PCREAPI(3)PCREAPI(3)NAME
PCRE - Perl-compatible regular expressions
#include <pcre.h>
PCRE NATIVE API BASIC FUNCTIONS
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
void pcre_free_study(pcre_extra *extra);
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
PCRE NATIVE API STRING EXTRACTION FUNCTIONS
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
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 *);
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 val‐
ues.
References to bytes and UTF-8 in this document should be read as refer‐
ences 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 cor‐
respond 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_mal‐
loc() 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 sim‐
plest 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 dis‐
cussed in the pcrejit documentation.
A second matching function, pcre_dfa_exec(), which is not Perl-compati‐
ble, is also provided. This uses a different algorithm for the match‐
ing. 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 docu‐
mentation.
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, respec‐
tively. 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 environ‐
ments 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 are always called in a stack-like manner (last
obtained, first freed), and always for memory blocks of the same size.
There is a discussion about PCRE's stack usage in the pcrestack docu‐
mentation.
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.
NEWLINES
PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (line‐
feed) character, the two-character sequence CRLF, any of the three pre‐
ceding, 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 stan‐
dard. 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 char‐
acter 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 advance‐
ment 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 function pointed to by pcre_callout, are shared by all threads.
The compiled form of a regular expression is not altered during match‐
ing, 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 sepa‐
rate 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 regu‐
lar expression with one version of PCRE for use with a different ver‐
sion 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 dis‐
cover which optional features have been compiled into the PCRE library.
The pcrebuild documentation has more details about these optional fea‐
tures.
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 avail‐
able; 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 avail‐
able; 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 avail‐
able; 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 architec‐
ture 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 corre‐
spond 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 pat‐
tern 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 spec‐
ified when PCRE is built; the default is 250.
PCRE_CONFIG_MATCH_LIMIT
The output is a long integer that gives the default limit for the num‐
ber 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 argu‐
ment, which is an address (see below).
The options argument contains various bit settings that affect the com‐
pilation. It should be zero if no options are required. The available
options are described below. Some of them (in 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 mes‐
sage. 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 char‐
acter.
If pcre_compile2() is used instead of pcre_compile(), and the error‐
codeptr 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_com‐
pile():
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 set‐
ting 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 com‐
piled 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 char‐
acter 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 set‐
ting 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. How‐
ever, 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 conven‐
tions" 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 intro‐
duces 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 alterna‐
tive 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 com‐
pile 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 new‐
lines 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 pre‐
vents 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 sep‐
arator, 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 sensi‐
ble. 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 # out‐
side 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 paren‐
theses 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 match‐
ing 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 prop‐
erty support.
PCRE_UNGREEDY
This option inverts the "greediness" of the quantifiers so that they
are not greedy by default, but become greedy if followed by "?". It is
not compatible with Perl. It can also be set by a (?U) option setting
within the pattern.
PCRE_UTF8
This option causes PCRE to regard both the pattern and the subject as
strings of UTF-8 characters instead of single-byte strings. However, it
is available only when PCRE is built to include UTF support. If not,
the use of this option provokes an error. Details of how this option
changes the behaviour of PCRE are given in the pcreunicode page.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
automatically checked. There is a discussion about the validity of
UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is
found, pcre_compile() returns an error. If you already know that your
pattern is valid, and you want to skip this check for performance rea‐
sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the
effect of passing an invalid UTF-8 string as a 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
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 pat‐
tern 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 con‐
tains 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 pat‐
terns 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 documenta‐
tion.
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 optimiza‐
tions 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 exe‐
cution time. However, if PCRE_NO_START_OPTIMIZE is passed to
pcre_exec(), (that is, after any JIT compilation has happened) JIT exe‐
cution is disabled. For JIT execution to work with PCRE_NO_START_OPTI‐
MIZE, 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 sup‐
port, 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 char‐
acters. However, when PCRE is built, it is possible to cause the inter‐
nal tables to be rebuilt in the default "C" locale of the local system,
which may cause them to be different.
The internal tables can always be overridden by tables supplied by the
application that calls PCRE. These may be created in a different locale
from the default. As more and more applications change to using Uni‐
code, 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 let‐
ters), 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 pat‐
tern, 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 discus‐
sion below in the section on matching a pattern). This facility is pro‐
vided for use with pre-compiled patterns that have been saved and
reloaded. Character tables are not saved with patterns, so if a non-
standard table was used at compile time, it must be provided again when
the reloaded pattern is matched. Attempting to use this facility to
match a pattern in a different locale from the one in which it was com‐
piled 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 pat‐
tern. 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 endi‐
anness 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() func‐
tion. External callers can cause PCRE to use its internal tables by
passing a NULL table pointer.
PCRE_INFO_FIRSTBYTE
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.
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.
Since for the 32-bit library using the non-UTF-32 mode, this function
is unable to return the full 32-bit range of the character, this value
is deprecated; instead the PCRE_INFO_FIRSTCHARACTERFLAGS and
PCRE_INFO_FIRSTCHARACTER values should be used.
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 par‐
ticular 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 argu‐
ment 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 look‐
behind. \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 parenthe‐
ses. The names are just an additional way of identifying the parenthe‐
ses, which still acquire numbers. Several convenience functions such as
pcre_get_named_substring() are provided for extracting captured sub‐
strings 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 num‐
ber 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_DUP‐
NAMES 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 match‐
ing.
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 argu‐
ment 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_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_FIRSTCHARACTER
Return the fixed first character value 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_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 fol‐
lows 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 argu‐
ments 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 option‐
ally 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 addi‐
tional 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 some‐
times 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 unlim‐
ited 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 match‐
ing 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 recur‐
sive. This limit is of use only if it is set smaller than match_limit.
Limiting the recursion depth limits the amount of machine stack that
can be used, or, when PCRE has been compiled to use memory on the heap
instead of the stack, the amount of heap memory that can be used. This
limit is not relevant, and is ignored, when matching is done using JIT
compiled code.
The default value for match_limit_recursion can be set when PCRE is
built; the default default is the same value as the default for
match_limit. You can override the default by suppling pcre_exec() with
a pcre_extra block in which match_limit_recursion is set, and
PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
A value for the recursion limit may also be supplied by an item at the
start of a pattern of the form
(*LIMIT_RECURSION=d)
where d is a decimal number. However, such a setting is ignored unless
d is less than the limit set by the caller of pcre_exec() or, if no
such limit is set, less than the default.
The callout_data field is used in conjunction with the "callout" fea‐
ture, 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 dis‐
cussion 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 auto‐
matically 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 back‐
tracking control verbs such as (*MARK:NAME), and the execution ends up
with a name to pass back, a pointer to the name string (zero termi‐
nated) 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 doc‐
umentation.
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 descrip‐
tion of pcre_compile() above. During matching, the newline choice
affects the behaviour of the dot, circumflex, and dollar metacharac‐
ters. 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 cur‐
rent 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 con‐
tains an explicit CR or LF reference, and so advances only by one char‐
acter 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 behav‐
iour 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 with‐
out 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 occur‐
rences 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 off‐
set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
fails, by advancing the starting offset (see below) and trying an ordi‐
nary 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 pat‐
tern 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 pat‐
tern 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 pat‐
tern 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 opti‐
mizations 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 enti‐
tled Error return values from pcre_exec() below). If startoffset con‐
tains 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 com‐
patibility, 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 consid‐
ered 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 sub‐
ject (in UTF-32 mode, one data unit equals one character, so all off‐
sets 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 suc‐
cess. 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 occur‐
rence 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 pcre‐
demo 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 sub‐
string. 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 vec‐
tor 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 sub‐
strings, each substring using a pair of integers. The remaining third
of the vector is used as workspace by pcre_exec() while matching cap‐
turing 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 off‐
sets, 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 pat‐
tern 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 over‐
flow) 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 tem‐
porary 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 corre‐
sponding 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 corre‐
spond to capturing parentheses in the pattern are never changed. That
is, if a pattern contains n capturing parentheses, no more than ovec‐
tor[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 ele‐
ment, 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 char‐
acter 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 charac‐
ter 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 implementa‐
tion of returned information; it is retained for backwards compatibil‐
ity.
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, can‐
not 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_SHORT‐
UTF8, 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 (origi‐
nally 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 cor‐
rect 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 sec‐
ond is 0). Such a byte can only validly occur as the second or subse‐
quent 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_sub‐
string_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_sub‐
string(). 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 func‐
tions: 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_sub‐
string(), 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 sub‐
strings 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 sub‐
string by inspecting the appropriate offset in ovector, which is nega‐
tive for unset substrings.
The two convenience functions pcre_free_substring() and pcre_free_sub‐
string_list() can be used to free the memory returned by a previous
call of pcre_get_substring() or pcre_get_substring_list(), respec‐
tively. They do nothing more than call the function pointed to by
pcre_free, which of course could be called directly from a C program.
However, PCRE is used in some situations where it is linked via a spe‐
cial interface to another programming language that cannot use
pcre_free directly; it is for these cases that the functions are pro‐
vided.
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 num‐
ber. 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 com‐
piled 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. Sec‐
ond, 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 appropri‐
ate. 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 subpat‐
terns 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 sec‐
tion entitled Information about a pattern above. Given all the rele‐
vant 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 documen‐
tation.
What you have to do is to insert a callout right at the end of the pat‐
tern. When your callout function is called, extract and save the cur‐
rent 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 call‐
ing 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 nega‐
tive 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. Never‐
theless, 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 documenta‐
tion.
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 differ‐
ent 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_NEW‐
LINE_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_PAR‐
TIAL_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 sub‐
ject 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 exam‐
ples, 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 alterna‐
tive 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 sub‐
string 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 long‐
est 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 possi‐
ble 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 pat‐
tern 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 ALSOpcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3),
pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcre‐
sample(3), pcrestack(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 12 November 2013
Copyright (c) 1997-2013 University of Cambridge.
PCRE 8.34 12 November 2013 PCREAPI(3)
