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PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
SYNOPSIS OF PCRE API
#include
pcre *pcre_compile(const char *pattern, int options,
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_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_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_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
PCRE has its own native API, which is described in this
document. There is also a set of wrapper functions that
correspond to the POSIX regular expression API. These are
described in the pcreposix documentation.
The native API function prototypes are defined in the
header file pcre.h, and on Unix systems the library itself
is called libpcre.a, so can be accessed by adding -lpcre
to the command for linking an application which calls it.
The header file defines the macros PCRE_MAJOR and
PCRE_MINOR to contain the major and minor release numbers
for the library. Applications can use these to include
support for different releases.
The functions pcre_compile(), pcre_study(), and
pcre_exec() are used for compiling and matching regular
expressions. A sample program that demonstrates the sim-
plest way of using them is given in the file pcredemo.c.
The pcresample documentation describes how to run it.
There are convenience functions for extracting captured
substrings from a matched subject string. They are:
pcre_copy_substring()
pcre_copy_named_substring()
pcre_get_substring()
pcre_get_named_substring()
pcre_get_substring_list()
pcre_free_substring() and pcre_free_substring_list() are
also provided, to free the memory used for extracted
strings.
The function pcre_maketables() is used (optionally) to
build a set of character tables in the current locale for
passing to pcre_compile().
The function pcre_fullinfo() is used to find out informa-
tion about a compiled pattern; pcre_info() is an obsolete
version which returns only some of the available informa-
tion, but is retained for backwards compatibility. The
function pcre_version() returns a pointer to a string con-
taining the version of PCRE and its date of release.
The global variables pcre_malloc and pcre_free initially
contain the entry points of the standard malloc() and
free() functions respectively. PCRE calls the memory man-
agement functions via these variables, so a calling pro-
gram 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 com-
piled to use the heap for remembering data, instead of
recursive function calls. This 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.
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 documenta-
tion.
MULTITHREADING
The PCRE functions can be used in multi-threading applica-
tions, with the proviso that the memory management func-
tions 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 matching, so the same compiled pattern can safely
be used by several threads at once.
CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);
The function pcre_config() makes it possible for a PCRE
client to discover which optional features have been com-
piled into the PCRE library. The pcrebuild documentation
has more details about these optional features.
The first argument for pcre_config() is an integer, speci-
fying which information is required; the second argument
is a pointer to a variable 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 sup-
port is available; otherwise it is set to zero.
PCRE_CONFIG_NEWLINE
The output is an integer that is set to the value of the
code that is used for the newline character. It is either
linefeed (10) or carriage return (13), and should normally
be the standard character for your operating system.
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 regu-
lar expressions to be compiled, at the expense of slower
matching. 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.
PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
The output is an integer that contains the threshold above
which the POSIX interface uses malloc() for output vec-
tors. Further details are given in the pcreposix documen-
tation.
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. Further details are given with
pcre_exec() below.
PCRE_CONFIG_STACKRECURSE
The output is an integer that is set to one if internal
recursion 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);
The function pcre_compile() is called to compile a pattern
into an internal form. The pattern is a C string termi-
nated by a binary zero, and is passed in the argument pat-
tern. 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 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 com-
plete pcre data block is not fully relocatable, because it
contains a copy of the tableptr argument, which is an
address (see below).
The options argument contains independent bits that affect
the compilation. It should be zero if no options are
required. Some of the options, in particular, those that
are compatible with Perl, can also be set and unset from
within the pattern (see the detailed description of regu-
lar expressions in the pcrepattern documentation). For
these options, the contents of the options argument speci-
fies their initial settings at the start of compilation
and execution. The PCRE_ANCHORED option can be set at the
time of matching as well as at compile time.
If errptr is NULL, pcre_compile() returns NULL immedi-
ately. Otherwise, if compilation of a pattern fails,
pcre_compile() returns NULL, and sets the variable pointed
to by errptr to point to a textual error message. The off-
set 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
immediate error is given.
If the final argument, tableptr, is NULL, PCRE uses a
default set of character tables which are built when it is
compiled, using the default C locale. Otherwise, tableptr
must be the result of a call to pcre_maketables(). See the
section on locale support below.
This code fragment shows a typical straightforward call to
pcre_compile():
pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */
The following option bits are defined:
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 which 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_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.
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 the
final character if it is a newline (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 newlines. Without it,
newlines are excluded. This option is equivalent to Perl's
/s option, and it can be changed within a pattern by a
(?s) option setting. A negative class such as [^a] always
matches a newline character, independent of the setting of
this option.
PCRE_EXTENDED
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 char-
acter (code 11). In addition, characters between an
unescaped # outside a character class and the next newline
character, inclusive, are also ignored. This is equivalent
to Perl's /x option, and it can be changed within a pat-
tern 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 back-
slash 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. 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_MULTILINE
By default, PCRE treats the subject string as consisting
of a single "line" of characters (even if it actually con-
tains several 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_DOLLAR_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 any newline 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
"\n" characters in a subject string, or no occurrences of
^ or $ in a pattern, setting PCRE_MULTILINE has no effect.
PCRE_NO_AUTO_CAPTURE
If this option is set, it disables the use of numbered
capturing parentheses in the pattern. Any opening paren-
thesis that is not followed by ? behaves as if it were
followed by ?: but named parentheses can still be used for
capturing (and they acquire numbers in the usual way).
There is no equivalent of this option in Perl.
PCRE_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 if
PCRE has been 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 sec-
tion 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. 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 performance reasons, you
can set the PCRE_NO_UTF8_CHECK option. When it is set, the
effect of passing an invalid UTF-8 string as a pattern is
undefined. It may cause your program to crash. Note that
there is a similar option for suppressing the checking of
subject strings passed to pcre_exec().
STUDYING A PATTERN
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
When a 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 argu-
ment. If studing the pattern produces additional informa-
tion 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 a pcre_study() can be passed
directly to pcre_exec(). However, the pcre_extra block
also contains other fields that can be set by the caller
before the block is passed; these are described below. If
studying the pattern does not produce any additional
information, pcre_study() returns NULL. In that circum-
stance, if the calling program wants to pass some of the
other fields to pcre_exec(), it must set up its own
pcre_extra block.
The second argument contains option bits. At present, no
options are defined for pcre_study(), 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. Oth-
erwise it points to a textual error message. You should
therefore test the error pointer for NULL after calling
pcre_study(), to be sure that it has run successfully.
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 patterns that do not have a single fixed starting
character. A bitmap of possible starting characters is
created.
LOCALE SUPPORT
PCRE handles caseless matching, and determines whether
characters are letters, digits, or whatever, by reference
to a set of tables. When running in UTF-8 mode, this
applies only to characters with codes less than 256. The
library contains a default set of tables that is created
in the default C locale when PCRE is compiled. This is
used when the final argument of pcre_compile() is NULL,
and is sufficient for many applications.
An alternative set of tables can, however, be supplied.
Such tables are built by calling the pcre_maketables()
function, which has no arguments, in the relevant locale.
The result can then be passed to pcre_compile() as often
as necessary. For example, to build and use tables that
are appropriate for the French locale (where accented
characters with codes greater than 128 are treated as let-
ters), the following code could be used:
setlocale(LC_CTYPE, "fr");
tables = pcre_maketables();
re = pcre_compile(..., tables);
The tables are built in memory that is obtained via
pcre_malloc. The pointer that is passed to pcre_compile is
saved with the compiled pattern, and the same tables are
used via this pointer by pcre_study() and pcre_exec().
Thus, for any single pattern, compilation, studying and
matching all happen in the same locale, but different pat-
terns can be compiled in different locales. It is the
caller's responsibility to ensure that the memory contain-
ing the tables remains available for as long as it is
needed.
INFORMATION ABOUT A PATTERN
int pcre_fullinfo(const pcre *code, const pcre_extra
*extra,
int what, void *where);
The pcre_fullinfo() function returns information about a
compiled pattern. It replaces the obsolete pcre_info()
function, which is nevertheless 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 vari-
able to receive the data. The yield of the function is
zero for success, or one of the following negative num-
bers:
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
Here is a typical call of pcre_fullinfo(), to obtain the
length of the compiled pattern:
int rc;
unsigned long int 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 vari-
able. 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_FIRSTBYTE
Return information about the first byte of any matched
string, for a non-anchored pattern. (This option used to
be called PCRE_INFO_FIRSTCHAR; the old name is still rec-
ognized for backwards compatibility.)
If there is a fixed first byte, e.g. from a pattern such
as (cat|cow|coyote), it is returned in the integer pointed
to by where. 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 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 NULL is returned. The
fourth argument should point to an unsigned char * vari-
able.
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 captur-
ing parentheses. The names are just an additional way of
identifying the parentheses, which still acquire a number.
A caller that wants to extract data from a named subpat-
tern must convert the name to a number in order to access
the correct pointers in the output vector (described with
pcre_exec() below). In order to do this, it must first use
these three values to obtain the name-to-number mapping
table for the pattern.
The map consists of a number of fixed-size entries.
PCRE_INFO_NAMECOUNT gives the number of entries, and
PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both
of these return an int value. The entry size depends on
the length of the longest name. PCRE_INFO_NAMETABLE
returns a pointer to the first entry of the table (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. For example, consider the following pattern (assume
PCRE_EXTENDED is set, so white space - including newlines
- is ignored):
(?P (?P(\d\d)?\d\d) -
(?P\d\d) - (?P\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
hex, 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,
remember that the length of each entry may be different
for each compiled pattern.
PCRE_INFO_OPTIONS
Return a copy of the options with which the pattern was
compiled. The fourth argument should point to an unsigned
long int variable. These option bits are those specified
in the call to pcre_compile(), modified by any top-level
option settings within the pattern itself.
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 com-
piled data. The fourth argument should point to a size_t
variable.
PCRE_INFO_STUDYSIZE
Returns 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 get-
ting 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 following
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 inte-
ger it points to (see PCRE_INFO_OPTIONS above).
If the pattern is not anchored and the firstcharptr argu-
ment is not NULL, it is used to pass back information
about the first character of any matched string (see
PCRE_INFO_FIRSTBYTE above).
MATCHING A PATTERN
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 pre-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 argu-
ment.
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 for substring information */
30); /* number of elements in the vector */
If the extra argument is not NULL, it must point to a
pcre_extra data block. The pcre_study() function returns
such a block (when it doesn't return NULL), but you can
also create one for yourself, and pass additional informa-
tion in it. The fields in the block are as follows:
unsigned long int flags;
void *study_data;
unsigned long int match_limit;
void *callout_data;
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_CALLOUT_DATA
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 can add to the block
by setting the other fields.
The match_limit 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 is
imposed on the number of times this function is called
during a match, which has the effect of limiting the
amount of recursion and backtracking that can take place.
For patterns that are not anchored, the count starts from
zero for each position in the subject string.
The default limit for the library can be set when PCRE is
built; the default default is 10 million, which handles
all but the most extreme cases. You can reduce the default
by suppling pcre_exec() with a pcre_extra block in which
match_limit is set to a smaller value, and
PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the
limit is exceeded, pcre_exec() returns PCRE_ERROR_MATCH-
LIMIT.
The pcre_callout field is used in conjunction with the
"callout" feature, which is described in the pcrecallout
documentation.
The PCRE_ANCHORED option can be passed in the options
argument, whose unused bits must be zero. This limits
pcre_exec() to matching at the first matching position.
However, 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.
When PCRE_UTF8 was set at compile time, the validity of
the subject as a UTF-8 string is automatically checked,
and the value of startoffset is also checked to ensure
that it points to the start of a UTF-8 character. If an
invalid UTF-8 sequence of bytes is found, pcre_exec()
returns the error PCRE_ERROR_BADUTF8. If startoffset
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 startoff-
set that does not point to the start of a UTF-8 character,
is undefined. Your program may crash.
There are also three further options that can be set only
at matching time:
PCRE_NOTBOL
The first character of the 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.
PCRE_NOTEOL
The end of the 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 dol-
lar never to match.
PCRE_NOTEMPTY
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
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 set, and then if that
fails by advancing the starting offset (see below) and
trying an ordinary match again.
The subject string is passed to pcre_exec() as a pointer
in subject, a length in length, and a starting byte offset
in startoffset. 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.
If the pattern was compiled with the PCRE_UTF8 option, the
subject must be a sequence of bytes that is a valid UTF-8
string, and the starting offset must point to the begin-
ning of a UTF-8 character. If an invalid UTF-8 string or
offset is passed, an error (either PCRE_ERROR_BADUTF8 or
PCRE_ERROR_BADUTF8_OFFSET) is returned, unless the option
PCRE_NO_UTF8_CHECK is set, in which case PCRE's behaviour
is not defined.
A non-zero starting offset is useful when searching for
another match in the same subject by calling pcre_exec()
again after a previous success. Setting startoffset dif-
fers 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 "Missis-
sipi" the first call to pcre_exec() finds the first occur-
rence. 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 sub-
ject, which is deemed to be a word boundary. However, if
pcre_exec() is passed the entire string again, but with
startoffset set to 4, it finds the second occurrence of
"iss" because it is able to look behind the starting point
to discover that it is preceded by a letter.
If a non-zero starting offset is passed when the pattern
is anchored, one attempt to match at the given offset is
tried. This can only succeed if the pattern does not
require the match to be at the start of the subject.
In general, a pattern matches a certain portion of the
subject, and in addition, further substrings from the sub-
ject may be picked out by parts of the pattern. Following
the usage in Jeffrey Friedl's book, this is called "cap-
turing" in what follows, and the phrase "capturing subpat-
tern" is used for a fragment of a pattern that picks out a
substring. PCRE supports several other kinds of parenthe-
sized subpattern that do not cause substrings to be cap-
tured.
Captured substrings are returned to the caller via a vec-
tor of integer offsets whose address is passed in ovector.
The number of elements in the vector is passed in ovec-
size. The first two-thirds of the vector is used to pass
back captured substrings, each substring using a pair of
integers. The remaining third of the vector is used as
workspace by pcre_exec() while matching capturing subpat-
terns, 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 has been successful, information about cap-
tured substrings is returned in pairs of integers, start-
ing 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 sub-
string, and the second is set to the offset of the first
character after the end of a substring. The first pair,
ovector[0] and ovector[1], identify the portion of the
subject string matched by the entire pattern. The next
pair is used for the first capturing subpattern, and so
on. The value returned by pcre_exec() is the number of
pairs that have been set. If there are no capturing sub-
patterns, the return value from a successful match is 1,
indicating that just the first pair of offsets has been
set.
Some convenience functions are provided for extracting the
captured substrings as separate strings. These are
described in the following section.
It is possible for an 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) subpatterns 1 and
3 are matched, but 2 is not. When this happens, both off-
set values corresponding to the unused subpattern are set
to -1.
If a capturing subpattern is matched repeatedly, it is the
last portion of the string that it matched that gets
returned.
If the vector is too small to hold all the captured sub-
strings, 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 inter-
est, pcre_exec() may be called with ovector passed as NULL
and ovecsize as zero. However, if the pattern contains
back references and the ovector isn't big enough to remem-
ber the related substrings, PCRE has to get additional
memory for use during matching. Thus it is usually advis-
able to supply an ovector.
Note that pcre_info() 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.
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. This is the error it gives when the magic number
isn't present.
PCRE_ERROR_UNKNOWN_NODE (-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 remem-
ber the referenced substrings, PCRE gets a block of memory
at the start of matching to use for this purpose. If the
call via pcre_malloc() fails, this error is given. The
memory is 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() func-
tions (see below). It is never returned by pcre_exec().
PCRE_ERROR_MATCHLIMIT (-8)
The recursion and 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.
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 conve-
nience, the functions pcre_copy_substring(), pcre_get_sub-
string(), and pcre_get_substring_list() are provided for
extracting captured substrings as new, separate, zero-ter-
minated strings. These functions identify substrings by
number. The next section describes functions for extract-
ing 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.
The first three arguments are the same for all three of
these functions: subject is the subject string which 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 size of the vector divided by three.
The functions pcre_copy_substring() and pcre_get_sub-
string() extract a single substring, whose number is given
as stringnumber. A value of zero extracts the substring
that matched the entire pattern, while 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 func-
tion is the length of the string, not including the termi-
nating zero, or one of
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 avail-
able substrings and builds a list of pointers to them. All
this is done in a single block of memory which 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
PCRE_ERROR_NOMEMORY (-6)
if the attempt to get the memory block failed.
When any of these functions encounter a substring that is
unset, which can happen when capturing subpattern number
n+1 matches some part of the subject, but subpattern n has
not been used at all, they return an empty string. This
can be distinguished from a genuine zero-length substring
by inspecting the appropriate offset in ovector, which is
negative for unset substrings.
The two convenience functions pcre_free_substring() and
pcre_free_substring_list() can be used to free the memory
returned by a previous call of pcre_get_substring() or
pcre_get_substring_list(), respectively. They do nothing
more than call the function pointed to by pcre_free, which
of course could be called directly from a C program. How-
ever, PCRE is used in some situations where it is linked
via a special interface to another programming language
which cannot use pcre_free directly; it is for these cases
that the functions are provided.
EXTRACTING CAPTURED SUBSTRINGS BY 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_stringnumber(const pcre *code,
const char *name);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
To extract a substring by name, you first have to find
associated number. This can be done by calling
pcre_get_stringnumber(). The first argument is the com-
piled pattern, and the second is the name. For example,
for this pattern
ab(?\d+)...
the number of the subpattern called "xxx" is 1. Given the
number, you can then extract the substring directly, or
use one of the functions described in the previous sec-
tion. 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
functions that extract by number, and so are not re-
described here. There are just two differences.
First, instead of a substring number, a substring name is
given. Second, there is an extra argument, given at the
start, which is a pointer to the compiled pattern. This is
needed in order to gain access to the name-to-number
translation table.
These functions call pcre_get_stringnumber(), and if it
succeeds, they then call pcre_copy_substring() or
pcre_get_substring(), as appropriate.
Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.
PCRE(3)