Provided by: libpcre3-dev_8.39-13ubuntu0.22.04.1_amd64 
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);
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);
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 otherwise. 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 functionality. They are described
in the pcreposix documentation. Both of these APIs define a set of C function calls. A C++
wrapper (again for the 8-bit library only) is also distributed 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 macros 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 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
patterns. 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 matching functions, there are convenience functions
for extracting 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 internal 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() maintains a reference count in a data block containing a
compiled pattern. This is provided for the benefit of object-oriented applications.
The global variables pcre_malloc and 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 calling program can replace them if it wishes to
intercept the calls. This should be done before calling any PCRE functions.
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() function. 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 pcrestack documentation.
The global variable pcre_callout initially contains NULL. It can be set by the caller to a
"callout" 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 compilation 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 compiled, 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 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 newline 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 controlled in a similar way, but
by separate options.
MULTITHREADING
The PCRE functions can be used in multi-threading applications, 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 functions 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
compiled 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
compiled. Details are given in the pcreprecompile documentation, which includes a
description of the pcre_pattern_to_host_byte_order() function. However, compiling 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 compiled into the PCRE library. The pcrebuild documentation has more
details about these optional features.
The first argument 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 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 version 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 support is available; otherwise it
is set to zero. This value should normally be given to the 16-bit version of this
function, 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 support is available; otherwise it
is set to zero. This value should normally be given to the 32-bit version of this
function, 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
configured, 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 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 corresponding 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 character 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 compiled 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 compiled, 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 application.
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
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 matching 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 recursive function calls that use the stack to remember their state.
This is the usual way that PCRE is compiled. The output 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 relocatable, because it may
contain a copy of the tableptr argument, which is an address (see below).
The options argument contains various bit settings that affect the compilation. It should
be zero if no options are required. The available options are described below. Some of
them (in particular, those that are compatible with Perl, but some others as well) can
also be set and unset from within the pattern (see the detailed description in the
pcrepattern documentation). For those options that can be different in different parts of
the pattern, the contents of the options argument specifies their settings at the start of
compilation 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 compilation 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 characters, 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 discussion, 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 */
&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 matching 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 pcrecallout
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 otherwise. 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 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 indicates a newline. However, it only ever matches one character, 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 setting. 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 subpatterns 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 pattern 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 newlines 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 pattern; escape sequences
that happen to represent a newline do not count.
This option makes it possible to include comments inside complicated patterns. Note,
however, that this applies only to data characters. White space characters may never
appear within special character sequences in a pattern, 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 characters).
(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).
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" metacharacter ($) 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
newline. 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 pattern, 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 patterns 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 preceding sequences should be recognized. Setting PCRE_NEWLINE_ANY
specifies that any Unicode newline sequence should be recognized.
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.
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 specially 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 character 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 newline 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 capturing 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 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
reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of passing
an invalid UTF-8 string as a pattern 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 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
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 subpattern
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 compatibility 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 property
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)
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 produce 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 information. It may still
return NULL, however, if an error occurs in pcre_study().
The second argument of pcre_study() contains option bits. There are three further options
in addition 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. 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 application
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 calling 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.
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 compilation
has happened) JIT execution is disabled. For JIT execution 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 characters 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. However, when PCRE is built, it is
possible to cause the internal 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
applications 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 the caller's responsibility to ensure that the memory containing 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 pattern, compilation, studying and matching all
happen in the same locale, but different patterns can be processed in different 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 provided 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 argument 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:
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 pattern. 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
(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 construction 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
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 function is unable to return
the full 32-bit range of characters, 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
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 information 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 registers 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 lookbehinds 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. Otherwise 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 identifying 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 converting 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
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 corresponding 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 subpattern 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
pattern.
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 documentation gives details of partial matching.
PCRE_INFO_OPTIONS
Return a copy of the options with which the pattern was compiled. 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 compiled 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
compiled 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 matching (see the
section entitled "Studying a pattern" above). The format of the study_data block is
private, but its 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 pattern 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,
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 alternative matching function, which is
described below in the section about the pcre_dfa_exec() function.
In most applications, the pattern will have been compiled (and optionally studied) in the
same process that calls pcre_exec(). However, it is possible to save compiled patterns and
study data, and then use them later in different processes, possibly 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;
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 possibilities 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 runaway matching that goes on for a very long time, and so the match_limit
value is also used in this case (but in a different 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 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 recursion. The
recursion depth is a smaller number than the total number of calls, because not all calls
to match() are recursive. 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 internal tables to be used.
Warning: The tables that pcre_exec() uses must be the same as those that were used when
the pattern was compiled. If this is not the case, the behaviour of pcre_exec() is
undefined. Therefore, when a pattern is compiled 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 pattern 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 compiled pattern; if you wish to retain such a name you must 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 backtracking 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 match
attempt for an unanchored pattern fails when the current position is at a CRLF sequence,
and the pattern contains 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 characters,
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).
Notwithstanding 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 pattern, 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 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 optimizations, possibly causing
performance to suffer, but ensuring that in cases where the result is "no match", the
callouts 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
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 subject 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 optimizations 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 subject 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 subject, 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 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 alternatives. 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 overrides 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 matching string. There is a more detailed discussion of partial
and multi-segment matching, with examples, in the pcrepartial 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 startoffset. 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 startoffset is negative or greater than the length of the subject, 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. Setting startoffset differs from
just passing over a shortened string and setting PCRE_NOTBOL in the case of a pattern that
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 newline, 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 pattern 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 subject, 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 subpattern 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
information. The number passed in ovecsize should always be a 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 captured, 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 substring 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 usually
advisable to supply an ovector of reasonable size.
There are some cases where zero is returned (indicating vector 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 substring) 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 substring matched by the whole pattern, is
(n+1)*3.
It is 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 expression 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 subpatterns (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 contains 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 following 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 compiled 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)
While running the pattern match, an unknown item was encountered 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() functions (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 compatibility, if PCRE_PARTIAL_HARD is set
and the problem is a truncated 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 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 subject.
PCRE_ERROR_PARTIAL (-12)
The subject string did not match, but it did match partially. 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
matching. 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 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 compatibility.
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
recursively for the second time at the same position in the subject 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 function, 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().
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 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 character have the binary value 0b10
(that is, the most significant 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 corrigendum #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 captured substrings as new,
separate, zero-terminated strings. These functions identify substrings by number. The next
section describes functions for extracting named substrings.
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 stringnumber. A value of zero extracts the substring that matched
the entire pattern, whereas higher values extract the captured 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)
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(), respectively. 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 programming 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 associated 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 function 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 convenience, 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-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 duplicate 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 different 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. (Duplicate 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 documentation.
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() function. 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.
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 pcrecallout 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 backtrack 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
estimate of the amount of stack that is used by pcre_exec(), to help them set recursion
limits, as described in the pcrestack documentation. The estimate that is output by
pcretest when called with the -m and -C options is obtained by calling 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 negative
error code PCRE_ERROR_NULL, but with this special combination 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 algorithm 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, 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 arguments provide workspace for the function. The workspace vector
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
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 subject is
reached, there have been no complete matches, but there is still at least one matching
possibility. The portion of the string that was inspected when the longest partial 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 pcrepartial
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
necessarily 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 substrings 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>
<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
function 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 internally). 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 condition item that uses a back
reference for the condition, or a test for recursion in a specific group. These are not
supported.
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 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 function 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), pcreposix(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.