pcreapi
PCREAPI(3) PCREAPI(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE NATIVE API
#include <pcre.h>
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);
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);
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);
const unsigned char *pcre_maketables(void);
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
int pcre_refcount(pcre *code, int adjust);
int pcre_config(int what, void *where);
char *pcre_version(void);
void *(*pcre_malloc)(size_t);
void (*pcre_free)(void *);
void *(*pcre_stack_malloc)(size_t);
void (*pcre_stack_free)(void *);
int (*pcre_callout)(pcre_callout_block *);
PCRE API OVERVIEW
PCRE has its own native API, which is described in this document.
There are also some wrapper functions that correspond to the POSIX
regular expression API. These are described in the pcreposix documen-
tation. Both of these APIs define a set of C function calls. A C++
wrapper is 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 systems the 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 num-
bers for the library. Applications can use these to include support
for different releases of PCRE.
The functions pcre_compile(), pcre_compile2(), pcre_study(), and
pcre_exec() are used for compiling and matching regular expressions in
a Perl-compatible manner. A sample program that demonstrates the sim-
plest way of using them is provided in the file called pcredemo.c in
the source distribution. The pcresample documentation describes how to
run it.
A second matching function, pcre_dfa_exec(), which is not Perl-compat-
ible, is also provided. This uses a different algorithm for the match-
ing. The alternative algorithm finds all possible matches (at a given
point in the subject), and scans the subject just once. 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 sub-
ject 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 pro-
vided, 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; pcre_info() is an obsolete version that returns only
some of the available information, but is retained for backwards com-
patibility. The function pcre_version() returns a pointer to a string
containing the version of PCRE and its date of release.
The function pcre_refcount() maintains a reference count in a data
block containing a compiled pattern. This is provided for the benefit
of object-oriented applications.
The global variables pcre_malloc and pcre_free initially contain the
entry points of the standard malloc() and free() functions, respec-
tively. PCRE calls the memory management functions via these vari-
ables, 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 pro-
vided so that special-purpose external code can be used for this case.
When used, these functions are always called in a stack-like manner
(last obtained, first freed), and always for memory blocks of the same
size. There is a discussion about PCRE’s stack usage in the pcrestack
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.
NEWLINES
PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (line-
feed) character, the two-character sequence CRLF, any of the three
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 (formfeed, U+000C), NEL (next line,
U+0085), LS (line separator, U+2028), and PS (paragraph separator,
U+2029).
Each of the first three conventions is used by at least one operating
system as its standard newline sequence. When PCRE is built, a default
can be specified. The default default is LF, which is the Unix stan-
dard. When PCRE is run, the default can be overridden, either when a
pattern is compiled, or when it is matched.
At compile time, the newline convention can be specified by the
options argument of pcre_compile(), or it can be specified by special
text at the start of the pattern itself; this overrides any other set-
tings. 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, circum-
flex, 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 function pointed to by pcre_callout, are shared by all
threads.
The compiled form of a regular expression is not altered during match-
ing, so the same compiled pattern can safely be used by several
threads at once.
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. However, compiling a regular expression
with one version of PCRE for use with a different version is not guar-
anteed to work and may cause crashes.
CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);
The function pcre_config() makes it possible for a PCRE client to dis-
cover which optional features have been compiled into the PCRE
library. The pcrebuild documentation has more details about these
optional features.
The first argument for pcre_config() is an integer, specifying which
information is required; the second argument is a pointer to a vari-
able into which the information is placed. The following information
is available:
PCRE_CONFIG_UTF8
The output is an integer that is set to one if UTF-8 support is avail-
able; otherwise it is set to zero.
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_NEWLINE
The output is an integer whose value specifies the default character
sequence that is recognized as meaning "newline". The four values that
are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANY-
CRLF, and -1 for ANY. The default should normally be 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 over-
ridden 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. The value is 2, 3,
or 4. Larger values allow larger regular expressions to be compiled,
at the expense of slower matching. The default value of 2 is suffi-
cient for all but the most massive patterns, since it allows the com-
piled pattern to be up to 64K in size.
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_MATCH_LIMIT
The output is an integer that gives the default limit for the number
of internal matching function calls in a pcre_exec() execution. Fur-
ther details are given with pcre_exec() below.
PCRE_CONFIG_MATCH_LIMIT_RECURSION
The output is an 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.
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 avail-
able options are described below. Some of them, in particular, those
that are compatible with Perl, can also be set and unset from within
the pattern (see the detailed description in the pcrepattern documen-
tation). For these options, the contents of the options argument spec-
ifies their initial settings at the start of compilation and execu-
tion. The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the
time of matching as well as at compile time.
If errptr is NULL, pcre_compile() returns NULL immediately. Other-
wise, 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. The offset from the start of the pattern to the
character where the error was discovered is placed in the variable
pointed to by erroffset, which must not be NULL. If it is, an immedi-
ate error is given.
If pcre_compile2() is used instead of pcre_compile(), and the error-
codeptr argument is not NULL, a non-zero error code number is returned
via this argument in the event of an error. This is in addition to the
textual error message. Error codes and messages are listed below.
If the final argument, tableptr, is NULL, PCRE uses a default set of
character tables that are built when PCRE is compiled, using the
default C locale. Otherwise, tableptr must be an address that is the
result of a call to pcre_maketables(). This value is stored with the
compiled pattern, and used again by pcre_exec(), unless another table
pointer is passed to it. For more discussion, see the section on
locale support below.
This code fragment shows a typical straightforward call to pcre_com-
pile():
pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */
The following names for option bits are defined in the pcre.h header
file:
PCRE_ANCHORED
If this bit is set, the pattern is forced to be "anchored", that is,
it is constrained to match only at the first matching point in the
string that is being searched (the "subject string"). This effect can
also be achieved by appropriate constructs in the pattern itself,
which is the only way to do it in Perl.
PCRE_AUTO_CALLOUT
If this bit is set, pcre_compile() automatically inserts callout
items, all with number 255, before each pattern item. For discussion
of the callout facility, see the pcrecallout documentation.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what the \R
escape sequence matches. The choice is either to match only CR, LF, or
CRLF, or to match any Unicode newline sequence. The default is speci-
fied 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 com-
piled with Unicode property support, but not otherwise. If you want to
use caseless matching for characters 128 and above, you must ensure
that PCRE is compiled with Unicode property support as well as with
UTF-8 support.
PCRE_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern matches only
at the end of the subject string. Without this option, a dollar also
matches immediately before a newline at the end of the string (but not
before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
if PCRE_MULTILINE is set. There is no equivalent to this option in
Perl, and no way to set it within a pattern.
PCRE_DOTALL
If this bit is set, a dot metacharater in the pattern matches all
characters, including those that indicate newline. Without it, 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 pat-
tern 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, whitespace data characters in the pattern are
totally ignored except when escaped or inside a character class.
Whitespace does not include the VT character (code 11). In addition,
characters between an unescaped # outside a character class and the
next newline, inclusive, are also ignored. This is equivalent to
Perl’s /x option, and it can be changed within a pattern by a (?x)
option setting.
This option makes it possible to include comments inside complicated
patterns. Note, however, that this applies only to data characters.
Whitespace characters may never appear within special character
sequences in a pattern, for example within the sequence (?( which
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 a warning for this.)
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_MULTILINE
By default, 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,
while the "end of line" metacharacter ($) matches only at the end of
the string, or before a terminating newline (unless PCRE_DOL-
LAR_ENDONLY is set). This is the same as Perl.
When PCRE_MULTILINE it is set, the "start of line" and "end of line"
constructs match immediately following or immediately before internal
newlines in the subject string, respectively, as well as at the very
start and end. This is equivalent to Perl’s /m option, and it can be
changed within a pattern by a (?m) option setting. If there are no
newlines in a subject string, or no occurrences of ^ or $ in a pat-
tern, setting PCRE_MULTILINE has no effect.
PCRE_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. Set-
ting PCRE_NEWLINE_ANY specifies that any Unicode newline sequence
should be recognized. The Unicode newline sequences are the three just
mentioned, plus the single characters VT (vertical tab, U+000B), FF
(formfeed, U+000C), NEL (next line, U+0085), LS (line separator,
U+2028), and PS (paragraph separator, U+2029). The last two are recog-
nized only in UTF-8 mode.
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 equiva-
lent to PCRE_NEWLINE_CRLF, but other combinations may yield unused
numbers and cause an error.
The only time that a line break is specially recognized when compiling
a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
character class is encountered. This indicates a comment that lasts
until after the next line break sequence. In other circumstances, line
break sequences are treated as literal data, except that in
PCRE_EXTENDED mode, both CR and LF are treated as whitespace charac-
ters and are therefore ignored.
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 over-
ridden.
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 fol-
lowed 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_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 character strings.
However, it is available only when PCRE is built to include UTF-8 sup-
port. If not, the use of this option provokes an error. Details of how
this option changes the behaviour of PCRE are given in the section on
UTF-8 support in the main pcre 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 main pcre page. If an invalid UTF-8 sequence of
bytes is found, pcre_compile() returns an error. If you already know
that your pattern is valid, and you want to skip this check for per-
formance 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. Note that this option
can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
UTF-8 validity checking of subject strings.
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. 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 (?
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 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 not compiled with PCRE_UTF8 support
33 [this code is not in use]
34 character value in \x{...} sequence is too large
35 invalid condition (?(0)
36 \C not allowed in lookbehind assertion
37 PCRE does not support \L, \l, \N, \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
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 10,000)
50 [this code is not in use]
51 octal value is greater than \377 (not in 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 name or an optionally braced
non-zero number
58 (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
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 addi-
tional 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(). 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 additional information
pcre_study() returns NULL. In that circumstance, if the calling pro-
gram wants to pass any of the other fields to pcre_exec(), it must set
up its own pcre_extra block.
The second argument of pcre_study() contains option bits. At present,
no options are defined, and this argument should always be zero.
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 success-
fully.
This is a typical call to pcre_study():
pcre_extra *pe;
pe = pcre_study(
re, /* result of pcre_compile() */
0, /* no options exist */
&error); /* set to NULL or points to a message */
At present, studying a pattern is useful only for non-anchored pat-
terns that do not have a single fixed starting character. A bitmap of
possible starting bytes is created.
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 value. When running in UTF-8 mode, this applies only to
characters with codes less than 128. Higher-valued codes never match
escapes such as \w or \d, but can be tested with \p if PCRE is built
with Unicode character property support. The use of locales with Uni-
code is discouraged. If you are handling characters with codes greater
than 128, you should either use UTF-8 and Unicode, 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() or pcre_exec() 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 com-
piled pattern, and the same tables are used via this pointer by
pcre_study() and normally also by pcre_exec(). Thus, by default, for
any single pattern, compilation, studying and matching all happen in
the same locale, but different patterns can be compiled in different
locales.
It is possible to pass a table pointer or NULL (indicating the use of
the internal tables) to pcre_exec(). Although not intended for this
purpose, this facility could be used to match a pattern in a different
locale from the one in which it was compiled. Passing table pointers
at run time is discussed below in the section on matching a pattern.
INFORMATION ABOUT A PATTERN
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
The pcre_fullinfo() function returns information about a compiled pat-
tern. It replaces the obsolete pcre_info() function, which is never-
theless retained for backwards compability (and is documented below).
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_BADOPTION the value of what was invalid
The "magic number" is placed at the start of each compiled pattern as
an simple check against passing an arbitrary memory pointer. 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() */
pe, /* 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
Return information about the first byte of any matched string, for a
non-anchored pattern. The fourth argument should point to an int vari-
able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
is still recognized for backwards compatibility.)
If there is a fixed first byte, for example, from a pattern such as
(cat|cow|coyote), its value is returned. Otherwise, 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_FIRSTTABLE
If the pattern was studied, and this resulted in the construction of a
256-bit table indicating a fixed set of bytes for the first byte 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) option setting is used in the pattern, otherwise
0. The fourth argument should point to an int variable. The (?J)
internal option setting changes the local PCRE_DUPNAMES option.
PCRE_INFO_LASTLITERAL
Return the value of the rightmost literal byte that must exist in any
matched string, other than at its start, if such a byte has been
recorded. The fourth argument should point to an int variable. If
there is no such byte, -1 is returned. For anchored patterns, a last
literal byte 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.
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 func-
tions 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_NAME-
COUNT 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 (a pointer to char).
The first two bytes of each entry are the number of the capturing
parenthesis, most significant byte first. The rest of the entry is the
corresponding name, zero terminated. The names are in alphabetical
order. When PCRE_DUPNAMES is set, duplicate names are in order of
their parentheses numbers. For example, consider the following pattern
(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, otherwise 0.
The fourth argument should point to an int variable. The pcrepartial
documentation lists the restrictions that apply to patterns when par-
tial matching is used.
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(), modi-
fied 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_SIZE
Return the size of the compiled pattern, that is, the value that was
passed as the argument to pcre_malloc() when PCRE was getting memory
in which to place the compiled data. The fourth argument should point
to a size_t variable.
PCRE_INFO_STUDYSIZE
Return the size of the data block pointed to by the study_data field
in a pcre_extra block. That is, it is the value that was passed to
pcre_malloc() when PCRE was getting memory into which to place the
data created by pcre_study(). The fourth argument should point to a
size_t variable.
OBSOLETE INFO FUNCTION
int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
The pcre_info() function is now obsolete because its interface is too
restrictive to return all the available data about a compiled pattern.
New programs should use pcre_fullinfo() instead. The yield of
pcre_info() is the number of capturing subpatterns, or one of the fol-
lowing negative numbers:
PCRE_ERROR_NULL the argument code was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
If the optptr argument is not NULL, a copy of the options with which
the pattern was compiled is placed in the integer it points to (see
PCRE_INFO_OPTIONS above).
If the pattern is not anchored and the firstcharptr argument is not
NULL, it is used to pass back information about the first character of
any matched string (see PCRE_INFO_FIRSTBYTE above).
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 com-
piled 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 sce-
nario.)
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
has been studied, the result of the study should be passed in the
extra argument. This function is the main matching facility of the
library, and it operates in a Perl-like manner. For specialist use
there is also an alternative matching function, which is described
below in the section about the pcre_dfa_exec() function.
In most applications, the pattern will have been compiled (and option-
ally studied) in the same process that calls pcre_exec(). However, it
is possible to save compiled patterns and study data, and then use
them later in different processes, possibly even on different hosts.
For a discussion about this, see the pcreprecompile documentation.
Here is an example of a simple call to pcre_exec():
int rc;
int ovector[30];
rc = pcre_exec(
re, /* result of pcre_compile() */
NULL, /* we didn’t study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information */
30); /* number of elements (NOT size in bytes) */
Extra data for pcre_exec()
If the extra argument is not NULL, it must point to a pcre_extra data
block. The pcre_study() function returns such a block (when it doesn’t
return NULL), but you can also create one for yourself, and pass addi-
tional information in it. The pcre_extra block contains the following
fields (not necessarily in this order):
unsigned long int flags;
void *study_data;
unsigned long int match_limit;
unsigned long int match_limit_recursion;
void *callout_data;
const unsigned char *tables;
The flags field is a bitmap that specifies which of the other fields
are set. The flag bits are:
PCRE_EXTRA_STUDY_DATA
PCRE_EXTRA_MATCH_LIMIT
PCRE_EXTRA_MATCH_LIMIT_RECURSION
PCRE_EXTRA_CALLOUT_DATA
PCRE_EXTRA_TABLES
Other flag bits should be set to zero. The study_data field is set in
the pcre_extra block that is returned by pcre_study(), together with
the appropriate flag bit. You should not set this yourself, but you
may add to the block by setting the other fields and their correspond-
ing 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 the use of nested unlimited
repeats.
Internally, PCRE uses a function called match() which it calls repeat-
edly (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.
The default value for the limit can be set when PCRE is built; the
default default is 10 million, which handles all but the most extreme
cases. You can override the default by suppling pcre_exec() with a
pcre_extra block in which match_limit is set, and
PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
The match_limit_recursion field is similar to match_limit, but instead
of limiting the total number of times that match() is called, it lim-
its 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 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.
The default value for match_limit_recursion can be set when PCRE is
built; the default default is the same value as the default for
match_limit. You can override the default by suppling pcre_exec() with
a pcre_extra block in which match_limit_recursion is set, and
PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
The pcre_callout field is used in conjunction with the "callout" fea-
ture, which is described in the pcrecallout documentation.
The tables field is used to pass a character tables pointer to
pcre_exec(); this overrides the value that is stored with the compiled
pattern. A non-NULL value is stored with the compiled pattern only if
custom tables were supplied to pcre_compile() via its tableptr argu-
ment. If NULL is passed to pcre_exec() using this mechanism, it
forces PCRE’s internal tables to be used. This facility is helpful
when re-using patterns that have been saved after compiling with an
external set of tables, because the external tables might be at a dif-
ferent address when pcre_exec() is called. See the pcreprecompile doc-
umentation for a discussion of saving compiled patterns for later use.
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_NO_UTF8_CHECK and
PCRE_PARTIAL.
PCRE_ANCHORED
The PCRE_ANCHORED option limits pcre_exec() to matching at the first
matching position. If a pattern was compiled with PCRE_ANCHORED, or
turned out to be anchored by virtue of its contents, it cannot be made
unachored at matching time.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what the \R
escape sequence matches. The choice is either to match only CR, LF, or
CRLF, or to match any Unicode newline sequence. These options override
the choice that was made or defaulted when the pattern was compiled.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the newline definition that was chosen or
defaulted when the pattern was compiled. For details, see the descrip-
tion of pcre_compile() above. During matching, the newline choice
affects the behaviour of the dot, circumflex, and dollar metacharac-
ters. It may also alter the way the match position is advanced after a
match failure for an unanchored pattern.
When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
set, and a match attempt for an unanchored pattern fails when the cur-
rent position is at a CRLF sequence, and the pattern contains no
explicit matches for CR or LF characters, the match position is
advanced by two characters instead of one, in other words, to after
the CRLF.
The above rule is a compromise that makes the most common cases work
as expected. For example, if the pattern is .+A (and the PCRE_DOTALL
option is not set), it does not match the string "\r\nA" because,
after failing at the start, it skips both the CR and the LF before
retrying. However, the pattern [\r\n]A does match that string, because
it 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 metachar-
acter. 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 the
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".
Perl has no direct equivalent of PCRE_NOTEMPTY, 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 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 demon-
strates how to do this in the pcredemo.c sample program.
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 subse-
quently called. The value of startoffset is also checked to ensure
that it points to the start of a UTF-8 character. There is a discus-
sion about the validity of UTF-8 strings in the section on UTF-8 sup-
port in the main pcre page. If an invalid UTF-8 sequence of bytes is
found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoff-
set contains an invalid value, 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 UTF-8 character. When PCRE_NO_UTF8_CHECK is
set, the effect of passing an invalid UTF-8 string as a subject, or a
value of startoffset that does not point to the start of a UTF-8 char-
acter, is undefined. Your program may crash.
PCRE_PARTIAL
This option turns on the partial matching feature. If the subject
string fails to match the pattern, but at some point during the match-
ing process the end of the subject was reached (that is, the subject
partially matches the pattern and the failure to match occurred only
because there were not enough subject characters), pcre_exec() returns
PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
used, there are restrictions on what may appear in the pattern. These
are discussed 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 byte offset in startoffset. In UTF-8
mode, the byte offset must point to the start of a UTF-8 character.
Unlike the pattern string, the subject may contain binary zero bytes.
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.
A non-zero starting offset is useful when searching for another match
in the same subject by calling pcre_exec() again after a previous suc-
cess. Setting startoffset differs from just passing over a shortened
string and setting PCRE_NOTBOL in the case of a pattern that begins
with any kind of lookbehind. For example, consider the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B matches
only if the current position in the subject is not a word boundary.)
When applied to the string "Mississipi" the first call to pcre_exec()
finds the first occurrence. If pcre_exec() is called again with just
the remainder of the subject, namely "issipi", it does not match,
because \B is always false at the start of the subject, which is
deemed to be a word boundary. However, if pcre_exec() is passed the
entire string again, but with startoffset set to 4, it finds the sec-
ond occurrence of "iss" because it is able to look behind the starting
point to discover that it is preceded by a letter.
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 suc-
ceed if the pattern does not require the match to be at the start of
the subject.
How pcre_exec() returns captured substrings
In general, a pattern matches a certain portion of the subject, and in
addition, further substrings from the subject may be picked out by
parts of the pattern. Following the usage in Jeffrey Friedl’s book,
this is called "capturing" in what follows, and the phrase "capturing
subpattern" is used for a fragment of a pattern that picks out a sub-
string. PCRE supports several other kinds of parenthesized subpattern
that do not cause substrings to be captured.
Captured substrings are returned to the caller via a vector of integer
offsets 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 sub-
strings, each substring using a pair of integers. The remaining third
of the vector is used as workspace by pcre_exec() while matching cap-
turing subpatterns, and is not available for passing back information.
The length 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 a 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. The first pair, ovector[0] and ovec-
tor[1], identify the portion of the subject string matched by the
entire pattern. The next pair is used for the first capturing subpat-
tern, 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 sub-
strings 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 por-
tion 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. In particular, if the substring
offsets are not 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.
The pcre_info() 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 exam-
ple, 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. However, you can refer to the off-
sets for the second and third capturing subpatterns if you wish
(assuming the vector is large enough, of course).
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 sub-
strings, 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.
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.
PCRE_ERROR_BADUTF8_OFFSET (-11)
The UTF-8 byte sequence that was passed as a subject was valid, but
the value of startoffset did not point to the beginning of a UTF-8
character.
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)
The PCRE_PARTIAL option was used with a compiled pattern containing
items that are not supported for partial matching. See the pcrepartial
documentation for details of 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_recur-
sion 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.
Error numbers -16 to -20 and -22 are not used by pcre_exec().
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
Captured substrings can be accessed directly by using the offsets
returned by pcre_exec() in ovector. For convenience, the functions
pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
string_list() are provided for extracting captured substrings as new,
separate, zero-terminated strings. These functions identify substrings
by number. The next section describes functions for extracting named
substrings.
A substring that contains a binary zero is correctly extracted and has
a further zero added on the end, but the result is not, of course, a C
string. However, you can process such a string by referring to the
length that is returned by pcre_copy_substring() and pcre_get_sub-
string(). Unfortunately, the interface to pcre_get_substring_list()
is not adequate for handling strings containing binary zeros, because
the end of the final string is not independently indicated.
The first three arguments are the same for all three of these func-
tions: subject is the subject string that has just been successfully
matched, ovector is a pointer to the vector of integer offsets that
was passed to pcre_exec(), and stringcount is the number of substrings
that were captured by the match, including the substring that matched
the entire regular expression. This is the value returned by
pcre_exec() if it is greater than zero. If pcre_exec() returned zero,
indicating that it ran out of space in ovector, the value passed as
stringcount should be the number of elements in the vector divided by
three.
The functions pcre_copy_substring() and pcre_get_substring() extract a
single substring, whose number is given as stringnumber. A value of
zero extracts the substring that matched the entire pattern, whereas
higher values extract the captured substrings. For pcre_copy_sub-
string(), the string is placed in buffer, whose length is given by
buffersize, while for pcre_get_substring() a new block of memory is
obtained via pcre_malloc, and its address is returned via stringptr.
The yield of the function is the length of the string, not including
the terminating zero, or one of these error codes:
PCRE_ERROR_NOMEMORY (-6)
The buffer was too small for pcre_copy_substring(), or the attempt to
get memory failed for pcre_get_substring().
PCRE_ERROR_NOSUBSTRING (-7)
There is no substring whose number is stringnumber.
The pcre_get_substring_list() function extracts all available sub-
strings and builds a list of pointers to them. All this is done in a
single block of memory that is obtained via pcre_malloc. The address
of the memory block is returned via listptr, which is also the start
of the list of string pointers. The end of the list is marked by a
NULL pointer. The yield of the function is zero if all went well, or
the error code
PCRE_ERROR_NOMEMORY (-6)
if the attempt to get the memory block failed.
When any of these functions encounter a substring that is unset, which
can happen when capturing subpattern number n+1 matches some part of
the subject, but subpattern n has not been used at all, they return an
empty string. This can be distinguished from a genuine zero-length
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 pro-
gram. 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 pro-
vided.
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
To extract a substring by name, you first have to find associated num-
ber. For example, for this pattern
(a+)b(?<xxx>\d+)...
the number of the subpattern called "xxx" is 2. If the name is known
to be unique (PCRE_DUPNAMES was not set), you can find the number from
the name by calling pcre_get_stringnumber(). The first argument is the
compiled pattern, and the second is the name. The yield of the func-
tion is the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there
is no subpattern of that name.
Given the number, you can extract the substring directly, or use one
of the functions described in the previous section. For convenience,
there are also two functions that do the whole job.
Most of the arguments of pcre_copy_named_substring() and
pcre_get_named_substring() are the same as those for the similarly
named functions that extract by number. As these are described in the
previous section, they are not re-described here. There are just two
differences:
First, instead of a substring number, a substring name is given. Sec-
ond, there is an extra argument, given at the start, which is a
pointer to the compiled pattern. This is needed in order to gain
access to the name-to-number translation table.
These functions call pcre_get_stringnumber(), and if it succeeds, they
then call pcre_copy_substring() or pcre_get_substring(), as appropri-
ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
behaviour may not be what you want (see the next section).
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. Normally, patterns with
duplicate names are such that in any one match, only one of the named
subpatterns participates. An example is shown in the pcrepattern docu-
mentation.
When duplicates are present, pcre_copy_named_substring() and
pcre_get_named_substring() return the first substring corresponding to
the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
(-7) is returned; no data is returned. The pcre_get_stringnumber()
function returns one of the numbers that are associated with the name,
but it is not defined which it is.
If you want to get full details of all captured substrings for a given
name, you must use the pcre_get_stringtable_entries() 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. Given all the rele-
vant entries for the name, you can extract each of their numbers, and
hence the captured data, if any.
FINDING ALL POSSIBLE MATCHES
The traditional matching function uses a similar algorithm to Perl,
which stops when it finds the first match, starting at a given point
in the subject. If you want to find all possible matches, or the
longest possible match, consider using the alternative matching func-
tion (see below) instead. If you cannot use the alternative function,
but still need to find all possible matches, you can kludge it up by
making use of the callout facility, which is described in the pcre-
callout documentation.
What you have to do is to insert a callout right at the end of the
pattern. When your callout function is called, extract and save the
current matched substring. Then return 1, which forces pcre_exec() to
backtrack and try other alternatives. Ultimately, when it runs out of
matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
The function pcre_dfa_exec() is called to match a subject string
against a compiled pattern, using a matching algorithm that scans the
subject string just once, and does not backtrack. This has different
characteristics to the normal algorithm, and is not compatible with
Perl. Some of the features of PCRE patterns are not supported. Never-
theless, there are times when this kind of matching can be useful. For
a discussion of the two matching algorithms, see the pcrematching doc-
umentation.
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 dif-
ferent way, and this is described below. The other common arguments
are used in the same way as for pcre_exec(), so their description is
not repeated here.
The two additional arguments provide workspace for the function. The
workspace vector should contain at least 20 elements. It is used for
keeping track of multiple paths through the pattern tree. More
workspace will be needed for patterns and subjects where there are a
lot of potential matches.
Here is an example of a simple call to pcre_dfa_exec():
int rc;
int ovector[10];
int wspace[20];
rc = pcre_dfa_exec(
re, /* result of pcre_compile() */
NULL, /* we didn’t study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information */
10, /* number of elements (NOT size in bytes) */
wspace, /* working space vector */
20); /* number of elements (NOT size in bytes) */
Option bits for pcre_dfa_exec()
The unused bits of the options argument for pcre_dfa_exec() must be
zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the
last three of these are the same as for pcre_exec(), so their descrip-
tion is not repeated here.
PCRE_PARTIAL
This has the same general effect as it does for pcre_exec(), but the
details are slightly different. When PCRE_PARTIAL is set for
pcre_dfa_exec(), 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 provided the partial match
is set as the first matching string.
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 alter-
native 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() is called with the PCRE_PARTIAL option, and
returns a partial match, it is possible to call it again, with addi-
tional subject characters, and have it continue with the same match.
The PCRE_DFA_RESTART option requests this action; when it is set, the
workspace and wscount options must reference the same vector as before
because data about the match so far is left in them after a partial
match. There is more discussion of this facility in the pcrepartial
documentation.
Successful returns from pcre_dfa_exec()
When pcre_dfa_exec() succeeds, it may have matched more than one sub-
string in the subject. Note, however, that all the matches from one
run of the function start at the same point in the subject. The
shorter matches are all initial substrings of the longer matches. For
example, if the pattern
<.*>
is matched against the string
This is <something> <something else> <something further> no more
the three matched strings are
<something>
<something> <something else>
<something> <something else> <something further>
On success, the yield of the function is a number greater than zero,
which is the number of matched substrings. The substrings themselves
are returned in ovector. Each string uses two elements; the first is
the offset to the start, and the second is the offset to the end. In
fact, all the strings have the same start offset. (Space could have
been saved by giving this only once, but it was decided to retain some
compatibility with the way pcre_exec() returns data, even though the
meaning of the strings is different.)
The strings are returned in reverse order of length; that is, the
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.
Error returns from pcre_dfa_exec()
The pcre_dfa_exec() function returns a negative number when it fails.
Many of the errors are the same as for pcre_exec(), and these are
described above. There are in addition the following errors that are
specific to pcre_dfa_exec():
PCRE_ERROR_DFA_UITEM (-16)
This return is given if pcre_dfa_exec() encounters an item in the pat-
tern that it does not support, for instance, the use of \C or a back
reference.
PCRE_ERROR_DFA_UCOND (-17)
This return is given if pcre_dfa_exec() encounters a condition item
that uses a back reference for the condition, or a test for recursion
in a specific group. These are not supported.
PCRE_ERROR_DFA_UMLIMIT (-18)
This return is given if pcre_dfa_exec() is called with an extra block
that contains a setting of the match_limit field. This is not sup-
ported (it is meaningless).
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.
SEE ALSO
pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 11 September 2007
Copyright (c) 1997-2007 University of Cambridge.
PCREAPI(3)
