Introduction to Library Functions PCREAPI(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE NATIVE API
#include
pcre *pcrecompile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcrecompile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcreextra *pcrestudy(const pcre *code, int options,
const char **errptr);
int pcreexec(const pcre *code, const pcreextra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
int pcredfaexec(const pcre *code, const pcreextra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
int pcrecopynamedsubstring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcrecopysubstring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcregetnamedsubstring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
int pcregetstringnumber(const pcre *code,
const char *name);
int pcregetstringtableentries(const pcre *code,
const char *name, char **first, char
int pcregetsubstring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
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int pcregetsubstringlist(const char *subject,
int *ovector, int stringcount, const char ***listptr);
void pcrefreesubstring(const char *stringptr);
void pcrefreesubstringlist(const char **stringptr);
const unsigned char *pcremaketables(void);
int pcrefullinfo(const pcre *code, const pcreextra *extra,
int what, void *where);
int pcreinfo(const pcre *code, int *optptr, *firstcharptr);
int pcrerefcount(pcre *code, int adjust);
int pcreconfig(int what, void *where);
char *pcreversion(void);
void *(*pcremalloc)(sizet);
void (*pcrefree)(void *);
void *(*pcrestackmalloc)(sizet);
void (*pcrestackfree)(void *);
int (*pcrecallout)(pcrecalloutblock *);
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 documentation. Both of these APIs
define a set of C function calls. A C] wrapper is distri-
buted 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 PCREMAJOR and
PCREMINOR to contain the major and minor release numbers
for the library. Applications can use these to include sup-
port for different releases of PCRE.
The functions pcrecompile(), pcrecompile2(), pcrestudy(),
and pcreexec() 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
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in the file called pcredemo.c in the source distribution.
The pcresample documentation describes how to compile and
run it.
A second matching function, pcredfaexec(), 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. 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 sub-
strings from a subject string that is matched by
pcreexec(). They are:
pcrecopysubstring()
pcrecopynamedsubstring()
pcregetsubstring()
pcregetnamedsubstring()
pcregetsubstringlist()
pcregetstringnumber()
pcregetstringtableentries()
pcrefreesubstring() and pcrefreesubstringlist() are
also provided, to free the memory used for extracted
strings.
The function pcremaketables() is used to build a set of
character tables in the current locale for passing to
pcrecompile(), pcreexec(), or pcredfaexec(). This is an
optional facility that is provided for specialist use. Most
commonly, no special tables are passed, in which case inter-
nal tables that are generated when PCRE is built are used.
The function pcrefullinfo() is used to find out information
about a compiled pattern; pcreinfo() is an obsolete version
that returns only some of the available information, but is
retained for backwards compatibility. The function
pcreversion() returns a pointer to a string containing the
version of PCRE and its date of release.
The function pcrerefcount() 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 pcremalloc and pcrefree 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
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replace them if it wishes to intercept the calls. This
should be done before calling any PCRE functions.
The global variables pcrestackmalloc and pcrestackfree
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 pcreexec() function. See the pcre-
build 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 are always
called in a stack-like manner (last obtained, first freed),
and always for memory blocks of the same size. There is a
discussion about PCRE's stack usage in the pcrestack docu-
mentation.
The global variable pcrecallout initially contains NUL. It
can be set by the caller to a "callout" function, which PCRE
will then call at specified points during a matching opera-
tion. 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 (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), F (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 standard. When PCRE is run, the
default can be overridden, either when a pattern is com-
piled, or when it is matched.
At compile time, the newline convention can be specified by
the options argument of pcrecompile(), or it can be speci-
fied 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
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handling of the dot, circumflex, and dollar metacharacters,
the handling of #-comments in /x mode, and, when CRLF is a
recognized line ending sequence, the match position advance-
ment for a non-anchored pattern. There is more detail about
this in the section on pcreexec() 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 applica-
tions, with the proviso that the memory management functions
pointed to by pcremalloc, pcrefree, pcrestackmalloc, and
pcrestackfree, and the callout function pointed to by
pcrecallout, 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.
SAVING PRECOMPILED PATERNS FOR LATER USE
The compiled form of a regular expression can be saved and
re-used at a later time, possibly by a different program,
and even on a host other than the one on which it was com-
piled. Details are given in the pcreprecompile documenta-
tion. However, compiling a regular expression with one ver-
sion of PCRE for use with a different version is not
guaranteed to work and may cause crashes.
CHECKING BUILD-TIME OPTIONS
int pcreconfig(int what, void *where);
The function pcreconfig() makes it possible for a PCRE
client to discover which optional features have been com-
piled into the PCRE library. The pcrebuild documentation has
more details about these optional features.
The first argument for pcreconfig() is an integer, specify-
ing which information is required; the second argument is a
pointer to a variable into which the information is placed.
The following information is available:
PCRECONFIGUTF8
The output is an integer that is set to one if UTF-8 support
is available; otherwise it is set to zero.
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PCRECONFIGUNICODEPROPERTIES
The output is an integer that is set to one if support for
Unicode character properties is available; otherwise it is
set to zero.
PCRECONFIGNEWLINE
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 ANYCRLF, and -1 for ANY. The
default should normally be the standard sequence for your
operating system.
PCRECONFIGBSR
The output is an integer whose value indicates what charac-
ter sequences the \R escape sequence matches by default. A
value of 0 means that \R matches any Unicode line ending
sequence; a value of 1 means that \R matches only CR, LF, or
CRLF. The default can be overridden when a pattern is com-
piled or matched.
PCRECONFIGLINKSIZE
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 match-
ing. 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.
PCRECONFIGPOSIXMALOCTHRESHOLD
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.
PCRECONFIGMATCHLIMIT
The output is an integer that gives the default limit for
the number of internal matching function calls in a
pcreexec() execution. Further details are given with
pcreexec() below.
PCRECONFIGMATCHLIMITRECURSION
The output is an integer that gives the default limit for
the depth of recursion when calling the internal matching
function in a pcreexec() execution. Further details are
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given with pcreexec() below.
PCRECONFIGSTACKRECURSE
The output is an integer that is set to one if internal
recursion when running pcreexec() is implemented by recur-
sive function calls that use the stack to remember their
state. This is the usual way that PCRE is compiled. The out-
put is zero if PCRE was compiled to use blocks of data on
the heap instead of recursive function calls. In this case,
pcrestackmalloc and pcrestackfree are called to manage
memory blocks on the heap, thus avoiding the use of the
stack.
COMPILING A PATERN
pcre *pcrecompile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcrecompile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
Either of the functions pcrecompile() or pcrecompile2()
can be called to compile a pattern into an internal form.
The only difference between the two interfaces is that
pcrecompile2() 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 pcremalloc 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
pcrefree) 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,
can also be set and unset from within the pattern (see the
detailed description in the pcrepattern documentation). For
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these options, the contents of the options argument speci-
fies their initial settings at the start of compilation and
execution. The PCREANCHORED and PCRENEWLINExxx options
can be set at the time of matching as well as at compile
time.
If errptr is NUL, pcrecompile() returns NUL immediately.
Otherwise, if compilation of a pattern fails, pcrecompile()
returns NUL, 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 NUL. If it is,
an immediate error is given.
If pcrecompile2() is used instead of pcrecompile(), and
the errorcodeptr argument is not NUL, 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 NUL, 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
pcremaketables(). This value is stored with the compiled
pattern, and used again by pcreexec(), unless another table
pointer is passed to it. For more discussion, see the sec-
tion on locale support below.
This code fragment shows a typical straightforward call to
pcrecompile():
pcre *re;
const char *error;
int erroffset;
re = pcrecompile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NUL); /* use default character tables */
The following names for option bits are defined in the
pcre.h header file:
PCREANCHORED
If this bit is set, the pattern is forced to be "anchored",
that is, it is constrained to match only at the first match-
ing point in the string that is being searched (the "subject
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string"). This effect can also be achieved by appropriate
constructs in the pattern itself, which is the only way to
do it in Perl.
PCREAUTOCALOUT
If this bit is set, pcrecompile() automatically inserts
callout items, all with number 255, before each pattern
item. For discussion of the callout facility, see the pcre-
callout documentation.
PCREBSRANYCRLF
PCREBSRUNICODE
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.
PCRECASELES
If this bit is set, letters in the pattern match both upper
and lower case letters. It is equivalent to Perl's /i
option, and it can be changed within a pattern by a (?i)
option setting. In UTF-8 mode, PCRE always understands the
concept of case for characters whose values are less than
128, so caseless matching is always possible. For characters
with higher values, the concept of case is supported if PCRE
is compiled with Unicode property support, but not other-
wise. If you want to use caseless matching for characters
128 and above, you must ensure that PCRE is compiled with
Unicode property support as well as with UTF-8 support.
PCREDOLARENDONLY
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 new-
lines). The PCREDOLARENDONLY option is ignored if
PCREMULTILINE is set. There is no equivalent to this
option in Perl, and no way to set it within a pattern.
PCREDOTAL
If this bit is set, a dot metacharater in the pattern
matches all characters, including those that indicate new-
line. 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 pattern by
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a (?s) option setting. A negative class such as [^a] always
matches newline characters, independent of the setting of
this option.
PCREDUPNAMES
If this bit is set, names used to identify capturing subpat-
terns need not be unique. This can be helpful for certain
types of pattern when it is known that only one instance of
the named subpattern can ever be matched. There are more
details of named subpatterns below; see also the pcrepattern
documentation.
PCREXTENDED
If this bit is set, whitespace data characters in the pat-
tern are totally ignored except when escaped or inside a
character class. Whitespace does not include the VT charac-
ter (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 set-
ting.
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 sub-
pattern.
PCREXTRA
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.
PCREFIRSTLINE
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.
PCREJAVASCRIPTCOMPAT
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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.
PCREMULTILINE
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 PCREDOLARENDONLY is
set). This is the same as Perl.
When PCREMULTILINE it is set, the "start of line" and "end
of line" constructs match immediately following or immedi-
ately 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 PCREMULTILINE has no effect.
PCRENEWLINECR
PCRENEWLINELF
PCRENEWLINECRLF
PCRENEWLINEANYCRLF
PCRENEWLINEANY
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
PCRENEWLINECRLF specifies that a newline is indicated by
the two-character CRLF sequence. Setting
PCRENEWLINEANYCRLF specifies that any of the three preced-
ing sequences should be recognized. Setting PCRENEWLINEANY
specifies that any Unicode newline sequence should be recog-
nized. The Unicode newline sequences are the three just men-
tioned, plus the single characters VT (vertical tab,
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U]000B), F (formfeed, U]000C), NEL (next line, U]0085), LS
(line separator, U]2028), and PS (paragraph separator,
U]2029). The last two are recognized 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, PCRENEWLINECR with PCRENEWLINELF is equivalent
to PCRENEWLINECRLF, 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 PCREXTENDED is set, and an unes-
caped # outside a character class is encountered. This indi-
cates 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 PCREXTENDED mode,
both CR and LF are treated as whitespace characters and are
therefore ignored.
The newline option that is set at compile time becomes the
default that is used for pcreexec() and pcredfaexec(),
but it can be overridden.
PCRENOAUTOCAPTURE
If this option is set, it disables the use of numbered cap-
turing parentheses in the pattern. Any opening parenthesis
that is not followed by ? behaves as if it were followed by
?: but named parentheses can still be used for capturing
(and they acquire numbers in the usual way). There is no
equivalent of this option in Perl.
PCREUNGREDY
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.
PCREUTF8
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 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 section on
UTF-8 support in the main pcre page.
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PCRENOUTF8CHECK
When PCREUTF8 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,
pcrecompile() 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 PCRENOUTF8CHECK
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 pcreexec() and pcredfaexec(), to suppress the UTF-8
validity checking of subject strings.
COMPILATION EROR CODES
The following table lists the error codes than may be
returned by pcrecompile2(), 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 (? or (?-
13 POSIX named classes are supported only within a class
14 missing )
15 reference to non-existent subpattern
16 erroffset passed as NUL
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 (?(
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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 PCREUTF8
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 10000)
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 subpat-
tern not found
54 DEFINE group contains more than one branch
55 repeating a DEFINE group is not allowed
56 inconsistent NEWLINE options
57 \g is not followed by a braced, angle-bracketed, or
quoted
name/number or by a plain number
58 a numbered reference must not be zero
59 (*VERB) with an argument is not supported
60 (*VERB) not recognized
61 number is too big
62 subpattern name expected
63 digit expected after (?]
64 ] is an invalid data character in JavaScript compati-
bility mode
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 PATERN
pcreextra *pcrestudy(const pcre *code, int options
const char **errptr);
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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 pcrestudy()
takes a pointer to a compiled pattern as its first argument.
If studying the pattern produces additional information that
will help speed up matching, pcrestudy() returns a pointer
to a pcreextra block, in which the studydata field points
to the results of the study.
The returned value from pcrestudy() can be passed directly
to pcreexec(). However, a pcreextra 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 pcrestudy() returns NUL. In that circumstance,
if the calling program wants to pass any of the other fields
to pcreexec(), it must set up its own pcreextra block.
The second argument of pcrestudy() contains option bits. At
present, no options are defined, and this argument should
always be zero.
The third argument for pcrestudy() is a pointer for an
error message. If studying succeeds (even if no data is
returned), the variable it points to is set to NUL. Other-
wise 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 NUL
after calling pcrestudy(), to be sure that it has run suc-
cessfully.
This is a typical call to pcrestudy():
pcreextra *pe;
pe = pcrestudy(
re, /* result of pcrecompile() */
0, /* no options exist */
&error); /* set to NUL or points to a message */
At present, studying a pattern is useful only for non-
anchored patterns that do not have a single fixed starting
character. A bitmap of possible starting bytes is created.
LOCALE SUPORT
PCRE handles caseless matching, and determines whether char-
acters 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
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or \d, but can be tested with \p if PCRE is built with
Unicode character property support. The use of locales with
Unicode 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 pcrecompile() is NUL. These are suf-
ficient for many applications. Normally, the internal
tables recognize only ASCI 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 sup-
plied 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 pcremaketables()
function, which has no arguments, in the relevant locale.
The result can then be passed to pcrecompile() or
pcreexec() 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(LCTYPE, "frFR");
tables = pcremaketables();
re = pcrecompile(..., tables);
The locale name "frFR" is used on Linux and other Unix-like
systems; if you are using Windows, the name for the French
locale is "french".
When pcremaketables() runs, the tables are built in memory
that is obtained via pcremalloc. It is the caller's respon-
sibility to ensure that the memory containing the tables
remains available for as long as it is needed.
The pointer that is passed to pcrecompile() is saved with
the compiled pattern, and the same tables are used via this
pointer by pcrestudy() and normally also by pcreexec().
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 NUL (indicating
the use of the internal tables) to pcreexec(). Although not
intended for this purpose, this facility could be used to
match a pattern in a different locale from the one in which
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it was compiled. Passing table pointers at run time is dis-
cussed below in the section on matching a pattern.
INFORMATION ABOUT A PATERN
int pcrefullinfo(const pcre *code, const pcreextra *extra,
int what, void *where);
The pcrefullinfo() function returns information about a
compiled pattern. It replaces the obsolete pcreinfo() func-
tion, which is nevertheless retained for backwards compabil-
ity (and is documented below).
The first argument for pcrefullinfo() is a pointer to the
compiled pattern. The second argument is the result of
pcrestudy(), or NUL 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:
PCRERORNUL the argument code was NUL
the argument where was NUL
PCRERORBADMAGIC the "magic number" was not found
PCRERORBADOPTION 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 pcrefullinfo(),
to obtain the length of the compiled pattern:
int rc;
sizet length;
rc = pcrefullinfo(
re, /* result of pcrecompile() */
pe, /* result of pcrestudy(), or NUL */
PCREINFOSIZE, /* 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:
PCREINFOBACKREFMAX
Return the number of the highest back reference in the pat-
tern. The fourth argument should point to an int variable.
Zero is returned if there are no back references.
PCREINFOCAPTURECOUNT
Return the number of capturing subpatterns in the pattern.
The fourth argument should point to an int variable.
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PCREINFODEFAULTABLES
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 pcrestudy() function. External callers
can cause PCRE to use its internal tables by passing a NUL
table pointer.
PCREINFOFIRSTBYTE
Return information about the first byte of any matched
string, for a non-anchored pattern. The fourth argument
should point to an int variable. (This option used to be
called PCREINFOFIRSTCHAR; the old name is still recognized
for backwards compatibility.)
If there is a fixed first byte, for example, from a pattern
such as (catcowcoyote), its value is returned. Otherwise,
if either
(a) the pattern was compiled with the PCREMULTILINE option,
and every branch starts with "^", or
(b) every branch of the pattern starts with ".*" and
PCREDOTAL 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.
PCREINFOFIRSTABLE
If the pattern was studied, and this resulted in the con-
struction 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 NUL is returned. The fourth
argument should point to an unsigned char * variable.
PCREINFOHASCRORLF
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.
PCREINFOJCHANGED
Return 1 if the (?J) or (?-J) option setting is used in the
pattern, otherwise 0. The fourth argument should point to an
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int variable. (?J) and (?-J) set and unset the local
PCREDUPNAMES option, respectively.
PCREINFOLASTLITERAL
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.
PCREINFONAMECOUNT
PCREINFONAMENTRYSIZE
PCREINFONAMETABLE
PCRE supports the use of named as well as numbered capturing
parentheses. The names are just an additional way of identi-
fying the parentheses, which still acquire numbers. Several
convenience functions such as pcregetnamedsubstring() are
provided for extracting captured substrings by name. It is
also possible to extract the data directly, by first con-
verting the name to a number in order to access the correct
pointers in the output vector (described with pcreexec()
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.
PCREINFONAMECOUNT gives the number of entries, and
PCREINFONAMENTRYSIZE gives the size of each entry; both
of these return an int value. The entry size depends on the
length of the longest name. PCREINFONAMETABLE 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 cap-
turing parenthesis, most significant byte first. The rest of
the entry is the corresponding name, zero terminated. The
names are in alphabetical order. When PCREDUPNAMES is set,
duplicate names are in order of their parentheses numbers.
For example, consider the following pattern (assume
PCREXTENDED is set, so white space - including newlines -
is ignored):
(? (?(\d\d)?\d\d) -
(?\d\d) - (?\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 ??:
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00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??
When writing code to extract data from named subpatterns
using the name-to-number map, remember that the length of
the entries is likely to be different for each compiled pat-
tern.
PCREINFOKPARTIAL
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 restric-
tions that apply to patterns when partial matching is used.
PCREINFOPTIONS
Return a copy of the options with which the pattern was com-
piled. The fourth argument should point to an unsigned long
int variable. These option bits are those specified in the
call to pcrecompile(), modified by any top-level option
settings at the start of the pattern itself. In other words,
they are the options that will be in force when matching
starts. For example, if the pattern /(?im)abc(?-i)d/ is com-
piled with the PCREXTENDED option, the result is
PCRECASELES, PCREMULTILINE, and PCREXTENDED.
A pattern is automatically anchored by PCRE if all of its
top-level alternatives begin with one of the following:
^ unless PCREMULTILINE is set
\A always
\G always
.* if PCREDOTAL is set and there are no back
references to the subpattern in which .* appears
For such patterns, the PCREANCHORED bit is set in the
options returned by pcrefullinfo().
PCREINFOSIZE
Return the size of the compiled pattern, that is, the value
that was passed as the argument to pcremalloc() when PCRE
was getting memory in which to place the compiled data. The
fourth argument should point to a sizet variable.
PCREINFOSTUDYSIZE
Return the size of the data block pointed to by the
studydata field in a pcreextra block. That is, it is the
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value that was passed to pcremalloc() when PCRE was getting
memory into which to place the data created by pcrestudy().
The fourth argument should point to a sizet variable.
OBSOLETE INFO FUNCTION
int pcreinfo(const pcre *code, int *optptr, *firstcharptr);
The pcreinfo() function is now obsolete because its inter-
face is too restrictive to return all the available data
about a compiled pattern. New programs should use
pcrefullinfo() instead. The yield of pcreinfo() is the
number of capturing subpatterns, or one of the following
negative numbers:
PCRERORNUL the argument code was NUL
PCRERORBADMAGIC the "magic number" was not found
If the optptr argument is not NUL, a copy of the options
with which the pattern was compiled is placed in the integer
it points to (see PCREINFOPTIONS above).
If the pattern is not anchored and the firstcharptr argument
is not NUL, it is used to pass back information about the
first character of any matched string (see
PCREINFOFIRSTBYTE above).
REFERENCE COUNTS
int pcrerefcount(pcre *code, int adjust);
The pcrerefcount() 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 con-
strained to lie between 0 and 65535, inclusive. If the new
value is outside these limits, it is forced to the appropri-
ate 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.)
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MATCHING A PATERN: THE TRADITIONAL FUNCTION
int pcreexec(const pcre *code, const pcreextra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
The function pcreexec() 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 pcredfaexec() function.
In most applications, the pattern will have been compiled
(and optionally studied) in the same process that calls
pcreexec(). However, it is possible to save compiled pat-
terns and study data, and then use them later in different
processes, possibly even on different hosts. For a discus-
sion about this, see the pcreprecompile documentation.
Here is an example of a simple call to pcreexec():
int rc;
int ovector[30];
rc = pcreexec(
re, /* result of pcrecompile() */
NUL, /* 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 pcreexec()
If the extra argument is not NUL, it must point to a
pcreextra data block. The pcrestudy() function returns
such a block (when it doesn't return NUL), but you can also
create one for yourself, and pass additional information in
it. The pcreextra block contains the following fields (not
necessarily in this order):
unsigned long int flags;
void *studydata;
unsigned long int matchlimit;
unsigned long int matchlimitrecursion;
void *calloutdata;
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const unsigned char *tables;
The flags field is a bitmap that specifies which of the
other fields are set. The flag bits are:
PCREXTRASTUDYDATA
PCREXTRAMATCHLIMIT
PCREXTRAMATCHLIMITRECURSION
PCREXTRACALOUTDATA
PCREXTRATABLES
Other flag bits should be set to zero. The studydata field
is set in the pcreextra block that is returned by
pcrestudy(), 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 corresponding flag
bits.
The matchlimit field provides a means of preventing PCRE
from using up a vast amount of resources when running pat-
terns 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 repeatedly (sometimes recursively). The limit set by
matchlimit 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 pcreexec() with a pcreextra block in which
matchlimit is set, and PCREXTRAMATCHLIMIT is set in the
flags field. If the limit is exceeded, pcreexec() returns
PCRERORMATCHLIMIT.
The matchlimitrecursion field is similar to matchlimit,
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 matchlimit.
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.
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The default value for matchlimitrecursion can be set when
PCRE is built; the default default is the same value as the
default for matchlimit. You can override the default by
suppling pcreexec() with a pcreextra block in which
matchlimitrecursion is set, and
PCREXTRAMATCHLIMITRECURSION is set in the flags field.
If the limit is exceeded, pcreexec() returns
PCRERORECURSIONLIMIT.
The pcrecallout field is used in conjunction with the "cal-
lout" feature, which is described in the pcrecallout docu-
mentation.
The tables field is used to pass a character tables pointer
to pcreexec(); this overrides the value that is stored with
the compiled pattern. A non-NUL value is stored with the
compiled pattern only if custom tables were supplied to
pcrecompile() via its tableptr argument. If NUL is passed
to pcreexec() using this mechanism, it forces PCRE's inter-
nal 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 different address when pcreexec() is called. See the
pcreprecompile documentation for a discussion of saving com-
piled patterns for later use.
Option bits for pcreexec()
The unused bits of the options argument for pcreexec() must
be zero. The only bits that may be set are PCREANCHORED,
PCRENEWLINExxx, PCRENOTBOL, PCRENOTEOL, PCRENOTEMPTY,
PCRENOUTF8CHECK and PCREPARTIAL.
PCREANCHORED
The PCREANCHORED option limits pcreexec() to matching at
the first matching position. If a pattern was compiled with
PCREANCHORED, or turned out to be anchored by virtue of its
contents, it cannot be made unachored at matching time.
PCREBSRANYCRLF
PCREBSRUNICODE
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.
PCRENEWLINECR
PCRENEWLINELF
PCRENEWLINECRLF
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PCRENEWLINEANYCRLF
PCRENEWLINEANY
These options override the newline definition that was
chosen or defaulted when the pattern was compiled. For
details, see the description of pcrecompile() 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 PCRENEWLINECRLF, PCRENEWLINEANYCRLF, or
PCRENEWLINEANY is set, and a match attempt for an unan-
chored 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 PCREDOTAL 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 appear-
ance 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 charac-
ters 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.
PCRENOTBOL
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 PCREMULTILINE (at compile time) causes circumflex
never to match. This option affects only the behaviour of
the circumflex metacharacter. It does not affect \A.
PCRENOTEOL
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
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immediately before it. Setting this without PCREMULTILINE
(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.
PCRENOTEMPTY
An empty string is not considered to be a valid match if
this option is set. If there are alternatives in the pat-
tern, they are tried. If all the alternatives match the
empty string, the entire match fails. For example, if the
pattern
a?b?
is applied to a string not beginning with "a" or "b", it
matches the empty string at the start of the subject. With
PCRENOTEMPTY 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 PCRENOTEMPTY, 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 PCRENOTEMPTY and PCREANCHORED, 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.c sample pro-
gram.
PCRENOUTF8CHECK
When PCREUTF8 is set at compile time, the validity of the
subject as a UTF-8 string is automatically checked when
pcreexec() is subsequently called. The value of star-
toffset 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 section on UTF-8 support in
the main pcre page. If an invalid UTF-8 sequence of bytes is
found, pcreexec() returns the error PCRERORBADUTF8. If
startoffset contains an invalid value,
PCRERORBADUTF8OFSET 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 PCRENOUTF8CHECK option when calling pcreexec(). You
might want to do this for the second and subsequent calls to
pcreexec() 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 PCRENOUTF8CHECK is set, the effect
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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
character, is undefined. Your program may crash.
PCREPARTIAL
This option turns on the partial matching feature. If the
subject string fails to match the pattern, but at some point
during the matching 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), pcreexec() returns
PCRERORPARTIAL instead of PCRERORNOMATCH. When
PCREPARTIAL is used, there are restrictions on what may
appear in the pattern. These are discussed in the pcrepar-
tial documentation.
The string to be matched by pcreexec()
The subject string is passed to pcreexec() as a pointer in
subject, a length (in bytes) 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 pcreexec()
again after a previous success. Setting startoffset differs
from just passing over a shortened string and setting
PCRENOTBOL 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 pcreexec() finds the first occurrence. If
pcreexec() 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 pcreexec() 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.
If a non-zero starting offset is passed when the pattern is
anchored, one attempt to match at the given offset is made.
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This can only succeed if the pattern does not require the
match to be at the start of the subject.
How pcreexec() returns captured substrings
In general, a pattern matches a certain portion of the sub-
ject, and in addition, further substrings from the subject
may be picked out by parts of the pattern. Following the
usage in Jeffrey Friedl's book, this is called "capturing"
in what follows, and the phrase "capturing subpattern" is
used for a fragment of a pattern that picks out a substring.
PCRE supports several other kinds of parenthesized subpat-
tern that do not cause substrings to be captured.
Captured substrings are returned to the caller via a vector
of integers whose address is passed in ovector. The number
of elements in the vector is passed in ovecsize, which must
be a non-negative number. Note: this argument is NOT the
size of ovector in bytes.
The first two-thirds of the vector is used to pass back cap-
tured substrings, each substring using a pair of integers.
The remaining third of the vector is used as workspace by
pcreexec() 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 sub-
strings 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 byte offset of the first character in a substring, and
the second is set to the byte offset of the first character
after the end of a substring. Note: these values are always
byte offsets, even in UTF-8 mode. They are not character
counts.
The first pair of integers, ovector[0] and ovector[1], iden-
tify the portion of the subject string matched by the entire
pattern. The next pair is used for the first capturing sub-
pattern, and so on. The value returned by pcreexec() 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.
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If the vector is too small to hold all the captured sub-
string offsets, it is used as far as possible (up to two-
thirds of its length), and the function returns a value of
zero. If the substring offsets are not of interest,
pcreexec() may be called with ovector passed as NUL 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 pcreinfo() 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 cap-
turing subpattern number is 1. However, you can refer to the
offsets 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 pcreexec()
If pcreexec() fails, it returns a negative number. The fol-
lowing are defined in the header file:
PCRERORNOMATCH (-1)
The subject string did not match the pattern.
PCRERORNUL (-2)
Either code or subject was passed as NUL, or ovector was
NUL and ovecsize was not zero.
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PCRERORBADOPTION (-3)
An unrecognized bit was set in the options argument.
PCRERORBADMAGIC (-4)
PCRE stores a 4-byte "magic number" at the start of the com-
piled code, to catch the case when it is passed a junk
pointer and to detect when a pattern that was compiled in an
environment of one endianness is run in an environment with
the other endianness. This is the error that PCRE gives when
the magic number is not present.
PCRERORUNKNOWNOPCODE (-5)
While running the pattern match, an unknown item was encoun-
tered in the compiled pattern. This error could be caused by
a bug in PCRE or by overwriting of the compiled pattern.
PCRERORNOMEMORY (-6)
If a pattern contains back references, but the ovector that
is passed to pcreexec() 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
pcremalloc() fails, this error is given. The memory is
automatically freed at the end of matching.
PCRERORNOSUBSTRING (-7)
This error is used by the pcrecopysubstring(),
pcregetsubstring(), and pcregetsubstringlist() func-
tions (see below). It is never returned by pcreexec().
PCRERORMATCHLIMIT (-8)
The backtracking limit, as specified by the matchlimit
field in a pcreextra structure (or defaulted) was reached.
See the description above.
PCRERORCALOUT (-9)
This error is never generated by pcreexec() itself. It is
provided for use by callout functions that want to yield a
distinctive error code. See the pcrecallout documentation
for details.
PCRERORBADUTF8 (-10)
A string that contains an invalid UTF-8 byte sequence was
passed as a subject.
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PCRERORBADUTF8OFSET (-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.
PCRERORPARTIAL (-12)
The subject string did not match, but it did match par-
tially. See the pcrepartial documentation for details of
partial matching.
PCRERORBADPARTIAL (-13)
The PCREPARTIAL option was used with a compiled pattern
containing items that are not supported for partial match-
ing. See the pcrepartial documentation for details of par-
tial matching.
PCRERORINTERNAL (-14)
An unexpected internal error has occurred. This error could
be caused by a bug in PCRE or by overwriting of the compiled
pattern.
PCRERORBADCOUNT (-15)
This error is given if the value of the ovecsize argument is
negative.
PCRERORECURSIONLIMIT (-21)
The internal recursion limit, as specified by the
matchlimitrecursion field in a pcreextra structure (or
defaulted) was reached. See the description above.
PCRERORBADNEWLINE (-23)
An invalid combination of PCRENEWLINExxx options was
given.
Error numbers -16 to -20 and -22 are not used by
pcreexec().
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcrecopysubstring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcregetsubstring(const char *subject, int *ovector,
int stringcount, int stringnumber,
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const char **stringptr);
int pcregetsubstringlist(const char *subject,
int *ovector, int stringcount, const char ***listptr);
Captured substrings can be accessed directly by using the
offsets returned by pcreexec() in ovector. For convenience,
the functions pcrecopysubstring(), pcregetsubstring(),
and pcregetsubstringlist() 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 sub-
strings.
A substring that contains a binary zero is correctly
extracted and has a further zero added on the end, but the
result is not, of course, a C string. However, you can pro-
cess such a string by referring to the length that is
returned by pcrecopysubstring() and pcregetsubstring().
Unfortunately, the interface to pcregetsubstringlist() 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 pcreexec(),
and stringcount is the number of substrings that were cap-
tured by the match, including the substring that matched the
entire regular expression. This is the value returned by
pcreexec() if it is greater than zero. If pcreexec()
returned zero, indicating that it ran out of space in ovec-
tor, the value passed as stringcount should be the number of
elements in the vector divided by three.
The functions pcrecopysubstring() and pcregetsubstring()
extract a single substring, whose number is given as string-
number. A value of zero extracts the substring that matched
the entire pattern, whereas higher values extract the cap-
tured substrings. For pcrecopysubstring(), the string is
placed in buffer, whose length is given by buffersize, while
for pcregetsubstring() a new block of memory is obtained
via pcremalloc, 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:
PCRERORNOMEMORY (-6)
The buffer was too small for pcrecopysubstring(), or the
attempt to get memory failed for pcregetsubstring().
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PCRERORNOSUBSTRING (-7)
There is no substring whose number is stringnumber.
The pcregetsubstringlist() function extracts all avail-
able substrings and builds a list of pointers to them. All
this is done in a single block of memory that is obtained
via pcremalloc. 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 NUL pointer.
The yield of the function is zero if all went well, or the
error code
PCRERORNOMEMORY (-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 nega-
tive for unset substrings.
The two convenience functions pcrefreesubstring() and
pcrefreesubstringlist() can be used to free the memory
returned by a previous call of pcregetsubstring() or
pcregetsubstringlist(), respectively. They do nothing
more than call the function pointed to by pcrefree, which
of course could be called directly from a C program. How-
ever, PCRE is used in some situations where it is linked via
a special interface to another programming language that
cannot use pcrefree directly; it is for these cases that
the functions are provided.
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcregetstringnumber(const pcre *code,
const char *name);
int pcrecopynamedsubstring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcregetnamedsubstring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
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To extract a substring by name, you first have to find asso-
ciated number. For example, for this pattern
(a])b(?\d])...
the number of the subpattern called "xxx" is 2. If the name
is known to be unique (PCREDUPNAMES was not set), you can
find the number from the name by calling
pcregetstringnumber(). The first argument is the compiled
pattern, and the second is the name. The yield of the func-
tion is the subpattern number, or PCRERORNOSUBSTRING (-
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 pcrecopynamedsubstring() and
pcregetnamedsubstring() 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 trans-
lation table.
These functions call pcregetstringnumber(), and if it
succeeds, they then call pcrecopysubstring() or
pcregetsubstring(), as appropriate. NOTE: If PCREDUPNAMES
is set and there are duplicate names, the behaviour may not
be what you want (see the next section).
DUPLICATE SUBPATERN NAMES
int pcregetstringtableentries(const pcre *code,
const char *name, char **first, char
When a pattern is compiled with the PCREDUPNAMES option,
names for subpatterns are not required to be unique. Nor-
mally, 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, pcrecopynamedsubstring() and
pcregetnamedsubstring() return the first substring
corresponding to the given name that is set. If none are
set, PCRERORNOSUBSTRING (-7) is returned; no data is
returned. The pcregetstringnumber() function returns one
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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
pcregetstringtableentries() 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 PCRERORNOSUBSTRING (-7) if there are none. The format
of the table is described above in the section entitled
Information about a pattern. Given all the relevant entries
for the name, you can extract each of their numbers, and
hence the captured data, if any.
FINDING AL POSIBLE MATCHES
The traditional matching function uses a similar algorithm
to Perl, which stops when it finds the first match, starting
at a given point in the subject. If you want to find all
possible matches, or the longest possible match, consider
using the alternative matching function (see below) instead.
If you cannot use the alternative function, but still need
to find all possible matches, you can kludge it up by making
use of the callout facility, which is described in the pcre-
callout documentation.
What you have to do is to insert a callout right at the end
of the pattern. When your callout function is called,
extract and save the current matched substring. Then return
1, which forces pcreexec() to backtrack and try other
alternatives. Ultimately, when it runs out of matches,
pcreexec() will yield PCRERORNOMATCH.
MATCHING A PATERN: THE ALTERNATIVE FUNCTION
int pcredfaexec(const pcre *code, const pcreextra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
The function pcredfaexec() is called to match a subject
string against a compiled pattern, using a matching algo-
rithm that scans the subject string just once, and does not
backtrack. This has different characteristics to the normal
algorithm, and is not compatible with Perl. Some of the
features of PCRE patterns are not supported. Nevertheless,
there are times when this kind of matching can be useful.
For a discussion of the two matching algorithms, see the
pcrematching documentation.
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The arguments for the pcredfaexec() function are the same
as for pcreexec(), 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
pcreexec(), so their description is not repeated here.
The two additional arguments provide workspace for the func-
tion. The workspace vector should contain at least 20 ele-
ments. 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 pcredfaexec():
int rc;
int ovector[10];
int wspace[20];
rc = pcredfaexec(
re, /* result of pcrecompile() */
NUL, /* 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 pcredfaexec()
The unused bits of the options argument for pcredfaexec()
must be zero. The only bits that may be set are
PCREANCHORED, PCRENEWLINExxx, PCRENOTBOL, PCRENOTEOL,
PCRENOTEMPTY, PCRENOUTF8CHECK, PCREPARTIAL,
PCREDFASHORTEST, and PCREDFARESTART. All but the last
three of these are the same as for pcreexec(), so their
description is not repeated here.
PCREPARTIAL
This has the same general effect as it does for pcreexec(),
but the details are slightly different. When PCREPARTIAL is
set for pcredfaexec(), the return code PCRERORNOMATCH
is converted into PCRERORPARTIAL if the end of the sub-
ject is reached, there have been no complete matches, but
there is still at least one matching possibility. The por-
tion of the string that provided the partial match is set as
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the first matching string.
PCREDFASHORTEST
Setting the PCREDFASHORTEST option causes the matching
algorithm to stop as soon as it has found one match. Because
of the way the alternative algorithm works, this is neces-
sarily the shortest possible match at the first possible
matching point in the subject string.
PCREDFARESTART
When pcredfaexec() is called with the PCREPARTIAL option,
and returns a partial match, it is possible to call it
again, with additional subject characters, and have it con-
tinue with the same match. The PCREDFARESTART 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 pcredfaexec()
When pcredfaexec() succeeds, it may have matched more than
one substring in the subject. Note, however, that all the
matches from one run of the function start at the same point
in the subject. The shorter matches are all initial sub-
strings of the longer matches. For example, if the pattern
<.*>
is matched against the string
This is
no more
the three matched strings are
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 pcreexec() returns data,
even though the meaning of the strings is different.)
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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 pcredfaexec()
The pcredfaexec() function returns a negative number when
it fails. Many of the errors are the same as for
pcreexec(), and these are described above. There are in
addition the following errors that are specific to
pcredfaexec():
PCRERORDFAUITEM (-16)
This return is given if pcredfaexec() encounters an item
in the pattern that it does not support, for instance, the
use of \C or a back reference.
PCRERORDFAUCOND (-17)
This return is given if pcredfaexec() encounters a condi-
tion item that uses a back reference for the condition, or a
test for recursion in a specific group. These are not sup-
ported.
PCRERORDFAUMLIMIT (-18)
This return is given if pcredfaexec() is called with an
extra block that contains a setting of the matchlimit
field. This is not supported (it is meaningless).
PCRERORDFAWSIZE (-19)
This return is given if pcredfaexec() runs out of space in
the workspace vector.
PCRERORDFARECURSE (-20)
When a recursive subpattern is processed, the matching func-
tion calls itself recursively, using private vectors for
ovector and workspace. This error is given if the output
vector is not large enough. This should be extremely rare,
as a vector of size 1000 is used.
SEE ALSO
pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematch-
ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3),
pcresample(3), pcrestack(3).
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AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 24 August 2008
Copyright (c) 1997-2008 University of Cambridge.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWpcre
Interface Stability Uncommitted
NOTES
Source for PCRE is available on http:/opensolaris.org.
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