PCREAPI(3) Introduction to Library Functions PCREAPI(3)

NAME

PCRE - Perl-compatible regular expressions

#include <pcre.h>

PCRE NATIVE API BASIC FUNCTIONS

pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);

pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);

pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);

void pcre_free_study(pcre_extra *extra);

int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);

int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);

PCRE NATIVE API STRING EXTRACTION FUNCTIONS

int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);

int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);

int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);

int pcre_get_stringnumber(const pcre *code,
const char *name);

int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);

int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);

int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);

void pcre_free_substring(const char *stringptr);

void pcre_free_substring_list(const char **stringptr);

PCRE NATIVE API AUXILIARY FUNCTIONS

int pcre_jit_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
pcre_jit_stack *jstack);

pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);

void pcre_jit_stack_free(pcre_jit_stack *stack);

void pcre_assign_jit_stack(pcre_extra *extra,
pcre_jit_callback callback, void *data);

const unsigned char *pcre_maketables(void);

int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);

int pcre_refcount(pcre *code, int adjust);

int pcre_config(int what, void *where);

const char *pcre_version(void);

int pcre_pattern_to_host_byte_order(pcre *code,
pcre_extra *extra, const unsigned char *tables);

PCRE NATIVE API INDIRECTED FUNCTIONS

void *(*pcre_malloc)(size_t);

void (*pcre_free)(void *);

void *(*pcre_stack_malloc)(size_t);

void (*pcre_stack_free)(void *);

int (*pcre_callout)(pcre_callout_block *);

int (*pcre_stack_guard)(void);

PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES
As well as support for 8-bit character strings, PCRE also supports
16-bit strings (from release 8.30) and 32-bit strings (from release
8.32), by means of two additional libraries. They can be built as
well as, or instead of, the 8-bit library. To avoid too much
complication, this document describes the 8-bit versions of the
functions, with only occasional references to the 16-bit and 32-bit
libraries.

The 16-bit and 32-bit functions operate in the same way as their
8-bit counterparts; they just use different data types for their
arguments and results, and their names start with pcre16_ or pcre32_
instead of pcre_. For every option that has UTF8 in its name (for
example, PCRE_UTF8), there are corresponding 16-bit and 32-bit names
with UTF8 replaced by UTF16 or UTF32, respectively. This facility is
in fact just cosmetic; the 16-bit and 32-bit option names define the
same bit values.

References to bytes and UTF-8 in this document should be read as
references to 16-bit data units and UTF-16 when using the 16-bit
library, or 32-bit data units and UTF-32 when using the 32-bit
library, unless specified otherwise. More details of the specific
differences for the 16-bit and 32-bit libraries are given in the
pcre16 and pcre32 pages.

PCRE API OVERVIEW

PCRE has its own native API, which is described in this document.
There are also some wrapper functions (for the 8-bit library only)
that correspond to the POSIX regular expression API, but they do not
give access to all the functionality. They are described in the
pcreposix documentation. Both of these APIs define a set of C
function calls. A C++ wrapper (again for the 8-bit library only) is
also distributed with PCRE. It is documented in the pcrecpp page.

The native API C function prototypes are defined in the header file
pcre.h, and on Unix-like systems the (8-bit) library itself is called
libpcre. It can normally be accessed by adding -lpcre to the command
for linking an application that uses PCRE. The header file defines
the macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor
release numbers for the library. Applications can use these to
include support for different releases of PCRE.

In a Windows environment, if you want to statically link an
application program against a non-dll pcre.a file, you must define
PCRE_STATIC before including pcre.h or pcrecpp.h, because otherwise
the pcre_malloc() and pcre_free() exported functions will be declared
__declspec(dllimport), with unwanted results.

The functions pcre_compile(), pcre_compile2(), pcre_study(), and
pcre_exec() are used for compiling and matching regular expressions
in a Perl-compatible manner. A sample program that demonstrates the
simplest way of using them is provided in the file called pcredemo.c
in the PCRE source distribution. A listing of this program is given
in the pcredemo documentation, and the pcresample documentation
describes how to compile and run it.

Just-in-time compiler support is an optional feature of PCRE that can
be built in appropriate hardware environments. It greatly speeds up
the matching performance of many patterns. Simple programs can easily
request that it be used if available, by setting an option that is
ignored when it is not relevant. More complicated programs might need
to make use of the functions pcre_jit_stack_alloc(),
pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to
control the JIT code's memory usage.

From release 8.32 there is also a direct interface for JIT execution,
which gives improved performance. The JIT-specific functions are
discussed in the pcrejit documentation.

A second matching function, pcre_dfa_exec(), which is not Perl-
compatible, is also provided. This uses a different algorithm for the
matching. The alternative algorithm finds all possible matches (at a
given point in the subject), and scans the subject just once (unless
there are lookbehind assertions). However, this algorithm does not
return captured substrings. A description of the two matching
algorithms and their advantages and disadvantages is given in the
pcrematching documentation.

In addition to the main compiling and matching functions, there are
convenience functions for extracting captured substrings from a
subject string that is matched by pcre_exec(). They are:

pcre_copy_substring()
pcre_copy_named_substring()
pcre_get_substring()
pcre_get_named_substring()
pcre_get_substring_list()
pcre_get_stringnumber()
pcre_get_stringtable_entries()

pcre_free_substring() and pcre_free_substring_list() are also
provided, to free the memory used for extracted strings.

The function pcre_maketables() is used to build a set of character
tables in the current locale for passing to pcre_compile(),
pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
provided for specialist use. Most commonly, no special tables are
passed, in which case internal tables that are generated when PCRE is
built are used.

The function pcre_fullinfo() is used to find out information about a
compiled pattern. The function pcre_version() returns a pointer to a
string containing the version of PCRE and its date of release.

The function pcre_refcount() maintains a reference count in a data
block containing a compiled pattern. This is provided for the benefit
of object-oriented applications.

The global variables pcre_malloc and pcre_free initially contain the
entry points of the standard malloc() and free() functions,
respectively. PCRE calls the memory management functions via these
variables, so a calling program can replace them if it wishes to
intercept the calls. This should be done before calling any PCRE
functions.

The global variables pcre_stack_malloc and pcre_stack_free are also
indirections to memory management functions. These special functions
are used only when PCRE is compiled to use the heap for remembering
data, instead of recursive function calls, when running the
pcre_exec() function. See the pcrebuild documentation for details of
how to do this. It is a non-standard way of building PCRE, for use in
environments that have limited stacks. Because of the greater use of
memory management, it runs more slowly. Separate functions are
provided so that special-purpose external code can be used for this
case. When used, these functions always allocate memory blocks of the
same size. There is a discussion about PCRE's stack usage in the
pcrestack documentation.

The global variable pcre_callout initially contains NULL. It can be
set by the caller to a "callout" function, which PCRE will then call
at specified points during a matching operation. Details are given in
the pcrecallout documentation.

The global variable pcre_stack_guard initially contains NULL. It can
be set by the caller to a function that is called by PCRE whenever it
starts to compile a parenthesized part of a pattern. When parentheses
are nested, PCRE uses recursive function calls, which use up the
system stack. This function is provided so that applications with
restricted stacks can force a compilation error if the stack runs
out. The function should return zero if all is well, or non-zero to
force an error.

NEWLINES

PCRE supports five different conventions for indicating line breaks
in strings: a single CR (carriage return) character, a single LF
(linefeed) character, the two-character sequence CRLF, any of the
three preceding, or any Unicode newline sequence. The Unicode newline
sequences are the three just mentioned, plus the single characters VT
(vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
U+0085), LS (line separator, U+2028), and PS (paragraph separator,
U+2029).

Each of the first three conventions is used by at least one operating
system as its standard newline sequence. When PCRE is built, a
default can be specified. The default default is LF, which is the
Unix standard. When PCRE is run, the default can be overridden,
either when a pattern is compiled, or when it is matched.

At compile time, the newline convention can be specified by the
options argument of pcre_compile(), or it can be specified by special
text at the start of the pattern itself; this overrides any other
settings. See the pcrepattern page for details of the special
character sequences.

In the PCRE documentation the word "newline" is used to mean "the
character or pair of characters that indicate a line break". The
choice of newline convention affects the handling of the dot,
circumflex, and dollar metacharacters, the handling of #-comments in
/x mode, and, when CRLF is a recognized line ending sequence, the
match position advancement for a non-anchored pattern. There is more
detail about this in the section on pcre_exec() options below.

The choice of newline convention does not affect the interpretation
of the \n or \r escape sequences, nor does it affect what \R matches,
which is controlled in a similar way, but by separate options.

MULTITHREADING

The PCRE functions can be used in multi-threading applications, with
the proviso that the memory management functions pointed to by
pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and
the callout and stack-checking functions pointed to by pcre_callout
and pcre_stack_guard, are shared by all threads.

The compiled form of a regular expression is not altered during
matching, so the same compiled pattern can safely be used by several
threads at once.

If the just-in-time optimization feature is being used, it needs
separate memory stack areas for each thread. See the pcrejit
documentation for more details.

SAVING PRECOMPILED PATTERNS FOR LATER USE

The compiled form of a regular expression can be saved and re-used at
a later time, possibly by a different program, and even on a host
other than the one on which it was compiled. Details are given in the
pcreprecompile documentation, which includes a description of the
pcre_pattern_to_host_byte_order() function. However, compiling a
regular expression with one version of PCRE for use with a different
version is not guaranteed to work and may cause crashes.

CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);

The function pcre_config() makes it possible for a PCRE client to
discover which optional features have been compiled into the PCRE
library. The pcrebuild documentation has more details about these
optional features.

The first argument for pcre_config() is an integer, specifying which
information is required; the second argument is a pointer to a
variable into which the information is placed. The returned value is
zero on success, or the negative error code PCRE_ERROR_BADOPTION if
the value in the first argument is not recognized. The following
information is available:

PCRE_CONFIG_UTF8

The output is an integer that is set to one if UTF-8 support is
available; otherwise it is set to zero. This value should normally be
given to the 8-bit version of this function, pcre_config(). If it is
given to the 16-bit or 32-bit version of this function, the result is
PCRE_ERROR_BADOPTION.

PCRE_CONFIG_UTF16

The output is an integer that is set to one if UTF-16 support is
available; otherwise it is set to zero. This value should normally be
given to the 16-bit version of this function, pcre16_config(). If it
is given to the 8-bit or 32-bit version of this function, the result
is PCRE_ERROR_BADOPTION.

PCRE_CONFIG_UTF32

The output is an integer that is set to one if UTF-32 support is
available; otherwise it is set to zero. This value should normally be
given to the 32-bit version of this function, pcre32_config(). If it
is given to the 8-bit or 16-bit version of this function, the result
is PCRE_ERROR_BADOPTION.

PCRE_CONFIG_UNICODE_PROPERTIES

The output is an integer that is set to one if support for Unicode
character properties is available; otherwise it is set to zero.

PCRE_CONFIG_JIT

The output is an integer that is set to one if support for just-in-
time compiling is available; otherwise it is set to zero.

PCRE_CONFIG_JITTARGET

The output is a pointer to a zero-terminated "const char *" string.
If JIT support is available, the string contains the name of the
architecture for which the JIT compiler is configured, for example
"x86 32bit (little endian + unaligned)". If JIT support is not
available, the result is NULL.

PCRE_CONFIG_NEWLINE

The output is an integer whose value specifies the default character
sequence that is recognized as meaning "newline". The values that are
supported in ASCII/Unicode environments are: 10 for LF, 13 for CR,
3338 for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC
environments, CR, ANYCRLF, and ANY yield the same values. However,
the value for LF is normally 21, though some EBCDIC environments use
37. The corresponding values for CRLF are 3349 and 3365. The default
should normally correspond to the standard sequence for your
operating system.

PCRE_CONFIG_BSR

The output is an integer whose value indicates what character
sequences the \R escape sequence matches by default. A value of 0
means that \R matches any Unicode line ending sequence; a value of 1
means that \R matches only CR, LF, or CRLF. The default can be
overridden when a pattern is compiled or matched.

PCRE_CONFIG_LINK_SIZE

The output is an integer that contains the number of bytes used for
internal linkage in compiled regular expressions. For the 8-bit
library, the value can be 2, 3, or 4. For the 16-bit library, the
value is either 2 or 4 and is still a number of bytes. For the 32-bit
library, the value is either 2 or 4 and is still a number of bytes.
The default value of 2 is sufficient for all but the most massive
patterns, since it allows the compiled pattern to be up to 64K in
size. Larger values allow larger regular expressions to be compiled,
at the expense of slower matching.

PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

The output is an integer that contains the threshold above which the
POSIX interface uses malloc() for output vectors. Further details are
given in the pcreposix documentation.

PCRE_CONFIG_PARENS_LIMIT

The output is a long integer that gives the maximum depth of nesting
of parentheses (of any kind) in a pattern. This limit is imposed to
cap the amount of system stack used when a pattern is compiled. It is
specified when PCRE is built; the default is 250. This limit does not
take into account the stack that may already be used by the calling
application. For finer control over compilation stack usage, you can
set a pointer to an external checking function in pcre_stack_guard.

PCRE_CONFIG_MATCH_LIMIT

The output is a long integer that gives the default limit for the
number of internal matching function calls in a pcre_exec()
execution. Further details are given with pcre_exec() below.

PCRE_CONFIG_MATCH_LIMIT_RECURSION

The output is a long integer that gives the default limit for the
depth of recursion when calling the internal matching function in a
pcre_exec() execution. Further details are given with pcre_exec()
below.

PCRE_CONFIG_STACKRECURSE

The output is an integer that is set to one if internal recursion
when running pcre_exec() is implemented by recursive function calls
that use the stack to remember their state. This is the usual way
that PCRE is compiled. The output is zero if PCRE was compiled to use
blocks of data on the heap instead of recursive function calls. In
this case, pcre_stack_malloc and pcre_stack_free are called to manage
memory blocks on the heap, thus avoiding the use of the stack.

COMPILING A PATTERN

pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);

pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);

Either of the functions pcre_compile() or pcre_compile2() can be
called to compile a pattern into an internal form. The only
difference between the two interfaces is that pcre_compile2() has an
additional argument, errorcodeptr, via which a numerical error code
can be returned. To avoid too much repetition, we refer just to
pcre_compile() below, but the information applies equally to
pcre_compile2().

The pattern is a C string terminated by a binary zero, and is passed
in the pattern argument. A pointer to a single block of memory that
is obtained via pcre_malloc is returned. This contains the compiled
code and related data. The pcre type is defined for the returned
block; this is a typedef for a structure whose contents are not
externally defined. It is up to the caller to free the memory (via
pcre_free) when it is no longer required.

Although the compiled code of a PCRE regex is relocatable, that is,
it does not depend on memory location, the complete pcre data block
is not fully relocatable, because it may contain a copy of the
tableptr argument, which is an address (see below).

The options argument contains various bit settings that affect the
compilation. It should be zero if no options are required. The
available options are described below. Some of them (in particular,
those that are compatible with Perl, but some others as well) can
also be set and unset from within the pattern (see the detailed
description in the pcrepattern documentation). For those options that
can be different in different parts of the pattern, the contents of
the options argument specifies their settings at the start of
compilation and execution. The PCRE_ANCHORED, PCRE_BSR_xxx,
PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and PCRE_NO_START_OPTIMIZE
options can be set at the time of matching as well as at compile
time.

If errptr is NULL, pcre_compile() returns NULL immediately.
Otherwise, if compilation of a pattern fails, pcre_compile() returns
NULL, and sets the variable pointed to by errptr to point to a
textual error message. This is a static string that is part of the
library. You must not try to free it. Normally, the offset from the
start of the pattern to the data unit that was being processed when
the error was discovered is placed in the variable pointed to by
erroffset, which must not be NULL (if it is, an immediate error is
given). However, for an invalid UTF-8 or UTF-16 string, the offset is
that of the first data unit of the failing character.

Some errors are not detected until the whole pattern has been
scanned; in these cases, the offset passed back is the length of the
pattern. Note that the offset is in data units, not characters, even
in a UTF mode. It may sometimes point into the middle of a UTF-8 or
UTF-16 character.

If pcre_compile2() is used instead of pcre_compile(), and the
errorcodeptr argument is not NULL, a non-zero error code number is
returned via this argument in the event of an error. This is in
addition to the textual error message. Error codes and messages are
listed below.

If the final argument, tableptr, is NULL, PCRE uses a default set of
character tables that are built when PCRE is compiled, using the
default C locale. Otherwise, tableptr must be an address that is the
result of a call to pcre_maketables(). This value is stored with the
compiled pattern, and used again by pcre_exec() and pcre_dfa_exec()
when the pattern is matched. For more discussion, see the section on
locale support below.

This code fragment shows a typical straightforward call to
pcre_compile():

pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */

The following names for option bits are defined in the pcre.h header
file:

PCRE_ANCHORED

If this bit is set, the pattern is forced to be "anchored", that is,
it is constrained to match only at the first matching point in the
string that is being searched (the "subject string"). This effect can
also be achieved by appropriate constructs in the pattern itself,
which is the only way to do it in Perl.

PCRE_AUTO_CALLOUT

If this bit is set, pcre_compile() automatically inserts callout
items, all with number 255, before each pattern item. For discussion
of the callout facility, see the pcrecallout documentation.

PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE

These options (which are mutually exclusive) control what the \R
escape sequence matches. The choice is either to match only CR, LF,
or CRLF, or to match any Unicode newline sequence. The default is
specified when PCRE is built. It can be overridden from within the
pattern, or by setting an option when a compiled pattern is matched.

PCRE_CASELESS

If this bit is set, letters in the pattern match both upper and lower
case letters. It is equivalent to Perl's /i option, and it can be
changed within a pattern by a (?i) option setting. In UTF-8 mode,
PCRE always understands the concept of case for characters whose
values are less than 128, so caseless matching is always possible.
For characters with higher values, the concept of case is supported
if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching for characters 128 and above,
you must ensure that PCRE is compiled with Unicode property support
as well as with UTF-8 support.

PCRE_DOLLAR_ENDONLY

If this bit is set, a dollar metacharacter in the pattern matches
only at the end of the subject string. Without this option, a dollar
also matches immediately before a newline at the end of the string
(but not before any other newlines). The PCRE_DOLLAR_ENDONLY option
is ignored if PCRE_MULTILINE is set. There is no equivalent to this
option in Perl, and no way to set it within a pattern.

PCRE_DOTALL

If this bit is set, a dot metacharacter in the pattern matches a
character of any value, including one that indicates a newline.
However, it only ever matches one character, even if newlines are
coded as CRLF. Without this option, a dot does not match when the
current position is at a newline. This option is equivalent to Perl's
/s option, and it can be changed within a pattern by a (?s) option
setting. A negative class such as [^a] always matches newline
characters, independent of the setting of this option.

PCRE_DUPNAMES

If this bit is set, names used to identify capturing subpatterns need
not be unique. This can be helpful for certain types of pattern when
it is known that only one instance of the named subpattern can ever
be matched. There are more details of named subpatterns below; see
also the pcrepattern documentation.

PCRE_EXTENDED

If this bit is set, most white space characters in the pattern are
totally ignored except when escaped or inside a character class.
However, white space is not allowed within sequences such as (?> that
introduce various parenthesized subpatterns, nor within a numerical
quantifier such as {1,3}. However, ignorable white space is
permitted between an item and a following quantifier and between a
quantifier and a following + that indicates possessiveness.

White space did not used to include the VT character (code 11),
because Perl did not treat this character as white space. However,
Perl changed at release 5.18, so PCRE followed at release 8.34, and
VT is now treated as white space.

PCRE_EXTENDED also causes characters between an unescaped # outside a
character class and the next newline, inclusive, to be ignored.
PCRE_EXTENDED is equivalent to Perl's /x option, and it can be
changed within a pattern by a (?x) option setting.

Which characters are interpreted as newlines is controlled by the
options passed to pcre_compile() or by a special sequence at the
start of the pattern, as described in the section entitled "Newline
conventions" in the pcrepattern documentation. Note that the end of
this type of comment is a literal newline sequence in the pattern;
escape sequences that happen to represent a newline do not count.

This option makes it possible to include comments inside complicated
patterns. Note, however, that this applies only to data characters.
White space characters may never appear within special character
sequences in a pattern, for example within the sequence (?( that
introduces a conditional subpattern.

PCRE_EXTRA

This option was invented in order to turn on additional functionality
of PCRE that is incompatible with Perl, but it is currently of very
little use. When set, any backslash in a pattern that is followed by
a letter that has no special meaning causes an error, thus reserving
these combinations for future expansion. By default, as in Perl, a
backslash followed by a letter with no special meaning is treated as
a literal. (Perl can, however, be persuaded to give an error for
this, by running it with the -w option.) There are at present no
other features controlled by this option. It can also be set by a
(?X) option setting within a pattern.

PCRE_FIRSTLINE

If this option is set, an unanchored pattern is required to match
before or at the first newline in the subject string, though the
matched text may continue over the newline.

PCRE_JAVASCRIPT_COMPAT

If this option is set, PCRE's behaviour is changed in some ways so
that it is compatible with JavaScript rather than Perl. The changes
are as follows:

(1) A lone closing square bracket in a pattern causes a compile-time
error, because this is illegal in JavaScript (by default it is
treated as a data character). Thus, the pattern AB]CD becomes illegal
when this option is set.

(2) At run time, a back reference to an unset subpattern group
matches an empty string (by default this causes the current matching
alternative to fail). A pattern such as (\1)(a) succeeds when this
option is set (assuming it can find an "a" in the subject), whereas
it fails by default, for Perl compatibility.

(3) \U matches an upper case "U" character; by default \U causes a
compile time error (Perl uses \U to upper case subsequent
characters).

(4) \u matches a lower case "u" character unless it is followed by
four hexadecimal digits, in which case the hexadecimal number defines
the code point to match. By default, \u causes a compile time error
(Perl uses it to upper case the following character).

(5) \x matches a lower case "x" character unless it is followed by
two hexadecimal digits, in which case the hexadecimal number defines
the code point to match. By default, as in Perl, a hexadecimal number
is always expected after \x, but it may have zero, one, or two digits
(so, for example, \xz matches a binary zero character followed by z).

PCRE_MULTILINE

By default, for the purposes of matching "start of line" and "end of
line", PCRE treats the subject string as consisting of a single line
of characters, even if it actually contains newlines. The "start of
line" metacharacter (^) matches only at the start of the string, and
the "end of line" metacharacter ($) matches only at the end of the
string, or before a terminating newline (except when
PCRE_DOLLAR_ENDONLY is set). Note, however, that unless PCRE_DOTALL
is set, the "any character" metacharacter (.) does not match at a
newline. This behaviour (for ^, $, and dot) is the same as Perl.

When PCRE_MULTILINE it is set, the "start of line" and "end of line"
constructs match immediately following or immediately before internal
newlines in the subject string, respectively, as well as at the very
start and end. This is equivalent to Perl's /m option, and it can be
changed within a pattern by a (?m) option setting. If there are no
newlines in a subject string, or no occurrences of ^ or $ in a
pattern, setting PCRE_MULTILINE has no effect.

PCRE_NEVER_UTF

This option locks out interpretation of the pattern as UTF-8 (or
UTF-16 or UTF-32 in the 16-bit and 32-bit libraries). In particular,
it prevents the creator of the pattern from switching to UTF
interpretation by starting the pattern with (*UTF). This may be
useful in applications that process patterns from external sources.
The combination of PCRE_UTF8 and PCRE_NEVER_UTF also causes an error.

PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY

These options override the default newline definition that was chosen
when PCRE was built. Setting the first or the second specifies that a
newline is indicated by a single character (CR or LF, respectively).
Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by
the two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF
specifies that any of the three preceding sequences should be
recognized. Setting PCRE_NEWLINE_ANY specifies that any Unicode
newline sequence should be recognized.

In an ASCII/Unicode environment, the Unicode newline sequences are
the three just mentioned, plus the single characters VT (vertical
tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS
(line separator, U+2028), and PS (paragraph separator, U+2029). For
the 8-bit library, the last two are recognized only in UTF-8 mode.

When PCRE is compiled to run in an EBCDIC (mainframe) environment,
the code for CR is 0x0d, the same as ASCII. However, the character
code for LF is normally 0x15, though in some EBCDIC environments 0x25
is used. Whichever of these is not LF is made to correspond to
Unicode's NEL character. EBCDIC codes are all less than 256. For more
details, see the pcrebuild documentation.

The newline setting in the options word uses three bits that are
treated as a number, giving eight possibilities. Currently only six
are used (default plus the five values above). This means that if you
set more than one newline option, the combination may or may not be
sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is
equivalent to PCRE_NEWLINE_CRLF, but other combinations may yield
unused numbers and cause an error.

The only time that a line break in a pattern is specially recognized
when compiling is when PCRE_EXTENDED is set. CR and LF are white
space characters, and so are ignored in this mode. Also, an unescaped
# outside a character class indicates a comment that lasts until
after the next line break sequence. In other circumstances, line
break sequences in patterns are treated as literal data.

The newline option that is set at compile time becomes the default
that is used for pcre_exec() and pcre_dfa_exec(), but it can be
overridden.

PCRE_NO_AUTO_CAPTURE

If this option is set, it disables the use of numbered capturing
parentheses in the pattern. Any opening parenthesis that is not
followed by ? behaves as if it were followed by ?: but named
parentheses can still be used for capturing (and they acquire numbers
in the usual way). There is no equivalent of this option in Perl.

PCRE_NO_AUTO_POSSESS

If this option is set, it disables "auto-possessification". This is
an optimization that, for example, turns a+b into a++b in order to
avoid backtracks into a+ that can never be successful. However, if
callouts are in use, auto-possessification means that some of them
are never taken. You can set this option if you want the matching
functions to do a full unoptimized search and run all the callouts,
but it is mainly provided for testing purposes.

PCRE_NO_START_OPTIMIZE

This is an option that acts at matching time; that is, it is really
an option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
time, it is remembered with the compiled pattern and assumed at
matching time. This is necessary if you want to use JIT execution,
because the JIT compiler needs to know whether or not this option is
set. For details see the discussion of PCRE_NO_START_OPTIMIZE below.

PCRE_UCP

This option changes the way PCRE processes \B, \b, \D, \d, \S, \s,
\W, \w, and some of the POSIX character classes. By default, only
ASCII characters are recognized, but if PCRE_UCP is set, Unicode
properties are used instead to classify characters. More details are
given in the section on generic character types in the pcrepattern
page. If you set PCRE_UCP, matching one of the items it affects takes
much longer. The option is available only if PCRE has been compiled
with Unicode property support.

PCRE_UNGREEDY

This option inverts the "greediness" of the quantifiers so that they
are not greedy by default, but become greedy if followed by "?". It
is not compatible with Perl. It can also be set by a (?U) option
setting within the pattern.

PCRE_UTF8

This option causes PCRE to regard both the pattern and the subject as
strings of UTF-8 characters instead of single-byte strings. However,
it is available only when PCRE is built to include UTF support. If
not, the use of this option provokes an error. Details of how this
option changes the behaviour of PCRE are given in the pcreunicode
page.

PCRE_NO_UTF8_CHECK

When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string
is automatically checked. There is a discussion about the validity of
UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence
is found, pcre_compile() returns an error. If you already know that
your pattern is valid, and you want to skip this check for
performance reasons, you can set the PCRE_NO_UTF8_CHECK option. When
it is set, the effect of passing an invalid UTF-8 string as a pattern
is undefined. It may cause your program to crash or loop. Note that
this option can also be passed to pcre_exec() and pcre_dfa_exec(), to
suppress the validity checking of subject strings only. If the same
string is being matched many times, the option can be safely set for
the second and subsequent matchings to improve performance.

COMPILATION ERROR CODES

The following table lists the error codes than may be returned by
pcre_compile2(), along with the error messages that may be returned
by both compiling functions. Note that error messages are always
8-bit ASCII strings, even in 16-bit or 32-bit mode. As PCRE has
developed, some error codes have fallen out of use. To avoid
confusion, they have not been re-used.

0 no error
1 \ at end of pattern
2 \c at end of pattern
3 unrecognized character follows \
4 numbers out of order in {} quantifier
5 number too big in {} quantifier
6 missing terminating ] for character class
7 invalid escape sequence in character class
8 range out of order in character class
9 nothing to repeat
10 [this code is not in use]
11 internal error: unexpected repeat
12 unrecognized character after (? or (?-
13 POSIX named classes are supported only within a class
14 missing )
15 reference to non-existent subpattern
16 erroffset passed as NULL
17 unknown option bit(s) set
18 missing ) after comment
19 [this code is not in use]
20 regular expression is too large
21 failed to get memory
22 unmatched parentheses
23 internal error: code overflow
24 unrecognized character after (?<
25 lookbehind assertion is not fixed length
26 malformed number or name after (?(
27 conditional group contains more than two branches
28 assertion expected after (?(
29 (?R or (?[+-]digits must be followed by )
30 unknown POSIX class name
31 POSIX collating elements are not supported
32 this version of PCRE is compiled without UTF support
33 [this code is not in use]
34 character value in \x{} or \o{} is too large
35 invalid condition (?(0)
36 \C not allowed in lookbehind assertion
37 PCRE does not support \L, \l, \N{name}, \U, or \u
38 number after (?C is > 255
39 closing ) for (?C expected
40 recursive call could loop indefinitely
41 unrecognized character after (?P
42 syntax error in subpattern name (missing terminator)
43 two named subpatterns have the same name
44 invalid UTF-8 string (specifically UTF-8)
45 support for \P, \p, and \X has not been compiled
46 malformed \P or \p sequence
47 unknown property name after \P or \p
48 subpattern name is too long (maximum 32 characters)
49 too many named subpatterns (maximum 10000)
50 [this code is not in use]
51 octal value is greater than \377 in 8-bit non-UTF-8 mode
52 internal error: overran compiling workspace
53 internal error: previously-checked referenced subpattern
not found
54 DEFINE group contains more than one branch
55 repeating a DEFINE group is not allowed
56 inconsistent NEWLINE options
57 \g is not followed by a braced, angle-bracketed, or quoted
name/number or by a plain number
58 a numbered reference must not be zero
59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
60 (*VERB) not recognized or malformed
61 number is too big
62 subpattern name expected
63 digit expected after (?+
64 ] is an invalid data character in JavaScript compatibility mode
65 different names for subpatterns of the same number are
not allowed
66 (*MARK) must have an argument
67 this version of PCRE is not compiled with Unicode property
support
68 \c must be followed by an ASCII character
69 \k is not followed by a braced, angle-bracketed, or quoted name
70 internal error: unknown opcode in find_fixedlength()
71 \N is not supported in a class
72 too many forward references
73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
74 invalid UTF-16 string (specifically UTF-16)
75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)
76 character value in \u.... sequence is too large
77 invalid UTF-32 string (specifically UTF-32)
78 setting UTF is disabled by the application
79 non-hex character in \x{} (closing brace missing?)
80 non-octal character in \o{} (closing brace missing?)
81 missing opening brace after \o
82 parentheses are too deeply nested
83 invalid range in character class
84 group name must start with a non-digit
85 parentheses are too deeply nested (stack check)

The numbers 32 and 10000 in errors 48 and 49 are defaults; different
values may be used if the limits were changed when PCRE was built.

STUDYING A PATTERN

pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);

If a compiled pattern is going to be used several times, it is worth
spending more time analyzing it in order to speed up the time taken
for matching. The function pcre_study() takes a pointer to a compiled
pattern as its first argument. If studying the pattern produces
additional information that will help speed up matching, pcre_study()
returns a pointer to a pcre_extra block, in which the study_data
field points to the results of the study.

The returned value from pcre_study() can be passed directly to
pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also
contains other fields that can be set by the caller before the block
is passed; these are described below in the section on matching a
pattern.

If studying the pattern does not produce any useful information,
pcre_study() returns NULL by default. In that circumstance, if the
calling program wants to pass any of the other fields to pcre_exec()
or pcre_dfa_exec(), it must set up its own pcre_extra block. However,
if pcre_study() is called with the PCRE_STUDY_EXTRA_NEEDED option, it
returns a pcre_extra block even if studying did not find any
additional information. It may still return NULL, however, if an
error occurs in pcre_study().

The second argument of pcre_study() contains option bits. There are
three further options in addition to PCRE_STUDY_EXTRA_NEEDED:

PCRE_STUDY_JIT_COMPILE
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

If any of these are set, and the just-in-time compiler is available,
the pattern is further compiled into machine code that executes much
faster than the pcre_exec() interpretive matching function. If the
just-in-time compiler is not available, these options are ignored.
All undefined bits in the options argument must be zero.

JIT compilation is a heavyweight optimization. It can take some time
for patterns to be analyzed, and for one-off matches and simple
patterns the benefit of faster execution might be offset by a much
slower study time. Not all patterns can be optimized by the JIT
compiler. For those that cannot be handled, matching automatically
falls back to the pcre_exec() interpreter. For more details, see the
pcrejit documentation.

The third argument for pcre_study() is a pointer for an error
message. If studying succeeds (even if no data is returned), the
variable it points to is set to NULL. Otherwise it is set to point to
a textual error message. This is a static string that is part of the
library. You must not try to free it. You should test the error
pointer for NULL after calling pcre_study(), to be sure that it has
run successfully.

When you are finished with a pattern, you can free the memory used
for the study data by calling pcre_free_study(). This function was
added to the API for release 8.20. For earlier versions, the memory
could be freed with pcre_free(), just like the pattern itself. This
will still work in cases where JIT optimization is not used, but it
is advisable to change to the new function when convenient.

This is a typical way in which pcre_study() is used (except that in a
real application there should be tests for errors):

int rc;
pcre *re;
pcre_extra *sd;
re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
sd = pcre_study(
re, /* result of pcre_compile() */
0, /* no options */
&error); /* set to NULL or points to a message */
rc = pcre_exec( /* see below for details of pcre_exec() options
*/
re, sd, "subject", 7, 0, 0, ovector, 30);
...
pcre_free_study(sd);
pcre_free(re);

Studying a pattern does two things: first, a lower bound for the
length of subject string that is needed to match the pattern is
computed. This does not mean that there are any strings of that
length that match, but it does guarantee that no shorter strings
match. The value is used to avoid wasting time by trying to match
strings that are shorter than the lower bound. You can find out the
value in a calling program via the pcre_fullinfo() function.

Studying a pattern is also useful for non-anchored patterns that do
not have a single fixed starting character. A bitmap of possible
starting bytes is created. This speeds up finding a position in the
subject at which to start matching. (In 16-bit mode, the bitmap is
used for 16-bit values less than 256. In 32-bit mode, the bitmap is
used for 32-bit values less than 256.)

These two optimizations apply to both pcre_exec() and
pcre_dfa_exec(), and the information is also used by the JIT
compiler. The optimizations can be disabled by setting the
PCRE_NO_START_OPTIMIZE option. You might want to do this if your
pattern contains callouts or (*MARK) and you want to make use of
these facilities in cases where matching fails.

PCRE_NO_START_OPTIMIZE can be specified at either compile time or
execution time. However, if PCRE_NO_START_OPTIMIZE is passed to
pcre_exec(), (that is, after any JIT compilation has happened) JIT
execution is disabled. For JIT execution to work with
PCRE_NO_START_OPTIMIZE, the option must be set at compile time.

There is a longer discussion of PCRE_NO_START_OPTIMIZE below.

LOCALE SUPPORT

PCRE handles caseless matching, and determines whether characters are
letters, digits, or whatever, by reference to a set of tables,
indexed by character code point. When running in UTF-8 mode, or in
the 16- or 32-bit libraries, this applies only to characters with
code points less than 256. By default, higher-valued code points
never match escapes such as \w or \d. However, if PCRE is built with
Unicode property support, all characters can be tested with \p and
\P, or, alternatively, the PCRE_UCP option can be set when a pattern
is compiled; this causes \w and friends to use Unicode property
support instead of the built-in tables.

The use of locales with Unicode is discouraged. If you are handling
characters with code points greater than 128, you should either use
Unicode support, or use locales, but not try to mix the two.

PCRE contains an internal set of tables that are used when the final
argument of pcre_compile() is NULL. These are sufficient for many
applications. Normally, the internal tables recognize only ASCII
characters. However, when PCRE is built, it is possible to cause the
internal tables to be rebuilt in the default "C" locale of the local
system, which may cause them to be different.

The internal tables can always be overridden by tables supplied by
the application that calls PCRE. These may be created in a different
locale from the default. As more and more applications change to
using Unicode, the need for this locale support is expected to die
away.

External tables are built by calling the pcre_maketables() function,
which has no arguments, in the relevant locale. The result can then
be passed to pcre_compile() as often as necessary. For example, to
build and use tables that are appropriate for the French locale
(where accented characters with values greater than 128 are treated
as letters), the following code could be used:

setlocale(LC_CTYPE, "fr_FR");
tables = pcre_maketables();
re = pcre_compile(..., tables);

The locale name "fr_FR" is used on Linux and other Unix-like systems;
if you are using Windows, the name for the French locale is "french".

When pcre_maketables() runs, the tables are built in memory that is
obtained via pcre_malloc. It is the caller's responsibility to ensure
that the memory containing the tables remains available for as long
as it is needed.

The pointer that is passed to pcre_compile() is saved with the
compiled pattern, and the same tables are used via this pointer by
pcre_study() and also by pcre_exec() and pcre_dfa_exec(). Thus, for
any single pattern, compilation, studying and matching all happen in
the same locale, but different patterns can be processed in different
locales.

It is possible to pass a table pointer or NULL (indicating the use of
the internal tables) to pcre_exec() or pcre_dfa_exec() (see the
discussion below in the section on matching a pattern). This facility
is provided for use with pre-compiled patterns that have been saved
and reloaded. Character tables are not saved with patterns, so if a
non-standard table was used at compile time, it must be provided
again when the reloaded pattern is matched. Attempting to use this
facility to match a pattern in a different locale from the one in
which it was compiled is likely to lead to anomalous (usually
incorrect) results.

INFORMATION ABOUT A PATTERN

int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);

The pcre_fullinfo() function returns information about a compiled
pattern. It replaces the pcre_info() function, which was removed from
the library at version 8.30, after more than 10 years of
obsolescence.

The first argument for pcre_fullinfo() is a pointer to the compiled
pattern. The second argument is the result of pcre_study(), or NULL
if the pattern was not studied. The third argument specifies which
piece of information is required, and the fourth argument is a
pointer to a variable to receive the data. The yield of the function
is zero for success, or one of the following negative numbers:

PCRE_ERROR_NULL the argument code was NULL
the argument where was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
endianness
PCRE_ERROR_BADOPTION the value of what was invalid
PCRE_ERROR_UNSET the requested field is not set

The "magic number" is placed at the start of each compiled pattern as
a simple check against passing an arbitrary memory pointer. The
endianness error can occur if a compiled pattern is saved and
reloaded on a different host. Here is a typical call of
pcre_fullinfo(), to obtain the length of the compiled pattern:

int rc;
size_t length;
rc = pcre_fullinfo(
re, /* result of pcre_compile() */
sd, /* result of pcre_study(), or NULL */
PCRE_INFO_SIZE, /* what is required */
&length); /* where to put the data */

The possible values for the third argument are defined in pcre.h, and
are as follows:

PCRE_INFO_BACKREFMAX

Return the number of the highest back reference in the pattern. The
fourth argument should point to an int variable. Zero is returned if
there are no back references.

PCRE_INFO_CAPTURECOUNT

Return the number of capturing subpatterns in the pattern. The fourth
argument should point to an int variable.

PCRE_INFO_DEFAULT_TABLES

Return a pointer to the internal default character tables within
PCRE. The fourth argument should point to an unsigned char *
variable. This information call is provided for internal use by the
pcre_study() function. External callers can cause PCRE to use its
internal tables by passing a NULL table pointer.

PCRE_INFO_FIRSTBYTE (deprecated)

Return information about the first data unit of any matched string,
for a non-anchored pattern. The name of this option refers to the
8-bit library, where data units are bytes. The fourth argument should
point to an int variable. Negative values are used for special cases.
However, this means that when the 32-bit library is in non-UTF-32
mode, the full 32-bit range of characters cannot be returned. For
this reason, this value is deprecated; use
PCRE_INFO_FIRSTCHARACTERFLAGS and PCRE_INFO_FIRSTCHARACTER instead.

If there is a fixed first value, for example, the letter "c" from a
pattern such as (cat|cow|coyote), its value is returned. In the 8-bit
library, the value is always less than 256. In the 16-bit library the
value can be up to 0xffff. In the 32-bit library the value can be up
to 0x10ffff.

If there is no fixed first value, and if either

(a) the pattern was compiled with the PCRE_MULTILINE option, and
every branch starts with "^", or

(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is
not set (if it were set, the pattern would be anchored),

-1 is returned, indicating that the pattern matches only at the start
of a subject string or after any newline within the string. Otherwise
-2 is returned. For anchored patterns, -2 is returned.

PCRE_INFO_FIRSTCHARACTER

Return the value of the first data unit (non-UTF character) of any
matched string in the situation where PCRE_INFO_FIRSTCHARACTERFLAGS
returns 1; otherwise return 0. The fourth argument should point to a
uint_t variable.

In the 8-bit library, the value is always less than 256. In the
16-bit library the value can be up to 0xffff. In the 32-bit library
in UTF-32 mode the value can be up to 0x10ffff, and up to 0xffffffff
when not using UTF-32 mode.

PCRE_INFO_FIRSTCHARACTERFLAGS

Return information about the first data unit of any matched string,
for a non-anchored pattern. The fourth argument should point to an
int variable.

If there is a fixed first value, for example, the letter "c" from a
pattern such as (cat|cow|coyote), 1 is returned, and the character
value can be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no
fixed first value, and if either

(a) the pattern was compiled with the PCRE_MULTILINE option, and
every branch starts with "^", or

(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is
not set (if it were set, the pattern would be anchored),

2 is returned, indicating that the pattern matches only at the start
of a subject string or after any newline within the string. Otherwise
0 is returned. For anchored patterns, 0 is returned.

PCRE_INFO_FIRSTTABLE

If the pattern was studied, and this resulted in the construction of
a 256-bit table indicating a fixed set of values for the first data
unit in any matching string, a pointer to the table is returned.
Otherwise NULL is returned. The fourth argument should point to an
unsigned char * variable.

PCRE_INFO_HASCRORLF

Return 1 if the pattern contains any explicit matches for CR or LF
characters, otherwise 0. The fourth argument should point to an int
variable. An explicit match is either a literal CR or LF character,
or \r or \n.

PCRE_INFO_JCHANGED

Return 1 if the (?J) or (?-J) option setting is used in the pattern,
otherwise 0. The fourth argument should point to an int variable.
(?J) and (?-J) set and unset the local PCRE_DUPNAMES option,
respectively.

PCRE_INFO_JIT

Return 1 if the pattern was studied with one of the JIT options, and
just-in-time compiling was successful. The fourth argument should
point to an int variable. A return value of 0 means that JIT support
is not available in this version of PCRE, or that the pattern was not
studied with a JIT option, or that the JIT compiler could not handle
this particular pattern. See the pcrejit documentation for details of
what can and cannot be handled.

PCRE_INFO_JITSIZE

If the pattern was successfully studied with a JIT option, return the
size of the JIT compiled code, otherwise return zero. The fourth
argument should point to a size_t variable.

PCRE_INFO_LASTLITERAL

Return the value of the rightmost literal data unit that must exist
in any matched string, other than at its start, if such a value has
been recorded. The fourth argument should point to an int variable.
If there is no such value, -1 is returned. For anchored patterns, a
last literal value is recorded only if it follows something of
variable length. For example, for the pattern /^a\d+z\d+/ the
returned value is "z", but for /^a\dz\d/ the returned value is -1.

Since for the 32-bit library using the non-UTF-32 mode, this function
is unable to return the full 32-bit range of characters, this value
is deprecated; instead the PCRE_INFO_REQUIREDCHARFLAGS and
PCRE_INFO_REQUIREDCHAR values should be used.

PCRE_INFO_MATCH_EMPTY

Return 1 if the pattern can match an empty string, otherwise 0. The
fourth argument should point to an int variable.

PCRE_INFO_MATCHLIMIT

If the pattern set a match limit by including an item of the form
(*LIMIT_MATCH=nnnn) at the start, the value is returned. The fourth
argument should point to an unsigned 32-bit integer. If no such value
has been set, the call to pcre_fullinfo() returns the error
PCRE_ERROR_UNSET.

PCRE_INFO_MAXLOOKBEHIND

Return the number of characters (NB not data units) in the longest
lookbehind assertion in the pattern. This information is useful when
doing multi-segment matching using the partial matching facilities.
Note that the simple assertions \b and \B require a one-character
lookbehind. \A also registers a one-character lookbehind, though it
does not actually inspect the previous character. This is to ensure
that at least one character from the old segment is retained when a
new segment is processed. Otherwise, if there are no lookbehinds in
the pattern, \A might match incorrectly at the start of a new
segment.

PCRE_INFO_MINLENGTH

If the pattern was studied and a minimum length for matching subject
strings was computed, its value is returned. Otherwise the returned
value is -1. The value is a number of characters, which in UTF mode
may be different from the number of data units. The fourth argument
should point to an int variable. A non-negative value is a lower
bound to the length of any matching string. There may not be any
strings of that length that do actually match, but every string that
does match is at least that long.

PCRE_INFO_NAMECOUNT
PCRE_INFO_NAMEENTRYSIZE
PCRE_INFO_NAMETABLE

PCRE supports the use of named as well as numbered capturing
parentheses. The names are just an additional way of identifying the
parentheses, which still acquire numbers. Several convenience
functions such as pcre_get_named_substring() are provided for
extracting captured substrings by name. It is also possible to
extract the data directly, by first converting the name to a number
in order to access the correct pointers in the output vector
(described with pcre_exec() below). To do the conversion, you need to
use the name-to-number map, which is described by these three values.

The map consists of a number of fixed-size entries.
PCRE_INFO_NAMECOUNT gives the number of entries, and
PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both of these
return an int value. The entry size depends on the length of the
longest name. PCRE_INFO_NAMETABLE returns a pointer to the first
entry of the table. This is a pointer to char in the 8-bit library,
where the first two bytes of each entry are the number of the
capturing parenthesis, most significant byte first. In the 16-bit
library, the pointer points to 16-bit data units, the first of which
contains the parenthesis number. In the 32-bit library, the pointer
points to 32-bit data units, the first of which contains the
parenthesis number. The rest of the entry is the corresponding name,
zero terminated.

The names are in alphabetical order. If (?| is used to create
multiple groups with the same number, as described in the section on
duplicate subpattern numbers in the pcrepattern page, the groups may
be given the same name, but there is only one entry in the table.
Different names for groups of the same number are not permitted.
Duplicate names for subpatterns with different numbers are permitted,
but only if PCRE_DUPNAMES is set. They appear in the table in the
order in which they were found in the pattern. In the absence of (?|
this is the order of increasing number; when (?| is used this is not
necessarily the case because later subpatterns may have lower
numbers.

As a simple example of the name/number table, consider the following
pattern after compilation by the 8-bit library (assume PCRE_EXTENDED
is set, so white space - including newlines - is ignored):

(?<date> (?<year>(\d\d)?\d\d) -
(?<month>\d\d) - (?<day>\d\d) )

There are four named subpatterns, so the table has four entries, and
each entry in the table is eight bytes long. The table is as follows,
with non-printing bytes shows in hexadecimal, and undefined bytes
shown as ??:

00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??

When writing code to extract data from named subpatterns using the
name-to-number map, remember that the length of the entries is likely
to be different for each compiled pattern.

PCRE_INFO_OKPARTIAL

Return 1 if the pattern can be used for partial matching with
pcre_exec(), otherwise 0. The fourth argument should point to an int
variable. From release 8.00, this always returns 1, because the
restrictions that previously applied to partial matching have been
lifted. The pcrepartial documentation gives details of partial
matching.

PCRE_INFO_OPTIONS

Return a copy of the options with which the pattern was compiled. The
fourth argument should point to an unsigned long int variable. These
option bits are those specified in the call to pcre_compile(),
modified by any top-level option settings at the start of the pattern
itself. In other words, they are the options that will be in force
when matching starts. For example, if the pattern /(?im)abc(?-i)d/ is
compiled with the PCRE_EXTENDED option, the result is PCRE_CASELESS,
PCRE_MULTILINE, and PCRE_EXTENDED.

A pattern is automatically anchored by PCRE if all of its top-level
alternatives begin with one of the following:

^ unless PCRE_MULTILINE is set
\A always
\G always
.* if PCRE_DOTALL is set and there are no back
references to the subpattern in which .* appears

For such patterns, the PCRE_ANCHORED bit is set in the options
returned by pcre_fullinfo().

PCRE_INFO_RECURSIONLIMIT

If the pattern set a recursion limit by including an item of the form
(*LIMIT_RECURSION=nnnn) at the start, the value is returned. The
fourth argument should point to an unsigned 32-bit integer. If no
such value has been set, the call to pcre_fullinfo() returns the
error PCRE_ERROR_UNSET.

PCRE_INFO_SIZE

Return the size of the compiled pattern in bytes (for all three
libraries). The fourth argument should point to a size_t variable.
This value does not include the size of the pcre structure that is
returned by pcre_compile(). The value that is passed as the argument
to pcre_malloc() when pcre_compile() is getting memory in which to
place the compiled data is the value returned by this option plus the
size of the pcre structure. Studying a compiled pattern, with or
without JIT, does not alter the value returned by this option.

PCRE_INFO_STUDYSIZE

Return the size in bytes (for all three libraries) of the data block
pointed to by the study_data field in a pcre_extra block. If
pcre_extra is NULL, or there is no study data, zero is returned. The
fourth argument should point to a size_t variable. The study_data
field is set by pcre_study() to record information that will speed up
matching (see the section entitled "Studying a pattern" above). The
format of the study_data block is private, but its length is made
available via this option so that it can be saved and restored (see
the pcreprecompile documentation for details).

PCRE_INFO_REQUIREDCHARFLAGS

Returns 1 if there is a rightmost literal data unit that must exist
in any matched string, other than at its start. The fourth argument
should point to an int variable. If there is no such value, 0 is
returned. If returning 1, the character value itself can be retrieved
using PCRE_INFO_REQUIREDCHAR.

For anchored patterns, a last literal value is recorded only if it
follows something of variable length. For example, for the pattern
/^a\d+z\d+/ the returned value 1 (with "z" returned from
PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0.

PCRE_INFO_REQUIREDCHAR

Return the value of the rightmost literal data unit that must exist
in any matched string, other than at its start, if such a value has
been recorded. The fourth argument should point to a uint32_t
variable. If there is no such value, 0 is returned.

REFERENCE COUNTS

int pcre_refcount(pcre *code, int adjust);

The pcre_refcount() function is used to maintain a reference count in
the data block that contains a compiled pattern. It is provided for
the benefit of applications that operate in an object-oriented
manner, where different parts of the application may be using the
same compiled pattern, but you want to free the block when they are
all done.

When a pattern is compiled, the reference count field is initialized
to zero. It is changed only by calling this function, whose action
is to add the adjust value (which may be positive or negative) to it.
The yield of the function is the new value. However, the value of the
count is constrained to lie between 0 and 65535, inclusive. If the
new value is outside these limits, it is forced to the appropriate
limit value.

Except when it is zero, the reference count is not correctly
preserved if a pattern is compiled on one host and then transferred
to a host whose byte-order is different. (This seems a highly
unlikely scenario.)

MATCHING A PATTERN: THE TRADITIONAL FUNCTION
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);

The function pcre_exec() is called to match a subject string against
a compiled pattern, which is passed in the code argument. If the
pattern was studied, the result of the study should be passed in the
extra argument. You can call pcre_exec() with the same code and extra
arguments as many times as you like, in order to match different
subject strings with the same pattern.

This function is the main matching facility of the library, and it
operates in a Perl-like manner. For specialist use there is also an
alternative matching function, which is described below in the
section about the pcre_dfa_exec() function.

In most applications, the pattern will have been compiled (and
optionally studied) in the same process that calls pcre_exec().
However, it is possible to save compiled patterns and study data, and
then use them later in different processes, possibly even on
different hosts. For a discussion about this, see the pcreprecompile
documentation.

Here is an example of a simple call to pcre_exec():

int rc;
int ovector[30];
rc = pcre_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information
*/
30); /* number of elements (NOT size in bytes) */

Extra data for pcre_exec()
If the extra argument is not NULL, it must point to a pcre_extra data
block. The pcre_study() function returns such a block (when it
doesn't return NULL), but you can also create one for yourself, and
pass additional information in it. The pcre_extra block contains the
following fields (not necessarily in this order):

unsigned long int flags;
void *study_data;
void *executable_jit;
unsigned long int match_limit;
unsigned long int match_limit_recursion;
void *callout_data;
const unsigned char *tables;
unsigned char **mark;

In the 16-bit version of this structure, the mark field has type
"PCRE_UCHAR16 **".

In the 32-bit version of this structure, the mark field has type
"PCRE_UCHAR32 **".

The flags field is used to specify which of the other fields are set.
The flag bits are:

PCRE_EXTRA_CALLOUT_DATA
PCRE_EXTRA_EXECUTABLE_JIT
PCRE_EXTRA_MARK
PCRE_EXTRA_MATCH_LIMIT
PCRE_EXTRA_MATCH_LIMIT_RECURSION
PCRE_EXTRA_STUDY_DATA
PCRE_EXTRA_TABLES

Other flag bits should be set to zero. The study_data field and
sometimes the executable_jit field are set in the pcre_extra block
that is returned by pcre_study(), together with the appropriate flag
bits. You should not set these yourself, but you may add to the block
by setting other fields and their corresponding flag bits.

The match_limit field provides a means of preventing PCRE from using
up a vast amount of resources when running patterns that are not
going to match, but which have a very large number of possibilities
in their search trees. The classic example is a pattern that uses
nested unlimited repeats.

Internally, pcre_exec() uses a function called match(), which it
calls repeatedly (sometimes recursively). The limit set by
match_limit is imposed on the number of times this function is called
during a match, which has the effect of limiting the amount of
backtracking that can take place. For patterns that are not anchored,
the count restarts from zero for each position in the subject string.

When pcre_exec() is called with a pattern that was successfully
studied with a JIT option, the way that the matching is executed is
entirely different. However, there is still the possibility of
runaway matching that goes on for a very long time, and so the
match_limit value is also used in this case (but in a different way)
to limit how long the matching can continue.

The default value for the limit can be set when PCRE is built; the
default default is 10 million, which handles all but the most extreme
cases. You can override the default by supplying pcre_exec() with a
pcre_extra block in which match_limit is set, and
PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

A value for the match limit may also be supplied by an item at the
start of a pattern of the form

(*LIMIT_MATCH=d)

where d is a decimal number. However, such a setting is ignored
unless d is less than the limit set by the caller of pcre_exec() or,
if no such limit is set, less than the default.

The match_limit_recursion field is similar to match_limit, but
instead of limiting the total number of times that match() is called,
it limits the depth of recursion. The recursion depth is a smaller
number than the total number of calls, because not all calls to
match() are recursive. This limit is of use only if it is set
smaller than match_limit.

Limiting the recursion depth limits the amount of machine stack that
can be used, or, when PCRE has been compiled to use memory on the
heap instead of the stack, the amount of heap memory that can be
used. This limit is not relevant, and is ignored, when matching is
done using JIT compiled code.

The default value for match_limit_recursion can be set when PCRE is
built; the default default is the same value as the default for
match_limit. You can override the default by supplying pcre_exec()
with a pcre_extra block in which match_limit_recursion is set, and
PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

A value for the recursion limit may also be supplied by an item at
the start of a pattern of the form

(*LIMIT_RECURSION=d)

where d is a decimal number. However, such a setting is ignored
unless d is less than the limit set by the caller of pcre_exec() or,
if no such limit is set, less than the default.

The callout_data field is used in conjunction with the "callout"
feature, and is described in the pcrecallout documentation.

The tables field is provided for use with patterns that have been
pre-compiled using custom character tables, saved to disc or
elsewhere, and then reloaded, because the tables that were used to
compile a pattern are not saved with it. See the pcreprecompile
documentation for a discussion of saving compiled patterns for later
use. If NULL is passed using this mechanism, it forces PCRE's
internal tables to be used.

Warning: The tables that pcre_exec() uses must be the same as those
that were used when the pattern was compiled. If this is not the
case, the behaviour of pcre_exec() is undefined. Therefore, when a
pattern is compiled and matched in the same process, this field
should never be set. In this (the most common) case, the correct
table pointer is automatically passed with the compiled pattern from
pcre_compile() to pcre_exec().

If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
set to point to a suitable variable. If the pattern contains any
backtracking control verbs such as (*MARK:NAME), and the execution
ends up with a name to pass back, a pointer to the name string (zero
terminated) is placed in the variable pointed to by the mark field.
The names are within the compiled pattern; if you wish to retain such
a name you must copy it before freeing the memory of a compiled
pattern. If there is no name to pass back, the variable pointed to by
the mark field is set to NULL. For details of the backtracking
control verbs, see the section entitled "Backtracking control" in the
pcrepattern documentation.

Option bits for pcre_exec()
The unused bits of the options argument for pcre_exec() must be zero.
The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and
PCRE_PARTIAL_SOFT.

If the pattern was successfully studied with one of the just-in-time
(JIT) compile options, the only supported options for JIT execution
are PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If
an unsupported option is used, JIT execution is disabled and the
normal interpretive code in pcre_exec() is run.

PCRE_ANCHORED

The PCRE_ANCHORED option limits pcre_exec() to matching at the first
matching position. If a pattern was compiled with PCRE_ANCHORED, or
turned out to be anchored by virtue of its contents, it cannot be
made unachored at matching time.

PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE

These options (which are mutually exclusive) control what the \R
escape sequence matches. The choice is either to match only CR, LF,
or CRLF, or to match any Unicode newline sequence. These options
override the choice that was made or defaulted when the pattern was
compiled.

PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY

These options override the newline definition that was chosen or
defaulted when the pattern was compiled. For details, see the
description of pcre_compile() above. During matching, the newline
choice affects the behaviour of the dot, circumflex, and dollar
metacharacters. It may also alter the way the match position is
advanced after a match failure for an unanchored pattern.

When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
set, and a match attempt for an unanchored pattern fails when the
current position is at a CRLF sequence, and the pattern contains no
explicit matches for CR or LF characters, the match position is
advanced by two characters instead of one, in other words, to after
the CRLF.

The above rule is a compromise that makes the most common cases work
as expected. For example, if the pattern is .+A (and the PCRE_DOTALL
option is not set), it does not match the string "\r\nA" because,
after failing at the start, it skips both the CR and the LF before
retrying. However, the pattern [\r\n]A does match that string,
because it contains an explicit CR or LF reference, and so advances
only by one character after the first failure.

An explicit match for CR of LF is either a literal appearance of one
of those characters, or one of the \r or \n escape sequences.
Implicit matches such as [^X] do not count, nor does \s (which
includes CR and LF in the characters that it matches).

Notwithstanding the above, anomalous effects may still occur when
CRLF is a valid newline sequence and explicit \r or \n escapes appear
in the pattern.

PCRE_NOTBOL

This option specifies that first character of the subject string is
not the beginning of a line, so the circumflex metacharacter should
not match before it. Setting this without PCRE_MULTILINE (at compile
time) causes circumflex never to match. This option affects only the
behaviour of the circumflex metacharacter. It does not affect \A.

PCRE_NOTEOL

This option specifies that the end of the subject string is not the
end of a line, so the dollar metacharacter should not match it nor
(except in multiline mode) a newline immediately before it. Setting
this without PCRE_MULTILINE (at compile time) causes dollar never to
match. This option affects only the behaviour of the dollar
metacharacter. It does not affect \Z or \z.

PCRE_NOTEMPTY

An empty string is not considered to be a valid match if this option
is set. If there are alternatives in the pattern, they are tried. If
all the alternatives match the empty string, the entire match fails.
For example, if the pattern

a?b?

is applied to a string not beginning with "a" or "b", it matches an
empty string at the start of the subject. With PCRE_NOTEMPTY set,
this match is not valid, so PCRE searches further into the string for
occurrences of "a" or "b".

PCRE_NOTEMPTY_ATSTART

This is like PCRE_NOTEMPTY, except that an empty string match that is
not at the start of the subject is permitted. If the pattern is
anchored, such a match can occur only if the pattern contains \K.

Perl has no direct equivalent of PCRE_NOTEMPTY or
PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
match of the empty string within its split() function, and when using
the /g modifier. It is possible to emulate Perl's behaviour after
matching a null string by first trying the match again at the same
offset with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
fails, by advancing the starting offset (see below) and trying an
ordinary match again. There is some code that demonstrates how to do
this in the pcredemo sample program. In the most general case, you
have to check to see if the newline convention recognizes CRLF as a
newline, and if so, and the current character is CR followed by LF,
advance the starting offset by two characters instead of one.

PCRE_NO_START_OPTIMIZE

There are a number of optimizations that pcre_exec() uses at the
start of a match, in order to speed up the process. For example, if
it is known that an unanchored match must start with a specific
character, it searches the subject for that character, and fails
immediately if it cannot find it, without actually running the main
matching function. This means that a special item such as (*COMMIT)
at the start of a pattern is not considered until after a suitable
starting point for the match has been found. Also, when callouts or
(*MARK) items are in use, these "start-up" optimizations can cause
them to be skipped if the pattern is never actually used. The start-
up optimizations are in effect a pre-scan of the subject that takes
place before the pattern is run.

The PCRE_NO_START_OPTIMIZE option disables the start-up
optimizations, possibly causing performance to suffer, but ensuring
that in cases where the result is "no match", the callouts do occur,
and that items such as (*COMMIT) and (*MARK) are considered at every
possible starting position in the subject string. If
PCRE_NO_START_OPTIMIZE is set at compile time, it cannot be unset at
matching time. The use of PCRE_NO_START_OPTIMIZE at matching time
(that is, passing it to pcre_exec()) disables JIT execution; in this
situation, matching is always done using interpretively.

Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
operation. Consider the pattern

(*COMMIT)ABC

When this is compiled, PCRE records the fact that a match must start
with the character "A". Suppose the subject string is "DEFABC". The
start-up optimization scans along the subject, finds "A" and runs the
first match attempt from there. The (*COMMIT) item means that the
pattern must match the current starting position, which in this case,
it does. However, if the same match is run with
PCRE_NO_START_OPTIMIZE set, the initial scan along the subject string
does not happen. The first match attempt is run starting from "D" and
when this fails, (*COMMIT) prevents any further matches being tried,
so the overall result is "no match". If the pattern is studied, more
start-up optimizations may be used. For example, a minimum length for
the subject may be recorded. Consider the pattern

(*MARK:A)(X|Y)

The minimum length for a match is one character. If the subject is
"ABC", there will be attempts to match "ABC", "BC", "C", and then
finally an empty string. If the pattern is studied, the final
attempt does not take place, because PCRE knows that the subject is
too short, and so the (*MARK) is never encountered. In this case,
studying the pattern does not affect the overall match result, which
is still "no match", but it does affect the auxiliary information
that is returned.

PCRE_NO_UTF8_CHECK

When PCRE_UTF8 is set at compile time, the validity of the subject as
a UTF-8 string is automatically checked when pcre_exec() is
subsequently called. The entire string is checked before any other
processing takes place. The value of startoffset is also checked to
ensure that it points to the start of a UTF-8 character. There is a
discussion about the validity of UTF-8 strings in the pcreunicode
page. If an invalid sequence of bytes is found, pcre_exec() returns
the error PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the
problem is a truncated character at the end of the subject,
PCRE_ERROR_SHORTUTF8. In both cases, information about the precise
nature of the error may also be returned (see the descriptions of
these errors in the section entitled Error return values from
pcre_exec() below). If startoffset contains a value that does not
point to the start of a UTF-8 character (or to the end of the
subject), PCRE_ERROR_BADUTF8_OFFSET is returned.

If you already know that your subject is valid, and you want to skip
these checks for performance reasons, you can set the
PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
do this for the second and subsequent calls to pcre_exec() if you are
making repeated calls to find all the matches in a single subject
string. However, you should be sure that the value of startoffset
points to the start of a character (or the end of the subject). When
PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as
a subject or an invalid value of startoffset is undefined. Your
program may crash or loop.

PCRE_PARTIAL_HARD
PCRE_PARTIAL_SOFT

These options turn on the partial matching feature. For backwards
compatibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A
partial match occurs if the end of the subject string is reached
successfully, but there are not enough subject characters to complete
the match. If this happens when PCRE_PARTIAL_SOFT (but not
PCRE_PARTIAL_HARD) is set, matching continues by testing any
remaining alternatives. Only if no complete match can be found is
PCRE_ERROR_PARTIAL returned instead of PCRE_ERROR_NOMATCH. In other
words, PCRE_PARTIAL_SOFT says that the caller is prepared to handle a
partial match, but only if no complete match can be found.

If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
case, if a partial match is found, pcre_exec() immediately returns
PCRE_ERROR_PARTIAL, without considering any other alternatives. In
other words, when PCRE_PARTIAL_HARD is set, a partial match is
considered to be more important that an alternative complete match.

In both cases, the portion of the string that was inspected when the
partial match was found is set as the first matching string. There is
a more detailed discussion of partial and multi-segment matching,
with examples, in the pcrepartial documentation.

The string to be matched by pcre_exec()
The subject string is passed to pcre_exec() as a pointer in subject,
a length in length, and a starting offset in startoffset. The units
for length and startoffset are bytes for the 8-bit library, 16-bit
data items for the 16-bit library, and 32-bit data items for the
32-bit library.

If startoffset is negative or greater than the length of the subject,
pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
zero, the search for a match starts at the beginning of the subject,
and this is by far the most common case. In UTF-8 or UTF-16 mode, the
offset must point to the start of a character, or the end of the
subject (in UTF-32 mode, one data unit equals one character, so all
offsets are valid). Unlike the pattern string, the subject may
contain binary zeroes.

A non-zero starting offset is useful when searching for another match
in the same subject by calling pcre_exec() again after a previous
success. Setting startoffset differs from just passing over a
shortened string and setting PCRE_NOTBOL in the case of a pattern
that begins with any kind of lookbehind. For example, consider the
pattern

\Biss\B

which finds occurrences of "iss" in the middle of words. (\B matches
only if the current position in the subject is not a word boundary.)
When applied to the string "Mississippi" the first call to
pcre_exec() finds the first occurrence. If pcre_exec() is called
again with just the remainder of the subject, namely "issippi", it
does not match, because \B is always false at the start of the
subject, which is deemed to be a word boundary. However, if
pcre_exec() is passed the entire string again, but with startoffset
set to 4, it finds the second occurrence of "iss" because it is able
to look behind the starting point to discover that it is preceded by
a letter.

Finding all the matches in a subject is tricky when the pattern can
match an empty string. It is possible to emulate Perl's /g behaviour
by first trying the match again at the same offset, with the
PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
fails, advancing the starting offset and trying an ordinary match
again. There is some code that demonstrates how to do this in the
pcredemo sample program. In the most general case, you have to check
to see if the newline convention recognizes CRLF as a newline, and if
so, and the current character is CR followed by LF, advance the
starting offset by two characters instead of one.

If a non-zero starting offset is passed when the pattern is anchored,
one attempt to match at the given offset is made. This can only
succeed if the pattern does not require the match to be at the start
of the subject.

How pcre_exec() returns captured substrings
In general, a pattern matches a certain portion of the subject, and
in addition, further substrings from the subject may be picked out by
parts of the pattern. Following the usage in Jeffrey Friedl's book,
this is called "capturing" in what follows, and the phrase "capturing
subpattern" is used for a fragment of a pattern that picks out a
substring. PCRE supports several other kinds of parenthesized
subpattern that do not cause substrings to be captured.

Captured substrings are returned to the caller via a vector of
integers whose address is passed in ovector. The number of elements
in the vector is passed in ovecsize, which must be a non-negative
number. Note: this argument is NOT the size of ovector in bytes.

The first two-thirds of the vector is used to pass back captured
substrings, each substring using a pair of integers. The remaining
third of the vector is used as workspace by pcre_exec() while
matching capturing subpatterns, and is not available for passing back
information. The number passed in ovecsize should always be a
multiple of three. If it is not, it is rounded down.

When a match is successful, information about captured substrings is
returned in pairs of integers, starting at the beginning of ovector,
and continuing up to two-thirds of its length at the most. The first
element of each pair is set to the offset of the first character in a
substring, and the second is set to the offset of the first character
after the end of a substring. These values are always data unit
offsets, even in UTF mode. They are byte offsets in the 8-bit
library, 16-bit data item offsets in the 16-bit library, and 32-bit
data item offsets in the 32-bit library. Note: they are not character
counts.

The first pair of integers, ovector[0] and ovector[1], identify the
portion of the subject string matched by the entire pattern. The next
pair is used for the first capturing subpattern, and so on. The value
returned by pcre_exec() is one more than the highest numbered pair
that has been set. For example, if two substrings have been
captured, the returned value is 3. If there are no capturing
subpatterns, the return value from a successful match is 1,
indicating that just the first pair of offsets has been set.

If a capturing subpattern is matched repeatedly, it is the last
portion of the string that it matched that is returned.

If the vector is too small to hold all the captured substring
offsets, it is used as far as possible (up to two-thirds of its
length), and the function returns a value of zero. If neither the
actual string matched nor any captured substrings are of interest,
pcre_exec() may be called with ovector passed as NULL and ovecsize as
zero. However, if the pattern contains back references and the
ovector is not big enough to remember the related substrings, PCRE
has to get additional memory for use during matching. Thus it is
usually advisable to supply an ovector of reasonable size.

There are some cases where zero is returned (indicating vector
overflow) when in fact the vector is exactly the right size for the
final match. For example, consider the pattern

(a)(?:(b)c|bd)

If a vector of 6 elements (allowing for only 1 captured substring) is
given with subject string "abd", pcre_exec() will try to set the
second captured string, thereby recording a vector overflow, before
failing to match "c" and backing up to try the second alternative.
The zero return, however, does correctly indicate that the maximum
number of slots (namely 2) have been filled. In similar cases where
there is temporary overflow, but the final number of used slots is
actually less than the maximum, a non-zero value is returned.

The pcre_fullinfo() function can be used to find out how many
capturing subpatterns there are in a compiled pattern. The smallest
size for ovector that will allow for n captured substrings, in
addition to the offsets of the substring matched by the whole
pattern, is (n+1)*3.

It is possible for capturing subpattern number n+1 to match some part
of the subject when subpattern n has not been used at all. For
example, if the string "abc" is matched against the pattern
(a|(z))(bc) the return from the function is 4, and subpatterns 1 and
3 are matched, but 2 is not. When this happens, both values in the
offset pairs corresponding to unused subpatterns are set to -1.

Offset values that correspond to unused subpatterns at the end of the
expression are also set to -1. For example, if the string "abc" is
matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are
not matched. The return from the function is 2, because the highest
used capturing subpattern number is 1, and the offsets for for the
second and third capturing subpatterns (assuming the vector is large
enough, of course) are set to -1.

Note: Elements in the first two-thirds of ovector that do not
correspond to capturing parentheses in the pattern are never changed.
That is, if a pattern contains n capturing parentheses, no more than
ovector[0] to ovector[2n+1] are set by pcre_exec(). The other
elements (in the first two-thirds) retain whatever values they
previously had.

Some convenience functions are provided for extracting the captured
substrings as separate strings. These are described below.

Error return values from pcre_exec()
If pcre_exec() fails, it returns a negative number. The following are
defined in the header file:

PCRE_ERROR_NOMATCH (-1)

The subject string did not match the pattern.

PCRE_ERROR_NULL (-2)

Either code or subject was passed as NULL, or ovector was NULL and
ovecsize was not zero.

PCRE_ERROR_BADOPTION (-3)

An unrecognized bit was set in the options argument.

PCRE_ERROR_BADMAGIC (-4)

PCRE stores a 4-byte "magic number" at the start of the compiled
code, to catch the case when it is passed a junk pointer and to
detect when a pattern that was compiled in an environment of one
endianness is run in an environment with the other endianness. This
is the error that PCRE gives when the magic number is not present.

PCRE_ERROR_UNKNOWN_OPCODE (-5)

While running the pattern match, an unknown item was encountered in
the compiled pattern. This error could be caused by a bug in PCRE or
by overwriting of the compiled pattern.

PCRE_ERROR_NOMEMORY (-6)

If a pattern contains back references, but the ovector that is passed
to pcre_exec() is not big enough to remember the referenced
substrings, PCRE gets a block of memory at the start of matching to
use for this purpose. If the call via pcre_malloc() fails, this error
is given. The memory is automatically freed at the end of matching.

This error is also given if pcre_stack_malloc() fails in pcre_exec().
This can happen only when PCRE has been compiled with --disable-
stack-for-recursion.

PCRE_ERROR_NOSUBSTRING (-7)

This error is used by the pcre_copy_substring(),
pcre_get_substring(), and pcre_get_substring_list() functions (see
below). It is never returned by pcre_exec().

PCRE_ERROR_MATCHLIMIT (-8)

The backtracking limit, as specified by the match_limit field in a
pcre_extra structure (or defaulted) was reached. See the description
above.

PCRE_ERROR_CALLOUT (-9)

This error is never generated by pcre_exec() itself. It is provided
for use by callout functions that want to yield a distinctive error
code. See the pcrecallout documentation for details.

PCRE_ERROR_BADUTF8 (-10)

A string that contains an invalid UTF-8 byte sequence was passed as a
subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size
of the output vector (ovecsize) is at least 2, the byte offset to the
start of the the invalid UTF-8 character is placed in the first
element, and a reason code is placed in the second element. The
reason codes are listed in the following section. For backward
compatibility, if PCRE_PARTIAL_HARD is set and the problem is a
truncated UTF-8 character at the end of the subject (reason codes 1
to 5), PCRE_ERROR_SHORTUTF8 is returned instead of
PCRE_ERROR_BADUTF8.

PCRE_ERROR_BADUTF8_OFFSET (-11)

The UTF-8 byte sequence that was passed as a subject was checked and
found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but
the value of startoffset did not point to the beginning of a UTF-8
character or the end of the subject.

PCRE_ERROR_PARTIAL (-12)

The subject string did not match, but it did match partially. See the
pcrepartial documentation for details of partial matching.

PCRE_ERROR_BADPARTIAL (-13)

This code is no longer in use. It was formerly returned when the
PCRE_PARTIAL option was used with a compiled pattern containing items
that were not supported for partial matching. From release 8.00
onwards, there are no restrictions on partial matching.

PCRE_ERROR_INTERNAL (-14)

An unexpected internal error has occurred. This error could be caused
by a bug in PCRE or by overwriting of the compiled pattern.

PCRE_ERROR_BADCOUNT (-15)

This error is given if the value of the ovecsize argument is
negative.

PCRE_ERROR_RECURSIONLIMIT (-21)

The internal recursion limit, as specified by the
match_limit_recursion field in a pcre_extra structure (or defaulted)
was reached. See the description above.

PCRE_ERROR_BADNEWLINE (-23)

An invalid combination of PCRE_NEWLINE_xxx options was given.

PCRE_ERROR_BADOFFSET (-24)

The value of startoffset was negative or greater than the length of
the subject, that is, the value in length.

PCRE_ERROR_SHORTUTF8 (-25)

This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
string ends with a truncated UTF-8 character and the
PCRE_PARTIAL_HARD option is set. Information about the failure is
returned as for PCRE_ERROR_BADUTF8. It is in fact sufficient to
detect this case, but this special error code for PCRE_PARTIAL_HARD
precedes the implementation of returned information; it is retained
for backwards compatibility.

PCRE_ERROR_RECURSELOOP (-26)

This error is returned when pcre_exec() detects a recursion loop
within the pattern. Specifically, it means that either the whole
pattern or a subpattern has been called recursively for the second
time at the same position in the subject string. Some simple patterns
that might do this are detected and faulted at compile time, but more
complicated cases, in particular mutual recursions between two
different subpatterns, cannot be detected until run time.

PCRE_ERROR_JIT_STACKLIMIT (-27)

This error is returned when a pattern that was successfully studied
using a JIT compile option is being matched, but the memory available
for the just-in-time processing stack is not large enough. See the
pcrejit documentation for more details.

PCRE_ERROR_BADMODE (-28)

This error is given if a pattern that was compiled by the 8-bit
library is passed to a 16-bit or 32-bit library function, or vice
versa.

PCRE_ERROR_BADENDIANNESS (-29)

This error is given if a pattern that was compiled and saved is
reloaded on a host with different endianness. The utility function
pcre_pattern_to_host_byte_order() can be used to convert such a
pattern so that it runs on the new host.

PCRE_ERROR_JIT_BADOPTION

This error is returned when a pattern that was successfully studied
using a JIT compile option is being matched, but the matching mode
(partial or complete match) does not correspond to any JIT
compilation mode. When the JIT fast path function is used, this error
may be also given for invalid options. See the pcrejit documentation
for more details.

PCRE_ERROR_BADLENGTH (-32)

This error is given if pcre_exec() is called with a negative value
for the length argument.

Error numbers -16 to -20, -22, and 30 are not used by pcre_exec().

Reason codes for invalid UTF-8 strings
This section applies only to the 8-bit library. The corresponding
information for the 16-bit and 32-bit libraries is given in the
pcre16 and pcre32 pages.

When pcre_exec() returns either PCRE_ERROR_BADUTF8 or
PCRE_ERROR_SHORTUTF8, and the size of the output vector (ovecsize) is
at least 2, the offset of the start of the invalid UTF-8 character is
placed in the first output vector element (ovector[0]) and a reason
code is placed in the second element (ovector[1]). The reason codes
are given names in the pcre.h header file:

PCRE_UTF8_ERR1
PCRE_UTF8_ERR2
PCRE_UTF8_ERR3
PCRE_UTF8_ERR4
PCRE_UTF8_ERR5

The string ends with a truncated UTF-8 character; the code specifies
how many bytes are missing (1 to 5). Although RFC 3629 restricts
UTF-8 characters to be no longer than 4 bytes, the encoding scheme
(originally defined by RFC 2279) allows for up to 6 bytes, and this
is checked first; hence the possibility of 4 or 5 missing bytes.

PCRE_UTF8_ERR6
PCRE_UTF8_ERR7
PCRE_UTF8_ERR8
PCRE_UTF8_ERR9
PCRE_UTF8_ERR10

The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte
of the character do not have the binary value 0b10 (that is, either
the most significant bit is 0, or the next bit is 1).

PCRE_UTF8_ERR11
PCRE_UTF8_ERR12

A character that is valid by the RFC 2279 rules is either 5 or 6
bytes long; these code points are excluded by RFC 3629.

PCRE_UTF8_ERR13

A 4-byte character has a value greater than 0x10fff; these code
points are excluded by RFC 3629.

PCRE_UTF8_ERR14

A 3-byte character has a value in the range 0xd800 to 0xdfff; this
range of code points are reserved by RFC 3629 for use with UTF-16,
and so are excluded from UTF-8.

PCRE_UTF8_ERR15
PCRE_UTF8_ERR16
PCRE_UTF8_ERR17
PCRE_UTF8_ERR18
PCRE_UTF8_ERR19

A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it
codes for a value that can be represented by fewer bytes, which is
invalid. For example, the two bytes 0xc0, 0xae give the value 0x2e,
whose correct coding uses just one byte.

PCRE_UTF8_ERR20

The two most significant bits of the first byte of a character have
the binary value 0b10 (that is, the most significant bit is 1 and the
second is 0). Such a byte can only validly occur as the second or
subsequent byte of a multi-byte character.

PCRE_UTF8_ERR21

The first byte of a character has the value 0xfe or 0xff. These
values can never occur in a valid UTF-8 string.

PCRE_UTF8_ERR22

This error code was formerly used when the presence of a so-called
"non-character" caused an error. Unicode corrigendum #9 makes it
clear that such characters should not cause a string to be rejected,
and so this code is no longer in use and is never returned.

EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);

int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);

int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);

Captured substrings can be accessed directly by using the offsets
returned by pcre_exec() in ovector. For convenience, the functions
pcre_copy_substring(), pcre_get_substring(), and
pcre_get_substring_list() are provided for extracting captured
substrings as new, separate, zero-terminated strings. These functions
identify substrings by number. The next section describes functions
for extracting named substrings.

A substring that contains a binary zero is correctly extracted and
has a further zero added on the end, but the result is not, of
course, a C string. However, you can process such a string by
referring to the length that is returned by pcre_copy_substring() and
pcre_get_substring(). Unfortunately, the interface to
pcre_get_substring_list() is not adequate for handling strings
containing binary zeros, because the end of the final string is not
independently indicated.

The first three arguments are the same for all three of these
functions: subject is the subject string that has just been
successfully matched, ovector is a pointer to the vector of integer
offsets that was passed to pcre_exec(), and stringcount is the number
of substrings that were captured by the match, including the
substring that matched the entire regular expression. This is the
value returned by pcre_exec() if it is greater than zero. If
pcre_exec() returned zero, indicating that it ran out of space in
ovector, the value passed as stringcount should be the number of
elements in the vector divided by three.

The functions pcre_copy_substring() and pcre_get_substring() extract
a single substring, whose number is given as stringnumber. A value of
zero extracts the substring that matched the entire pattern, whereas
higher values extract the captured substrings. For
pcre_copy_substring(), the string is placed in buffer, whose length
is given by buffersize, while for pcre_get_substring() a new block of
memory is obtained via pcre_malloc, and its address is returned via
stringptr. The yield of the function is the length of the string, not
including the terminating zero, or one of these error codes:

PCRE_ERROR_NOMEMORY (-6)

The buffer was too small for pcre_copy_substring(), or the attempt to
get memory failed for pcre_get_substring().

PCRE_ERROR_NOSUBSTRING (-7)

There is no substring whose number is stringnumber.

The pcre_get_substring_list() function extracts all available
substrings and builds a list of pointers to them. All this is done in
a single block of memory that is obtained via pcre_malloc. The
address of the memory block is returned via listptr, which is also
the start of the list of string pointers. The end of the list is
marked by a NULL pointer. The yield of the function is zero if all
went well, or the error code

PCRE_ERROR_NOMEMORY (-6)

if the attempt to get the memory block failed.

When any of these functions encounter a substring that is unset,
which can happen when capturing subpattern number n+1 matches some
part of the subject, but subpattern n has not been used at all, they
return an empty string. This can be distinguished from a genuine
zero-length substring by inspecting the appropriate offset in
ovector, which is negative for unset substrings.

The two convenience functions pcre_free_substring() and
pcre_free_substring_list() can be used to free the memory returned by
a previous call of pcre_get_substring() or pcre_get_substring_list(),
respectively. They do nothing more than call the function pointed to
by pcre_free, which of course could be called directly from a C
program. However, PCRE is used in some situations where it is linked
via a special interface to another programming language that cannot
use pcre_free directly; it is for these cases that the functions are
provided.

EXTRACTING CAPTURED SUBSTRINGS BY NAME

int pcre_get_stringnumber(const pcre *code,
const char *name);

int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);

int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);

To extract a substring by name, you first have to find associated
number. For example, for this pattern

(a+)b(?<xxx>\d+)...

the number of the subpattern called "xxx" is 2. If the name is known
to be unique (PCRE_DUPNAMES was not set), you can find the number
from the name by calling pcre_get_stringnumber(). The first argument
is the compiled pattern, and the second is the name. The yield of the
function is the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if
there is no subpattern of that name.

Given the number, you can extract the substring directly, or use one
of the functions described in the previous section. For convenience,
there are also two functions that do the whole job.

Most of the arguments of pcre_copy_named_substring() and
pcre_get_named_substring() are the same as those for the similarly
named functions that extract by number. As these are described in the
previous section, they are not re-described here. There are just two
differences:

First, instead of a substring number, a substring name is given.
Second, there is an extra argument, given at the start, which is a
pointer to the compiled pattern. This is needed in order to gain
access to the name-to-number translation table.

These functions call pcre_get_stringnumber(), and if it succeeds,
they then call pcre_copy_substring() or pcre_get_substring(), as
appropriate. NOTE: If PCRE_DUPNAMES is set and there are duplicate
names, the behaviour may not be what you want (see the next section).

Warning: If the pattern uses the (?| feature to set up multiple
subpatterns with the same number, as described in the section on
duplicate subpattern numbers in the pcrepattern page, you cannot use
names to distinguish the different subpatterns, because names are not
included in the compiled code. The matching process uses only
numbers. For this reason, the use of different names for subpatterns
of the same number causes an error at compile time.

DUPLICATE SUBPATTERN NAMES

int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);

When a pattern is compiled with the PCRE_DUPNAMES option, names for
subpatterns are not required to be unique. (Duplicate names are
always allowed for subpatterns with the same number, created by using
the (?| feature. Indeed, if such subpatterns are named, they are
required to use the same names.)

Normally, patterns with duplicate names are such that in any one
match, only one of the named subpatterns participates. An example is
shown in the pcrepattern documentation.

When duplicates are present, pcre_copy_named_substring() and
pcre_get_named_substring() return the first substring corresponding
to the given name that is set. If none are set,
PCRE_ERROR_NOSUBSTRING (-7) is returned; no data is returned. The
pcre_get_stringnumber() function returns one of the numbers that are
associated with the name, but it is not defined which it is.

If you want to get full details of all captured substrings for a
given name, you must use the pcre_get_stringtable_entries() function.
The first argument is the compiled pattern, and the second is the
name. The third and fourth are pointers to variables which are
updated by the function. After it has run, they point to the first
and last entries in the name-to-number table for the given name. The
function itself returns the length of each entry, or
PCRE_ERROR_NOSUBSTRING (-7) if there are none. The format of the
table is described above in the section entitled Information about a
pattern above. Given all the relevant entries for the name, you can
extract each of their numbers, and hence the captured data, if any.

FINDING ALL POSSIBLE MATCHES

The traditional matching function uses a similar algorithm to Perl,
which stops when it finds the first match, starting at a given point
in the subject. If you want to find all possible matches, or the
longest possible match, consider using the alternative matching
function (see below) instead. If you cannot use the alternative
function, but still need to find all possible matches, you can kludge
it up by making use of the callout facility, which is described in
the pcrecallout documentation.

What you have to do is to insert a callout right at the end of the
pattern. When your callout function is called, extract and save the
current matched substring. Then return 1, which forces pcre_exec() to
backtrack and try other alternatives. Ultimately, when it runs out of
matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.

OBTAINING AN ESTIMATE OF STACK USAGE

Matching certain patterns using pcre_exec() can use a lot of process
stack, which in certain environments can be rather limited in size.
Some users find it helpful to have an estimate of the amount of stack
that is used by pcre_exec(), to help them set recursion limits, as
described in the pcrestack documentation. The estimate that is output
by pcretest when called with the -m and -C options is obtained by
calling pcre_exec with the values NULL, NULL, NULL, -999, and -999
for its first five arguments.

Normally, if its first argument is NULL, pcre_exec() immediately
returns the negative error code PCRE_ERROR_NULL, but with this
special combination of arguments, it returns instead a negative
number whose absolute value is the approximate stack frame size in
bytes. (A negative number is used so that it is clear that no match
has happened.) The value is approximate because in some cases,
recursive calls to pcre_exec() occur when there are one or two
additional variables on the stack.

If PCRE has been compiled to use the heap instead of the stack for
recursion, the value returned is the size of each block that is
obtained from the heap.

MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);

The function pcre_dfa_exec() is called to match a subject string
against a compiled pattern, using a matching algorithm that scans the
subject string just once, and does not backtrack. This has different
characteristics to the normal algorithm, and is not compatible with
Perl. Some of the features of PCRE patterns are not supported.
Nevertheless, there are times when this kind of matching can be
useful. For a discussion of the two matching algorithms, and a list
of features that pcre_dfa_exec() does not support, see the
pcrematching documentation.

The arguments for the pcre_dfa_exec() function are the same as for
pcre_exec(), plus two extras. The ovector argument is used in a
different way, and this is described below. The other common
arguments are used in the same way as for pcre_exec(), so their
description is not repeated here.

The two additional arguments provide workspace for the function. The
workspace vector should contain at least 20 elements. It is used for
keeping track of multiple paths through the pattern tree. More
workspace will be needed for patterns and subjects where there are a
lot of potential matches.

Here is an example of a simple call to pcre_dfa_exec():

int rc;
int ovector[10];
int wspace[20];
rc = pcre_dfa_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information
*/
10, /* number of elements (NOT size in bytes) */
wspace, /* working space vector */
20); /* number of elements (NOT size in bytes) */

Option bits for pcre_dfa_exec()
The unused bits of the options argument for pcre_dfa_exec() must be
zero. The only bits that may be set are PCRE_ANCHORED,
PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF,
PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD,
PCRE_PARTIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but
the last four of these are exactly the same as for pcre_exec(), so
their description is not repeated here.

PCRE_PARTIAL_HARD
PCRE_PARTIAL_SOFT

These have the same general effect as they do for pcre_exec(), but
the details are slightly different. When PCRE_PARTIAL_HARD is set for
pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the
subject is reached and there is still at least one matching
possibility that requires additional characters. This happens even if
some complete matches have also been found. When PCRE_PARTIAL_SOFT is
set, the return code PCRE_ERROR_NOMATCH is converted into
PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
been no complete matches, but there is still at least one matching
possibility. The portion of the string that was inspected when the
longest partial match was found is set as the first matching string
in both cases. There is a more detailed discussion of partial and
multi-segment matching, with examples, in the pcrepartial
documentation.

PCRE_DFA_SHORTEST

Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
stop as soon as it has found one match. Because of the way the
alternative algorithm works, this is necessarily the shortest
possible match at the first possible matching point in the subject
string.

PCRE_DFA_RESTART

When pcre_dfa_exec() returns a partial match, it is possible to call
it again, with additional subject characters, and have it continue
with the same match. The PCRE_DFA_RESTART option requests this
action; when it is set, the workspace and wscount options must
reference the same vector as before because data about the match so
far is left in them after a partial match. There is more discussion
of this facility in the pcrepartial documentation.

Successful returns from pcre_dfa_exec()
When pcre_dfa_exec() succeeds, it may have matched more than one
substring in the subject. Note, however, that all the matches from
one run of the function start at the same point in the subject. The
shorter matches are all initial substrings of the longer matches. For
example, if the pattern

<.*>

is matched against the string

This is <something> <something else> <something further> no more

the three matched strings are

<something>
<something> <something else>
<something> <something else> <something further>

On success, the yield of the function is a number greater than zero,
which is the number of matched substrings. The substrings themselves
are returned in ovector. Each string uses two elements; the first is
the offset to the start, and the second is the offset to the end. In
fact, all the strings have the same start offset. (Space could have
been saved by giving this only once, but it was decided to retain
some compatibility with the way pcre_exec() returns data, even though
the meaning of the strings is different.)

The strings are returned in reverse order of length; that is, the
longest matching string is given first. If there were too many
matches to fit into ovector, the yield of the function is zero, and
the vector is filled with the longest matches. Unlike pcre_exec(),
pcre_dfa_exec() can use the entire ovector for returning matched
strings.

NOTE: PCRE's "auto-possessification" optimization usually applies to
character repeats at the end of a pattern (as well as internally).
For example, the pattern "a\d+" is compiled as if it were "a\d++"
because there is no point even considering the possibility of
backtracking into the repeated digits. For DFA matching, this means
that only one possible match is found. If you really do want multiple
matches in such cases, either use an ungreedy repeat ("a\d+?") or set
the PCRE_NO_AUTO_POSSESS option when compiling.

Error returns from pcre_dfa_exec()
The pcre_dfa_exec() function returns a negative number when it fails.
Many of the errors are the same as for pcre_exec(), and these are
described above. There are in addition the following errors that are
specific to pcre_dfa_exec():

PCRE_ERROR_DFA_UITEM (-16)

This return is given if pcre_dfa_exec() encounters an item in the
pattern that it does not support, for instance, the use of \C or a
back reference.

PCRE_ERROR_DFA_UCOND (-17)

This return is given if pcre_dfa_exec() encounters a condition item
that uses a back reference for the condition, or a test for recursion
in a specific group. These are not supported.

PCRE_ERROR_DFA_UMLIMIT (-18)

This return is given if pcre_dfa_exec() is called with an extra block
that contains a setting of the match_limit or match_limit_recursion
fields. This is not supported (these fields are meaningless for DFA
matching).

PCRE_ERROR_DFA_WSSIZE (-19)

This return is given if pcre_dfa_exec() runs out of space in the
workspace vector.

PCRE_ERROR_DFA_RECURSE (-20)

When a recursive subpattern is processed, the matching function calls
itself recursively, using private vectors for ovector and workspace.
This error is given if the output vector is not large enough. This
should be extremely rare, as a vector of size 1000 is used.

PCRE_ERROR_DFA_BADRESTART (-30)

When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some
plausibility checks are made on the contents of the workspace, which
should contain data about the previous partial match. If any of these
checks fail, this error is given.

SEE ALSO

pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3),
pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3),
pcresample(3), pcrestack(3).

AUTHOR

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.

REVISION

Last updated: 18 December 2015
Copyright (c) 1997-2015 University of Cambridge.

PCRE 8.39 18 December 2015 PCREAPI(3)