until after a suitable starting
point for the match has been found. Also, when callouts or
(*MARK) items are in use, these "start-up" optimizations can
cause them to be skipped if the pattern is never actually
used. The start-up optimizations are in effect a pre-scan of
the subject that takes place before the pattern is run. The
PCRE_NO_START_OPTIMIZE option disables the start-up optimi-
zations, possibly causing performance to suffer, but ensur-
ing that in cases where the result is "no match", the cal-
louts do occur, and that items such as (*COMMIT) and (*MARK)
are considered at every possible starting position in the
subject string. If PCRE_NO_START_OPTIMIZE is set at compile
time, it cannot be unset at matching time. The use of
PCRE_NO_START_OPTIMIZE at matching time (that is, passing it
to pcre_exec()) disables JIT execution; in this situation,
matching is always done using interpretively. Setting
PCRE_NO_START_OPTIMIZE can change the outcome of a matching
operation. Consider the pattern
(*COMMIT)ABC
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
When this is compiled, PCRE records the fact that a match
must start with the character "A". Suppose the subject
string is "DEFABC". The start-up optimization scans along
the subject, finds "A" and runs the first match attempt from
there. The (*COMMIT) item means that the pattern must match
the current starting position, which in this case, it does.
However, if the same match is run with
PCRE_NO_START_OPTIMIZE set, the initial scan along the sub-
ject string does not happen. The first match attempt is run
starting from "D" and when this fails, (*COMMIT) prevents
any further matches being tried, so the overall result is
"no match". If the pattern is studied, more start-up optimi-
zations may be used. For example, a minimum length for the
subject may be recorded. Consider the pattern
(*MARK:A)(X|Y)
The minimum length for a match is one character. If the sub-
ject is "ABC", there will be attempts to match "ABC", "BC",
"C", and then finally an empty string. If the pattern is
studied, the final attempt does not take place, because PCRE
knows that the subject is too short, and so the (*MARK) is
never encountered. In this case, studying the pattern does
not affect the overall match result, which is still "no
match", but it does affect the auxiliary information that is
returned.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set at compile time, the validity of the
subject as a UTF-8 string is automatically checked when
pcre_exec() is subsequently called. The entire string is
checked before any other processing takes place. The value
of startoffset is also checked to ensure that it points to
the start of a UTF-8 character. There is a discussion about
the validity of UTF-8 strings in the pcreunicode page. If an
invalid sequence of bytes is found, pcre_exec() returns the
error PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and
the problem is a truncated character at the end of the sub-
ject, PCRE_ERROR_SHORTUTF8. In both cases, information about
the precise nature of the error may also be returned (see
the descriptions of these errors in the section entitled
Error return values from pcre_exec() below). If startoffset
contains a value that does not point to the start of a UTF-8
character (or to the end of the subject),
PCRE_ERROR_BADUTF8_OFFSET is returned. If you already know
that your subject is valid, and you want to skip these
checks for performance reasons, you can set the
PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You
might want to do this for the second and subsequent calls to
pcre_exec() if you are making repeated calls to find all the
matches in a single subject string. However, you should be
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sure that the value of startoffset points to the start of a
character (or the end of the subject). When
PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid
string as a subject or an invalid value of startoffset is
undefined. Your program may crash or loop.
PCRE_PARTIAL_HARD
PCRE_PARTIAL_SOFT
These options turn on the partial matching feature. For
backwards compatibility, PCRE_PARTIAL is a synonym for
PCRE_PARTIAL_SOFT. A partial match occurs if the end of the
subject string is reached successfully, but there are not
enough subject characters to complete the match. If this
happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD)
is set, matching continues by testing any remaining alterna-
tives. Only if no complete match can be found is
PCRE_ERROR_PARTIAL returned instead of PCRE_ERROR_NOMATCH.
In other words, PCRE_PARTIAL_SOFT says that the caller is
prepared to handle a partial match, but only if no complete
match can be found. If PCRE_PARTIAL_HARD is set, it over-
rides PCRE_PARTIAL_SOFT. In this case, if a partial match is
found, pcre_exec() immediately returns PCRE_ERROR_PARTIAL,
without considering any other alternatives. In other words,
when PCRE_PARTIAL_HARD is set, a partial match is considered
to be more important that an alternative complete match. In
both cases, the portion of the string that was inspected
when the partial match was found is set as the first match-
ing string. There is a more detailed discussion of partial
and multi-segment matching, with examples, in the pcrepar-
tial documentation.
The string to be matched by pcre_exec()
The subject string is passed to pcre_exec() as a pointer in
subject, a length in length, and a starting offset in star-
toffset. The units for length and startoffset are bytes for
the 8-bit library, 16-bit data items for the 16-bit library,
and 32-bit data items for the 32-bit library. If star-
toffset is negative or greater than the length of the sub-
ject, pcre_exec() returns PCRE_ERROR_BADOFFSET. When the
starting offset is zero, the search for a match starts at
the beginning of the subject, and this is by far the most
common case. In UTF-8 or UTF-16 mode, the offset must point
to the start of a character, or the end of the subject (in
UTF-32 mode, one data unit equals one character, so all
offsets are valid). Unlike the pattern string, the subject
may contain binary zeroes. A non-zero starting offset is
useful when searching for another match in the same subject
by calling pcre_exec() again after a previous success. Set-
ting startoffset differs from just passing over a shortened
string and setting PCRE_NOTBOL in the case of a pattern that
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
begins with any kind of lookbehind. For example, consider
the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B
matches only if the current position in the subject is not a
word boundary.) When applied to the string "Mississipi" the
first call to pcre_exec() finds the first occurrence. If
pcre_exec() is called again with just the remainder of the
subject, namely "issipi", it does not match, because \B is
always false at the start of the subject, which is deemed to
be a word boundary. However, if pcre_exec() is passed the
entire string again, but with startoffset set to 4, it finds
the second occurrence of "iss" because it is able to look
behind the starting point to discover that it is preceded by
a letter. Finding all the matches in a subject is tricky
when the pattern can match an empty string. It is possible
to emulate Perl's /g behaviour by first trying the match
again at the same offset, with the PCRE_NOTEMPTY_ATSTART and
PCRE_ANCHORED options, and then if that fails, advancing the
starting offset and trying an ordinary match again. There is
some code that demonstrates how to do this in the pcredemo
sample program. In the most general case, you have to check
to see if the newline convention recognizes CRLF as a new-
line, and if so, and the current character is CR followed by
LF, advance the starting offset by two characters instead of
one. If a non-zero starting offset is passed when the pat-
tern is anchored, one attempt to match at the given offset
is made. This can only succeed if the pattern does not
require the match to be at the start of the subject.
How pcre_exec() returns captured substrings
In general, a pattern matches a certain portion of the sub-
ject, and in addition, further substrings from the subject
may be picked out by parts of the pattern. Following the
usage in Jeffrey Friedl's book, this is called "capturing"
in what follows, and the phrase "capturing subpattern" is
used for a fragment of a pattern that picks out a substring.
PCRE supports several other kinds of parenthesized subpat-
tern that do not cause substrings to be captured. Captured
substrings are returned to the caller via a vector of
integers whose address is passed in ovector. The number of
elements in the vector is passed in ovecsize, which must be
a non-negative number. Note: this argument is NOT the size
of ovector in bytes. The first two-thirds of the vector is
used to pass back captured substrings, each substring using
a pair of integers. The remaining third of the vector is
used as workspace by pcre_exec() while matching capturing
subpatterns, and is not available for passing back informa-
tion. The number passed in ovecsize should always be a
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
multiple of three. If it is not, it is rounded down. When a
match is successful, information about captured substrings
is returned in pairs of integers, starting at the beginning
of ovector, and continuing up to two-thirds of its length at
the most. The first element of each pair is set to the
offset of the first character in a substring, and the second
is set to the offset of the first character after the end of
a substring. These values are always data unit offsets, even
in UTF mode. They are byte offsets in the 8-bit library,
16-bit data item offsets in the 16-bit library, and 32-bit
data item offsets in the 32-bit library. Note: they are not
character counts. The first pair of integers, ovector[0]
and ovector[1], identify the portion of the subject string
matched by the entire pattern. The next pair is used for the
first capturing subpattern, and so on. The value returned by
pcre_exec() is one more than the highest numbered pair that
has been set. For example, if two substrings have been cap-
tured, the returned value is 3. If there are no capturing
subpatterns, the return value from a successful match is 1,
indicating that just the first pair of offsets has been set.
If a capturing subpattern is matched repeatedly, it is the
last portion of the string that it matched that is returned.
If the vector is too small to hold all the captured sub-
string offsets, it is used as far as possible (up to two-
thirds of its length), and the function returns a value of
zero. If neither the actual string matched nor any captured
substrings are of interest, pcre_exec() may be called with
ovector passed as NULL and ovecsize as zero. However, if the
pattern contains back references and the ovector is not big
enough to remember the related substrings, PCRE has to get
additional memory for use during matching. Thus it is usu-
ally advisable to supply an ovector of reasonable size.
There are some cases where zero is returned (indicating vec-
tor overflow) when in fact the vector is exactly the right
size for the final match. For example, consider the pattern
(a)(?:(b)c|bd)
If a vector of 6 elements (allowing for only 1 captured sub-
string) is given with subject string "abd", pcre_exec() will
try to set the second captured string, thereby recording a
vector overflow, before failing to match "c" and backing up
to try the second alternative. The zero return, however,
does correctly indicate that the maximum number of slots
(namely 2) have been filled. In similar cases where there is
temporary overflow, but the final number of used slots is
actually less than the maximum, a non-zero value is
returned. The pcre_fullinfo() function can be used to find
out how many capturing subpatterns there are in a compiled
pattern. The smallest size for ovector that will allow for n
captured substrings, in addition to the offsets of the sub-
string matched by the whole pattern, is (n+1)*3. It is
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
possible for capturing subpattern number n+1 to match some
part of the subject when subpattern n has not been used at
all. For example, if the string "abc" is matched against the
pattern (a|(z))(bc) the return from the function is 4, and
subpatterns 1 and 3 are matched, but 2 is not. When this
happens, both values in the offset pairs corresponding to
unused subpatterns are set to -1. Offset values that
correspond to unused subpatterns at the end of the expres-
sion are also set to -1. For example, if the string "abc" is
matched against the pattern (abc)(x(yz)?)? subpatterns 2 and
3 are not matched. The return from the function is 2,
because the highest used capturing subpattern number is 1,
and the offsets for for the second and third capturing sub-
patterns (assuming the vector is large enough, of course)
are set to -1. Note: Elements in the first two-thirds of
ovector that do not correspond to capturing parentheses in
the pattern are never changed. That is, if a pattern con-
tains n capturing parentheses, no more than ovector[0] to
ovector[2n+1] are set by pcre_exec(). The other elements (in
the first two-thirds) retain whatever values they previously
had. Some convenience functions are provided for extracting
the captured substrings as separate strings. These are
described below.
Error return values from pcre_exec()
If pcre_exec() fails, it returns a negative number. The fol-
lowing are defined in the header file:
PCRE_ERROR_NOMATCH (-1)
The subject string did not match the pattern.
PCRE_ERROR_NULL (-2)
Either code or subject was passed as NULL, or ovector was
NULL and ovecsize was not zero.
PCRE_ERROR_BADOPTION (-3)
An unrecognized bit was set in the options argument.
PCRE_ERROR_BADMAGIC (-4)
PCRE stores a 4-byte "magic number" at the start of the com-
piled code, to catch the case when it is passed a junk
pointer and to detect when a pattern that was compiled in an
environment of one endianness is run in an environment with
the other endianness. This is the error that PCRE gives when
the magic number is not present.
PCRE_ERROR_UNKNOWN_OPCODE (-5)
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
While running the pattern match, an unknown item was encoun-
tered in the compiled pattern. This error could be caused by
a bug in PCRE or by overwriting of the compiled pattern.
PCRE_ERROR_NOMEMORY (-6)
If a pattern contains back references, but the ovector that
is passed to pcre_exec() is not big enough to remember the
referenced substrings, PCRE gets a block of memory at the
start of matching to use for this purpose. If the call via
pcre_malloc() fails, this error is given. The memory is
automatically freed at the end of matching. This error is
also given if pcre_stack_malloc() fails in pcre_exec(). This
can happen only when PCRE has been compiled with --disable-
stack-for-recursion.
PCRE_ERROR_NOSUBSTRING (-7)
This error is used by the pcre_copy_substring(),
pcre_get_substring(), and pcre_get_substring_list() func-
tions (see below). It is never returned by pcre_exec().
PCRE_ERROR_MATCHLIMIT (-8)
The backtracking limit, as specified by the match_limit
field in a pcre_extra structure (or defaulted) was reached.
See the description above.
PCRE_ERROR_CALLOUT (-9)
This error is never generated by pcre_exec() itself. It is
provided for use by callout functions that want to yield a
distinctive error code. See the pcrecallout documentation
for details.
PCRE_ERROR_BADUTF8 (-10)
A string that contains an invalid UTF-8 byte sequence was
passed as a subject, and the PCRE_NO_UTF8_CHECK option was
not set. If the size of the output vector (ovecsize) is at
least 2, the byte offset to the start of the the invalid
UTF-8 character is placed in the first element, and a reason
code is placed in the second element. The reason codes are
listed in the following section. For backward compatibil-
ity, if PCRE_PARTIAL_HARD is set and the problem is a trun-
cated UTF-8 character at the end of the subject (reason
codes 1 to 5), PCRE_ERROR_SHORTUTF8 is returned instead of
PCRE_ERROR_BADUTF8.
PCRE_ERROR_BADUTF8_OFFSET (-11)
The UTF-8 byte sequence that was passed as a subject was
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
checked and found to be valid (the PCRE_NO_UTF8_CHECK option
was not set), but the value of startoffset did not point to
the beginning of a UTF-8 character or the end of the sub-
ject.
PCRE_ERROR_PARTIAL (-12)
The subject string did not match, but it did match par-
tially. See the pcrepartial documentation for details of
partial matching.
PCRE_ERROR_BADPARTIAL (-13)
This code is no longer in use. It was formerly returned when
the PCRE_PARTIAL option was used with a compiled pattern
containing items that were not supported for partial match-
ing. From release 8.00 onwards, there are no restrictions on
partial matching.
PCRE_ERROR_INTERNAL (-14)
An unexpected internal error has occurred. This error could
be caused by a bug in PCRE or by overwriting of the compiled
pattern.
PCRE_ERROR_BADCOUNT (-15)
This error is given if the value of the ovecsize argument is
negative.
PCRE_ERROR_RECURSIONLIMIT (-21)
The internal recursion limit, as specified by the
match_limit_recursion field in a pcre_extra structure (or
defaulted) was reached. See the description above.
PCRE_ERROR_BADNEWLINE (-23)
An invalid combination of PCRE_NEWLINE_xxx options was
given.
PCRE_ERROR_BADOFFSET (-24)
The value of startoffset was negative or greater than the
length of the subject, that is, the value in length.
PCRE_ERROR_SHORTUTF8 (-25)
This error is returned instead of PCRE_ERROR_BADUTF8 when
the subject string ends with a truncated UTF-8 character and
the PCRE_PARTIAL_HARD option is set. Information about the
failure is returned as for PCRE_ERROR_BADUTF8. It is in fact
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
sufficient to detect this case, but this special error code
for PCRE_PARTIAL_HARD precedes the implementation of
returned information; it is retained for backwards compati-
bility.
PCRE_ERROR_RECURSELOOP (-26)
This error is returned when pcre_exec() detects a recursion
loop within the pattern. Specifically, it means that either
the whole pattern or a subpattern has been called recur-
sively for the second time at the same position in the sub-
ject string. Some simple patterns that might do this are
detected and faulted at compile time, but more complicated
cases, in particular mutual recursions between two different
subpatterns, cannot be detected until run time.
PCRE_ERROR_JIT_STACKLIMIT (-27)
This error is returned when a pattern that was successfully
studied using a JIT compile option is being matched, but the
memory available for the just-in-time processing stack is
not large enough. See the pcrejit documentation for more
details.
PCRE_ERROR_BADMODE (-28)
This error is given if a pattern that was compiled by the
8-bit library is passed to a 16-bit or 32-bit library func-
tion, or vice versa.
PCRE_ERROR_BADENDIANNESS (-29)
This error is given if a pattern that was compiled and saved
is reloaded on a host with different endianness. The utility
function pcre_pattern_to_host_byte_order() can be used to
convert such a pattern so that it runs on the new host.
PCRE_ERROR_JIT_BADOPTION
This error is returned when a pattern that was successfully
studied using a JIT compile option is being matched, but the
matching mode (partial or complete match) does not
correspond to any JIT compilation mode. When the JIT fast
path function is used, this error may be also given for
invalid options. See the pcrejit documentation for more
details.
PCRE_ERROR_BADLENGTH (-32)
This error is given if pcre_exec() is called with a negative
value for the length argument. Error numbers -16 to -20,
-22, and 30 are not used by pcre_exec().
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
Reason codes for invalid UTF-8 strings
This section applies only to the 8-bit library. The
corresponding information for the 16-bit and 32-bit
libraries is given in the pcre16 and pcre32 pages. When
pcre_exec() returns either PCRE_ERROR_BADUTF8 or
PCRE_ERROR_SHORTUTF8, and the size of the output vector
(ovecsize) is at least 2, the offset of the start of the
invalid UTF-8 character is placed in the first output vector
element (ovector[0]) and a reason code is placed in the
second element (ovector[1]). The reason codes are given
names in the pcre.h header file:
PCRE_UTF8_ERR1
PCRE_UTF8_ERR2
PCRE_UTF8_ERR3
PCRE_UTF8_ERR4
PCRE_UTF8_ERR5
The string ends with a truncated UTF-8 character; the code
specifies how many bytes are missing (1 to 5). Although RFC
3629 restricts UTF-8 characters to be no longer than 4
bytes, the encoding scheme (originally defined by RFC 2279)
allows for up to 6 bytes, and this is checked first; hence
the possibility of 4 or 5 missing bytes.
PCRE_UTF8_ERR6
PCRE_UTF8_ERR7
PCRE_UTF8_ERR8
PCRE_UTF8_ERR9
PCRE_UTF8_ERR10
The two most significant bits of the 2nd, 3rd, 4th, 5th, or
6th byte of the character do not have the binary value 0b10
(that is, either the most significant bit is 0, or the next
bit is 1).
PCRE_UTF8_ERR11
PCRE_UTF8_ERR12
A character that is valid by the RFC 2279 rules is either 5
or 6 bytes long; these code points are excluded by RFC 3629.
PCRE_UTF8_ERR13
A 4-byte character has a value greater than 0x10fff; these
code points are excluded by RFC 3629.
PCRE_UTF8_ERR14
A 3-byte character has a value in the range 0xd800 to
0xdfff; this range of code points are reserved by RFC 3629
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
for use with UTF-16, and so are excluded from UTF-8.
PCRE_UTF8_ERR15
PCRE_UTF8_ERR16
PCRE_UTF8_ERR17
PCRE_UTF8_ERR18
PCRE_UTF8_ERR19
A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that
is, it codes for a value that can be represented by fewer
bytes, which is invalid. For example, the two bytes 0xc0,
0xae give the value 0x2e, whose correct coding uses just one
byte.
PCRE_UTF8_ERR20
The two most significant bits of the first byte of a charac-
ter have the binary value 0b10 (that is, the most signifi-
cant bit is 1 and the second is 0). Such a byte can only
validly occur as the second or subsequent byte of a multi-
byte character.
PCRE_UTF8_ERR21
The first byte of a character has the value 0xfe or 0xff.
These values can never occur in a valid UTF-8 string.
PCRE_UTF8_ERR22
This error code was formerly used when the presence of a
so-called "non-character" caused an error. Unicode corrigen-
dum #9 makes it clear that such characters should not cause
a string to be rejected, and so this code is no longer in
use and is never returned.
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
Captured substrings can be accessed directly by using the
offsets returned by pcre_exec() in ovector. For convenience,
the functions pcre_copy_substring(), pcre_get_substring(),
and pcre_get_substring_list() are provided for extracting
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
captured substrings as new, separate, zero-terminated
strings. These functions identify substrings by number. The
next section describes functions for extracting named sub-
strings. A substring that contains a binary zero is
correctly extracted and has a further zero added on the end,
but the result is not, of course, a C string. However, you
can process such a string by referring to the length that is
returned by pcre_copy_substring() and pcre_get_substring().
Unfortunately, the interface to pcre_get_substring_list() is
not adequate for handling strings containing binary zeros,
because the end of the final string is not independently
indicated. The first three arguments are the same for all
three of these functions: subject is the subject string
that has just been successfully matched, ovector is a
pointer to the vector of integer offsets that was passed to
pcre_exec(), and stringcount is the number of substrings
that were captured by the match, including the substring
that matched the entire regular expression. This is the
value returned by pcre_exec() if it is greater than zero. If
pcre_exec() returned zero, indicating that it ran out of
space in ovector, the value passed as stringcount should be
the number of elements in the vector divided by three. The
functions pcre_copy_substring() and pcre_get_substring()
extract a single substring, whose number is given as string-
number. A value of zero extracts the substring that matched
the entire pattern, whereas higher values extract the cap-
tured substrings. For pcre_copy_substring(), the string is
placed in buffer, whose length is given by buffersize, while
for pcre_get_substring() a new block of memory is obtained
via pcre_malloc, and its address is returned via stringptr.
The yield of the function is the length of the string, not
including the terminating zero, or one of these error codes:
PCRE_ERROR_NOMEMORY (-6)
The buffer was too small for pcre_copy_substring(), or the
attempt to get memory failed for pcre_get_substring().
PCRE_ERROR_NOSUBSTRING (-7)
There is no substring whose number is stringnumber. The
pcre_get_substring_list() function extracts all available
substrings and builds a list of pointers to them. All this
is done in a single block of memory that is obtained via
pcre_malloc. The address of the memory block is returned via
listptr, which is also the start of the list of string
pointers. The end of the list is marked by a NULL pointer.
The yield of the function is zero if all went well, or the
error code
PCRE_ERROR_NOMEMORY (-6)
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
if the attempt to get the memory block failed. When any of
these functions encounter a substring that is unset, which
can happen when capturing subpattern number n+1 matches some
part of the subject, but subpattern n has not been used at
all, they return an empty string. This can be distinguished
from a genuine zero-length substring by inspecting the
appropriate offset in ovector, which is negative for unset
substrings. The two convenience functions
pcre_free_substring() and pcre_free_substring_list() can be
used to free the memory returned by a previous call of
pcre_get_substring() or pcre_get_substring_list(), respec-
tively. They do nothing more than call the function pointed
to by pcre_free, which of course could be called directly
from a C program. However, PCRE is used in some situations
where it is linked via a special interface to another pro-
gramming language that cannot use pcre_free directly; it is
for these cases that the functions are provided.
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
To extract a substring by name, you first have to find asso-
ciated number. For example, for this pattern
(a+)b(?<xxx>\d+)...
the number of the subpattern called "xxx" is 2. If the name
is known to be unique (PCRE_DUPNAMES was not set), you can
find the number from the name by calling
pcre_get_stringnumber(). The first argument is the compiled
pattern, and the second is the name. The yield of the func-
tion is the subpattern number, or PCRE_ERROR_NOSUBSTRING (-
7) if there is no subpattern of that name. Given the
number, you can extract the substring directly, or use one
of the functions described in the previous section. For con-
venience, there are also two functions that do the whole
job. Most of the arguments of pcre_copy_named_substring()
and pcre_get_named_substring() are the same as those for the
similarly named functions that extract by number. As these
are described in the previous section, they are not re-
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
described here. There are just two differences: First,
instead of a substring number, a substring name is given.
Second, there is an extra argument, given at the start,
which is a pointer to the compiled pattern. This is needed
in order to gain access to the name-to-number translation
table. These functions call pcre_get_stringnumber(), and if
it succeeds, they then call pcre_copy_substring() or
pcre_get_substring(), as appropriate. NOTE: If PCRE_DUPNAMES
is set and there are duplicate names, the behaviour may not
be what you want (see the next section). Warning: If the
pattern uses the (?| feature to set up multiple subpatterns
with the same number, as described in the section on dupli-
cate subpattern numbers in the pcrepattern page, you cannot
use names to distinguish the different subpatterns, because
names are not included in the compiled code. The matching
process uses only numbers. For this reason, the use of dif-
ferent names for subpatterns of the same number causes an
error at compile time.
DUPLICATE SUBPATTERN NAMES
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
When a pattern is compiled with the PCRE_DUPNAMES option,
names for subpatterns are not required to be unique. (Dupli-
cate names are always allowed for subpatterns with the same
number, created by using the (?| feature. Indeed, if such
subpatterns are named, they are required to use the same
names.) Normally, patterns with duplicate names are such
that in any one match, only one of the named subpatterns
participates. An example is shown in the pcrepattern docu-
mentation. When duplicates are present,
pcre_copy_named_substring() and pcre_get_named_substring()
return the first substring corresponding to the given name
that is set. If none are set, PCRE_ERROR_NOSUBSTRING (-7) is
returned; no data is returned. The pcre_get_stringnumber()
function returns one of the numbers that are associated with
the name, but it is not defined which it is. If you want to
get full details of all captured substrings for a given
name, you must use the pcre_get_stringtable_entries() func-
tion. The first argument is the compiled pattern, and the
second is the name. The third and fourth are pointers to
variables which are updated by the function. After it has
run, they point to the first and last entries in the name-
to-number table for the given name. The function itself
returns the length of each entry, or PCRE_ERROR_NOSUBSTRING
(-7) if there are none. The format of the table is described
above in the section entitled Information about a pattern
above. Given all the relevant entries for the name, you can
extract each of their numbers, and hence the captured data,
if any.
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PCREAPI(3) C LIBRARY FUNCTIONS PCREAPI(3)
FINDING ALL POSSIBLE MATCHES
The traditional matching function uses a similar algorithm
to Perl, which stops when it finds the first match, starting
at a given point in the subject. If you want to find all
possible matches, or the longest possible match, consider
using the alternative matching function (see below) instead.
If you cannot use the alternative function, but still need
to find all possible matches, you can kludge it up by making
use of the callout facility, which is described in the pcre-
callout documentation. What you have to do is to insert a
callout right at the end of the pattern. When your callout
function is called, extract and save the current matched
substring. Then return 1, which forces pcre_exec() to back-
track and try other alternatives. Ultimately, when it runs
out of matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
OBTAINING AN ESTIMATE OF STACK USAGE
Matching certain patterns using pcre_exec() can use a lot of
process stack, which in certain environments can be rather
limited in size. Some users find it helpful to have an esti-
mate of the amount of stack that is used by pcre_exec(), to
help them set recursion limits, as described in the pcres-
tack documentation. The estimate that is output by pcretest
when called with the -m and -C options is obtained by cal-
ling pcre_exec with the values NULL, NULL, NULL, -999, and
-999 for its first five arguments. Normally, if its first
argument is NULL, pcre_exec() immediately returns the nega-
tive error code PCRE_ERROR_NULL, but with this special com-
bination of arguments, it returns instead a negative number
whose absolute value is the approximate stack frame size in
bytes. (A negative number is used so that it is clear that
no match has happened.) The value is approximate because in
some cases, recursive calls to pcre_exec() occur when there
are one or two additional variables on the stack. If PCRE
has been compiled to use the heap instead of the stack for
recursion, the value returned is the size of each block that
is obtained from the heap.
MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
The function pcre_dfa_exec() is called to match a subject
string against a compiled pattern, using a matching algo-
rithm that scans the subject string just once, and does not
backtrack. This has different characteristics to the normal
algorithm, and is not compatible with Perl. Some of the
features of PCRE patterns are not supported. Nevertheless,
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there are times when this kind of matching can be useful.
For a discussion of the two matching algorithms, and a list
of features that pcre_dfa_exec() does not support, see the
pcrematching documentation. The arguments for the
pcre_dfa_exec() function are the same as for pcre_exec(),
plus two extras. The ovector argument is used in a different
way, and this is described below. The other common arguments
are used in the same way as for pcre_exec(), so their
description is not repeated here. The two additional argu-
ments provide workspace for the function. The workspace vec-
tor should contain at least 20 elements. It is used for
keeping track of multiple paths through the pattern tree.
More workspace will be needed for patterns and subjects
where there are a lot of potential matches. Here is an
example of a simple call to pcre_dfa_exec():
int rc;
int ovector[10];
int wspace[20];
rc = pcre_dfa_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring
information */
10, /* number of elements (NOT size in
bytes) */
wspace, /* working space vector */
20); /* number of elements (NOT size in
bytes) */
Option bits for pcre_dfa_exec()
The unused bits of the options argument for pcre_dfa_exec()
must be zero. The only bits that may be set are
PCRE_ANCHORED, PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL,
PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK,
PCRE_BSR_ANYCRLF, PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE,
PCRE_PARTIAL_HARD, PCRE_PARTIAL_SOFT, PCRE_DFA_SHORTEST, and
PCRE_DFA_RESTART. All but the last four of these are
exactly the same as for pcre_exec(), so their description is
not repeated here.
PCRE_PARTIAL_HARD
PCRE_PARTIAL_SOFT
These have the same general effect as they do for
pcre_exec(), but the details are slightly different. When
PCRE_PARTIAL_HARD is set for pcre_dfa_exec(), it returns
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PCRE_ERROR_PARTIAL if the end of the subject is reached and
there is still at least one matching possibility that
requires additional characters. This happens even if some
complete matches have also been found. When
PCRE_PARTIAL_SOFT is set, the return code PCRE_ERROR_NOMATCH
is converted into PCRE_ERROR_PARTIAL if the end of the sub-
ject is reached, there have been no complete matches, but
there is still at least one matching possibility. The por-
tion of the string that was inspected when the longest par-
tial match was found is set as the first matching string in
both cases. There is a more detailed discussion of partial
and multi-segment matching, with examples, in the pcrepar-
tial documentation.
PCRE_DFA_SHORTEST
Setting the PCRE_DFA_SHORTEST option causes the matching
algorithm to stop as soon as it has found one match. Because
of the way the alternative algorithm works, this is neces-
sarily the shortest possible match at the first possible
matching point in the subject string.
PCRE_DFA_RESTART
When pcre_dfa_exec() returns a partial match, it is possible
to call it again, with additional subject characters, and
have it continue with the same match. The PCRE_DFA_RESTART
option requests this action; when it is set, the workspace
and wscount options must reference the same vector as before
because data about the match so far is left in them after a
partial match. There is more discussion of this facility in
the pcrepartial documentation.
Successful returns from pcre_dfa_exec()
When pcre_dfa_exec() succeeds, it may have matched more than
one substring in the subject. Note, however, that all the
matches from one run of the function start at the same point
in the subject. The shorter matches are all initial sub-
strings of the longer matches. For example, if the pattern
<.*>
is matched against the string
This is <something> <something else> <something further>
no more
the three matched strings are
<something>
<something> <something else>
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<something> <something else> <something further>
On success, the yield of the function is a number greater
than zero, which is the number of matched substrings. The
substrings themselves are returned in ovector. Each string
uses two elements; the first is the offset to the start, and
the second is the offset to the end. In fact, all the
strings have the same start offset. (Space could have been
saved by giving this only once, but it was decided to retain
some compatibility with the way pcre_exec() returns data,
even though the meaning of the strings is different.) The
strings are returned in reverse order of length; that is,
the longest matching string is given first. If there were
too many matches to fit into ovector, the yield of the func-
tion is zero, and the vector is filled with the longest
matches. Unlike pcre_exec(), pcre_dfa_exec() can use the
entire ovector for returning matched strings. NOTE: PCRE's
"auto-possessification" optimization usually applies to
character repeats at the end of a pattern (as well as inter-
nally). For example, the pattern "a\d+" is compiled as if it
were "a\d++" because there is no point even considering the
possibility of backtracking into the repeated digits. For
DFA matching, this means that only one possible match is
found. If you really do want multiple matches in such cases,
either use an ungreedy repeat ("a\d+?") or set the
PCRE_NO_AUTO_POSSESS option when compiling.
Error returns from pcre_dfa_exec()
The pcre_dfa_exec() function returns a negative number when
it fails. Many of the errors are the same as for
pcre_exec(), and these are described above. There are in
addition the following errors that are specific to
pcre_dfa_exec():
PCRE_ERROR_DFA_UITEM (-16)
This return is given if pcre_dfa_exec() encounters an item
in the pattern that it does not support, for instance, the
use of \C or a back reference.
PCRE_ERROR_DFA_UCOND (-17)
This return is given if pcre_dfa_exec() encounters a condi-
tion item that uses a back reference for the condition, or a
test for recursion in a specific group. These are not sup-
ported.
PCRE_ERROR_DFA_UMLIMIT (-18)
This return is given if pcre_dfa_exec() is called with an
extra block that contains a setting of the match_limit or
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match_limit_recursion fields. This is not supported (these
fields are meaningless for DFA matching).
PCRE_ERROR_DFA_WSSIZE (-19)
This return is given if pcre_dfa_exec() runs out of space in
the workspace vector.
PCRE_ERROR_DFA_RECURSE (-20)
When a recursive subpattern is processed, the matching func-
tion calls itself recursively, using private vectors for
ovector and workspace. This error is given if the output
vector is not large enough. This should be extremely rare,
as a vector of size 1000 is used.
PCRE_ERROR_DFA_BADRESTART (-30)
When pcre_dfa_exec() is called with the PCRE_DFA_RESTART
option, some plausibility checks are made on the contents of
the workspace, which should contain data about the previous
partial match. If any of these checks fail, this error is
given.
SEE ALSO
pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3),
pcrecpp(3)(3), pcrematching(3), pcrepartial(3), pcrepo-
six(3), pcreprecompile(3), pcresample(3), pcrestack(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 18 December 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE 8.39 Last change: 18 December 2015 53
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