trunk/doc/pcre.3

.TH PCRE 3
.SH NAME
pcre - Perl-compatible regular expressions.
.SH SYNOPSIS
.B #include <pcre.h>
.PP
.SM
.br
.B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
.ti +5n
.B const char **\fIerrptr\fR, int *\fIerroffset\fR,
.ti +5n
.B const unsigned char *\fItableptr\fR);
.PP
.br
.B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
.ti +5n
.B const char **\fIerrptr\fR);
.PP
.br
.B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
.ti +5n
.B "const char *\fIsubject\fR," int \fIlength\fR, int \fIstartoffset\fR,
.ti +5n
.B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR);
.PP
.br
.B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
.ti +5n
.B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
.ti +5n
.B int \fIbuffersize\fR);
.PP
.br
.B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
.ti +5n
.B int \fIstringcount\fR, int \fIstringnumber\fR,
.ti +5n
.B const char **\fIstringptr\fR);
.PP
.br
.B int pcre_get_substring_list(const char *\fIsubject\fR,
.ti +5n
.B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
.PP
.br
.B const unsigned char *pcre_maketables(void);
.PP
.br
.B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
.B *\fIfirstcharptr\fR);
.PP
.br
.B char *pcre_version(void);
.PP
.br
.B void *(*pcre_malloc)(size_t);
.PP
.br
.B void (*pcre_free)(void *);


.SH DESCRIPTION
The PCRE library is a set of functions that implement regular expression
pattern matching using the same syntax and semantics as Perl 5, with just a few
differences (see below). The current implementation corresponds to Perl 5.005.

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 API. These are
described in the \fBpcreposix\fR documentation.

The native API function prototypes are defined in the header file \fBpcre.h\fR,
and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
accessed by adding \fB-lpcre\fR to the command for linking an application which
calls it.

The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
are used for compiling and matching regular expressions, while
\fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
\fBpcre_get_substring_list()\fR are convenience functions for extracting
captured substrings from a matched subject string. The function
\fBpcre_maketables()\fR is used (optionally) to build a set of character tables
in the current locale for passing to \fBpcre_compile()\fR.

The function \fBpcre_info()\fR is used to find out information about a compiled
pattern, while the function \fBpcre_version()\fR returns a pointer to a string
containing the version of PCRE and its date of release.

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


.SH MULTI-THREADING
The PCRE functions can be used in multi-threading applications, with the
proviso that the memory management functions pointed to by \fBpcre_malloc\fR
and \fBpcre_free\fR 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.


.SH COMPILING A PATTERN
The function \fBpcre_compile()\fR is called to compile a pattern into an
internal form. The pattern is a C string terminated by a binary zero, and
is passed in the argument \fIpattern\fR. A pointer to a single block of memory
that is obtained via \fBpcre_malloc\fR is returned. This contains the
compiled code and related data. The \fBpcre\fR type is defined for this for
convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the
contents of the block are not externally defined. It is up to the caller to
free the memory when it is no longer required.
.PP
The size of a compiled pattern is roughly proportional to the length of the
pattern string, except that each character class (other than those containing
just a single character, negated or not) requires 33 bytes, and repeat
quantifiers with a minimum greater than one or a bounded maximum cause the
relevant portions of the compiled pattern to be replicated.
.PP
The \fIoptions\fR 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 regular expressions
below). For these options, the contents of the \fIoptions\fR argument specifies
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.
.PP
If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
error message. The offset from the start of the pattern to the character where
the error was discovered is placed in the variable pointed to by
\fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
.PP
If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
character tables which are built when it is compiled, using the default C
locale. Otherwise, \fItableptr\fR must be the result of a call to
\fBpcre_maketables()\fR. See the section on locale support below.
.PP
The following option bits are defined in the header file:

  PCRE_ANCHORED

If this bit is set, the pattern is forced to be "anchored", that is, it is
constrained to match only at the start of 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.

  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.

  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. 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 pattern are totally
ignored except when escaped or inside a character class, and 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 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 turns on additional functionality of PCRE that is incompatible with
Perl. 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. There are at present no other features
controlled by this option.

  PCRE_MULTILINE

By default, PCRE treats the subject string as consisting of a single "line" of
characters (even if it actually contains 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. 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_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.


.SH STUDYING A PATTERN
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 \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
argument, and returns a pointer to a \fBpcre_extra\fR block (another \fBvoid\fR
typedef) containing additional information about the pattern; this can be
passed to \fBpcre_exec()\fR. If no additional information is available, NULL
is returned.

The second argument contains option bits. At present, no options are defined
for \fBpcre_study()\fR, and this argument should always be zero.

The third argument for \fBpcre_study()\fR is a pointer to an error message. If
studying succeeds (even if no data is returned), the variable it points to is
set to NULL. Otherwise it points to a textual error 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.


.SH LOCALE SUPPORT
PCRE handles caseless matching, and determines whether characters are letters,
digits, or whatever, by reference to a set of tables. The library contains a
default set of tables which is created in the default C locale when PCRE is
compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
and is sufficient for many applications.

An alternative set of tables can, however, be supplied. Such tables are built
by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
relevant locale. The result can then be passed to \fBpcre_compile()\fR 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 letters), 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 \fBpcre_malloc\fR. The
pointer that is passed to \fBpcre_compile\fR is saved with the compiled
pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
and \fBpcre_exec()\fR. Thus 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 the caller's responsibility to ensure that the
memory containing the tables remains available for as long as it is needed.


.SH INFORMATION ABOUT A PATTERN
The \fBpcre_info()\fR function returns information about a compiled pattern.
Its yield is the number of capturing subpatterns, or one of the following
negative numbers:

  PCRE_ERROR_NULL       the argument \fIcode\fR was NULL
  PCRE_ERROR_BADMAGIC   the "magic number" was not found

If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
pattern was compiled is placed in the integer it points to. These option bits
are those specified in the call to \fBpcre_compile()\fR, modified by any
top-level option settings within the pattern itself, and with the PCRE_ANCHORED
bit set if the form of the pattern implies that it can match only at the start
of a subject string.

If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
it is used to pass back information about the first character of any matched
string. If there is a fixed first character, e.g. from a pattern such as
(cat|cow|coyote), then it is returned in the integer pointed to by
\fIfirstcharptr\fR. 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),

then -1 is returned, indicating that the pattern matches only at the
start of a subject string or after any "\\n" within the string. Otherwise -2 is
returned.


.SH MATCHING A PATTERN
The function \fBpcre_exec()\fR is called to match a subject string against a
pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
pattern has been studied, the result of the study should be passed in the
\fIextra\fR argument. Otherwise this must be NULL.

The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
unused bits must be zero. 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.

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 dollar 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 pattern, they are tried. If all the alternatives
match the empty string, the entire match fails. For example, if the pattern

  a?b?

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

Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case
of a pattern match of the empty string within its \fBsplit()\fR 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 as a pointer in \fIsubject\fR, a length in
\fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern
string, it may contain binary zero characters. 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 \fBpcre_exec()\fR again after a previous success.
Setting \fIstartoffset\fR differs from just passing over a shortened string and
setting PCRE_NOTBOL in the case of a pattern that begins with any kind of
lookbehind. For example, consider the pattern

  \\Biss\\B

which finds occurrences of "iss" in the middle of words. (\\B matches only if
the current position in the subject is not a word boundary.) When applied to
the string "Mississipi" the first call to \fBpcre_exec()\fR finds the first
occurrence. If \fBpcre_exec()\fR 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
\fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR
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 subject may be picked out by parts of the
pattern. Following the usage in Jeffrey Friedl's book, this is called
"capturing" in what follows, and the phrase "capturing subpattern" is used for
a fragment of a pattern that picks out a substring. PCRE supports several other
kinds of parenthesized subpattern that do not cause substrings to be captured.

Captured substrings are returned to the caller via a vector of integer offsets
whose address is passed in \fIovector\fR. The number of elements in the vector
is passed in \fIovecsize\fR. 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 \fBpcre_exec()\fR while
matching capturing subpatterns, and is not available for passing back
information. The length passed in \fIovecsize\fR should always be a multiple of
three. If it is not, it is rounded down.

When a match has been successful, information about captured substrings is
returned in pairs of integers, starting at the beginning of \fIovector\fR, and
continuing up to two-thirds of its length at the most. The first element of a
pair is set to the offset of the first character in a substring, and the second
is set to the offset of the first character after the end of a substring. The
first pair, \fIovector[0]\fR and \fIovector[1]\fR, 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 \fBpcre_exec()\fR
is the number of pairs that have been set. 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.

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 \fIn+1\fR to match some
part of the subject when subpattern \fIn\fR 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 offset
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 substrings, it is used as
far as possible (up to two-thirds of its length), and the function returns a
value of zero. In particular, if the substring offsets are not of interest,
\fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
\fIovecsize\fR as zero. However, if the pattern contains back references and
the \fIovector\fR isn't 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 \fIovector\fR.

Note that \fBpcre_info()\fR can be used to find out how many capturing
subpatterns there are in a compiled pattern. The smallest size for
\fIovector\fR that will allow for \fIn\fR captured substrings in addition to
the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.

If \fBpcre_exec()\fR 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 \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
NULL and \fIovecsize\fR was not zero.

  PCRE_ERROR_BADOPTION      (-3)

An unrecognized bit was set in the \fIoptions\fR 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 \fIovector\fR that is passed to
\fBpcre_exec()\fR 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 \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
the end of matching.


.SH EXTRACTING CAPTURED SUBSTRINGS
Captured substrings can be accessed directly by using the offsets returned by
\fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
\fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
\fBpcre_get_substring_list()\fR are provided for extracting captured substrings
as new, separate, zero-terminated strings. A substring that contains a binary
zero is correctly extracted and has a further zero added on the end, but the
result does not, of course, function as a C string.

The first three arguments are the same for all three functions: \fIsubject\fR
is the subject string which has just been successfully matched, \fIovector\fR
is a pointer to the vector of integer offsets that was passed to
\fBpcre_exec()\fR, and \fIstringcount\fR 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 \fBpcre_exec\fR if it
is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
ran out of space in \fIovector\fR, then the value passed as
\fIstringcount\fR should be the size of the vector divided by three.

The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
extract a single substring, whose number is given as \fIstringnumber\fR. A
value of zero extracts the substring that matched the entire pattern, while
higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
the string is placed in \fIbuffer\fR, whose length is given by
\fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
obtained via \fBpcre_malloc\fR, and its address is returned via
\fIstringptr\fR. The yield of the function is the length of the string, not
including the terminating zero, or one of

  PCRE_ERROR_NOMEMORY       (-6)

The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
memory failed for \fBpcre_get_substring()\fR.

  PCRE_ERROR_NOSUBSTRING    (-7)

There is no substring whose number is \fIstringnumber\fR.

The \fBpcre_get_substring_list()\fR function extracts all available substrings
and builds a list of pointers to them. All this is done in a single block of
memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
is returned via \fIlistptr\fR, 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 \fIn+1\fR matches some part of the
subject, but subpattern \fIn\fR 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 \fIovector\fR, which is negative for unset
substrings.


.SH LIMITATIONS
There are some size limitations in PCRE but it is hoped that they will never in
practice be relevant.
The maximum length of a compiled pattern is 65539 (sic) bytes.
All values in repeating quantifiers must be less than 65536.
The maximum number of capturing subpatterns is 99.
The maximum number of all parenthesized subpatterns, including capturing
subpatterns, assertions, and other types of subpattern, is 200.

The maximum length of a subject string is the largest positive number that an
integer variable can hold. However, PCRE uses recursion to handle subpatterns
and indefinite repetition. This means that the available stack space may limit
the size of a subject string that can be processed by certain patterns.


.SH DIFFERENCES FROM PERL
The differences described here are with respect to Perl 5.005.

1. By default, a whitespace character is any character that the C library
function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
alternative character type tables. Normally \fBisspace()\fR matches space,
formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
no longer includes vertical tab in its set of whitespace characters. The \\v
escape that was in the Perl documentation for a long time was never in fact
recognized. However, the character itself was treated as whitespace at least
up to 5.002. In 5.004 and 5.005 it does not match \\s.

2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
them, but they do not mean what you might think. For example, (?!a){3} does
not assert that the next three characters are not "a". It just asserts that the
next character is not "a" three times.

3. Capturing subpatterns that occur inside negative lookahead assertions are
counted, but their entries in the offsets vector are never set. Perl sets its
numerical variables from any such patterns that are matched before the
assertion fails to match something (thereby succeeding), but only if the
negative lookahead assertion contains just one branch.

4. Though binary zero characters are supported in the subject string, they are
not allowed in a pattern string because it is passed as a normal C string,
terminated by zero. The escape sequence "\\0" can be used in the pattern to
represent a binary zero.

5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
\\E, \\Q. In fact these are implemented by Perl's general string-handling and
are not part of its pattern matching engine.

6. The Perl \\G assertion is not supported as it is not relevant to single
pattern matches.

7. Fairly obviously, PCRE does not support the (?{code}) construction.

8. There are at the time of writing some oddities in Perl 5.005_02 concerned
with the settings of captured strings when part of a pattern is repeated. For
example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
"b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.

In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
future Perl changes to a consistent state that is different, PCRE may change to
follow.

9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.

10. PCRE provides some extensions to the Perl regular expression facilities:

(a) Although lookbehind assertions must match fixed length strings, each
alternative branch of a lookbehind assertion can match a different length of
string. Perl 5.005 requires them all to have the same length.

(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
character matches only at the very end of the string.

(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
meaning is faulted.

(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
inverted, that is, by default they are not greedy, but if followed by a
question mark they are.

(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start
of the subject.

(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
\fBpcre_exec()\fR have no Perl equivalents.


.SH REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions supported by PCRE are
described below. Regular expressions are also described in the Perl
documentation and in a number of other books, some of which have copious
examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
here is intended as reference documentation.

A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match the
corresponding characters in the subject. As a trivial example, the pattern

  The quick brown fox

matches a portion of a subject string that is identical to itself. The power of
regular expressions comes from the ability to include alternatives and
repetitions in the pattern. These are encoded in the pattern by the use of
\fImeta-characters\fR, which do not stand for themselves but instead are
interpreted in some special way.

There are two different sets of meta-characters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized in square brackets. Outside square brackets, the meta-characters are
as follows:

  \\      general escape character with several uses
  ^      assert start of subject (or line, in multiline mode)
  $      assert end of subject (or line, in multiline mode)
  .      match any character except newline (by default)
  [      start character class definition
  |      start of alternative branch
  (      start subpattern
  )      end subpattern
  ?      extends the meaning of (
         also 0 or 1 quantifier
         also quantifier minimizer
  *      0 or more quantifier
  +      1 or more quantifier
  {      start min/max quantifier

Part of a pattern that is in square brackets is called a "character class". In
a character class the only meta-characters are:

  \\      general escape character
  ^      negate the class, but only if the first character
  -      indicates character range
  ]      terminates the character class

The following sections describe the use of each of the meta-characters.


.SH BACKSLASH
The backslash character has several uses. Firstly, if it is followed by a
non-alphameric character, it takes away any special meaning that character may
have. This use of backslash as an escape character applies both inside and
outside character classes.

For example, if you want to match a "*" character, you write "\\*" in the
pattern. This applies whether or not the following character would otherwise be
interpreted as a meta-character, so it is always safe to precede a
non-alphameric with "\\" to specify that it stands for itself. In particular,
if you want to match a backslash, you write "\\\\".

If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
pattern (other than in a character class) and characters between a "#" outside
a character class and the next newline character are ignored. An escaping
backslash can be used to include a whitespace or "#" character as part of the
pattern.

A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually easier to
use one of the following escape sequences than the binary character it
represents:

  \\a     alarm, that is, the BEL character (hex 07)
  \\cx    "control-x", where x is any character
  \\e     escape (hex 1B)
  \\f     formfeed (hex 0C)
  \\n     newline (hex 0A)
  \\r     carriage return (hex 0D)
  \\t     tab (hex 09)
  \\xhh   character with hex code hh
  \\ddd   character with octal code ddd, or backreference

The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
7B.

After "\\x", up to two hexadecimal digits are read (letters can be in upper or
lower case).

After "\\0" up to two further octal digits are read. In both cases, if there
are fewer than two digits, just those that are present are used. Thus the
sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
Make sure you supply two digits after the initial zero if the character that
follows is itself an octal digit.

The handling of a backslash followed by a digit other than 0 is complicated.
Outside a character class, PCRE reads it and any following digits as a decimal
number. If the number is less than 10, or if there have been at least that many
previous capturing left parentheses in the expression, the entire sequence is
taken as a \fIback reference\fR. A description of how this works is given
later, following the discussion of parenthesized subpatterns.

Inside a character class, or if the decimal number is greater than 9 and there
have not been that many capturing subpatterns, PCRE re-reads up to three octal
digits following the backslash, and generates a single byte from the least
significant 8 bits of the value. Any subsequent digits stand for themselves.
For example:

  \\040   is another way of writing a space
  \\40    is the same, provided there are fewer than 40
            previous capturing subpatterns
  \\7     is always a back reference
  \\11    might be a back reference, or another way of
            writing a tab
  \\011   is always a tab
  \\0113  is a tab followed by the character "3"
  \\113   is the character with octal code 113 (since there
            can be no more than 99 back references)
  \\377   is a byte consisting entirely of 1 bits
  \\81    is either a back reference, or a binary zero
            followed by the two characters "8" and "1"

Note that octal values of 100 or greater must not be introduced by a leading
zero, because no more than three octal digits are ever read.

All the sequences that define a single byte value can be used both inside and
outside character classes. In addition, inside a character class, the sequence
"\\b" is interpreted as the backspace character (hex 08). Outside a character
class it has a different meaning (see below).

The third use of backslash is for specifying generic character types:

  \\d     any decimal digit
  \\D     any character that is not a decimal digit
  \\s     any whitespace character
  \\S     any character that is not a whitespace character
  \\w     any "word" character
  \\W     any "non-word" character

Each pair of escape sequences partitions the complete set of characters into
two disjoint sets. Any given character matches one, and only one, of each pair.

A "word" character is any letter or digit or the underscore character, that is,
any character which can be part of a Perl "word". The definition of letters and
digits is controlled by PCRE's character tables, and may vary if locale-
specific matching is taking place (see "Locale support" above). For example, in
the "fr" (French) locale, some character codes greater than 128 are used for
accented letters, and these are matched by \\w.

These character type sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the current
matching point is at the end of the subject string, all of them fail, since
there is no character to match.

The fourth use of backslash is for certain simple assertions. An assertion
specifies a condition that has to be met at a particular point in a match,
without consuming any characters from the subject string. The use of
subpatterns for more complicated assertions is described below. The backslashed
assertions are

  \\b     word boundary
  \\B     not a word boundary
  \\A     start of subject (independent of multiline mode)
  \\Z     end of subject or newline at end (independent of multiline mode)
  \\z     end of subject (independent of multiline mode)

These assertions may not appear in character classes (but note that "\\b" has a
different meaning, namely the backspace character, inside a character class).

A word boundary is a position in the subject string where the current character
and the previous character do not both match \\w or \\W (i.e. one matches
\\w and the other matches \\W), or the start or end of the string if the
first or last character matches \\w, respectively.

The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
dollar (described below) in that they only ever match at the very start and end
of the subject string, whatever options are set. They are not affected by the
PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of
\fBpcre_exec()\fR is non-zero, \\A can never match. The difference between \\Z
and \\z is that \\Z matches before a newline that is the last character of the
string as well as at the end of the string, whereas \\z matches only at the
end.


.SH CIRCUMFLEX AND DOLLAR
Outside a character class, in the default matching mode, the circumflex
character is an assertion which is true only if the current matching point is
at the start of the subject string. If the \fIstartoffset\fR argument of
\fBpcre_exec()\fR is non-zero, circumflex can never match. Inside a character
class, circumflex has an entirely different meaning (see below).

Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)

A dollar character is an assertion which is true only if the current matching
point is at the end of the subject string, or immediately before a newline
character that is the last character in the string (by default). Dollar need
not be the last character of the pattern if a number of alternatives are
involved, but it should be the last item in any branch in which it appears.
Dollar has no special meaning in a character class.

The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
time. This does not affect the \\Z assertion.

The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, they match immediately
after and immediately before an internal "\\n" character, respectively, in
addition to matching at the start and end of the subject string. For example,
the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
but not otherwise. Consequently, patterns that are anchored in single line mode
because all branches start with "^" are not anchored in multiline mode, and a
match for circumflex is possible when the \fIstartoffset\fR argument of
\fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.

Note that the sequences \\A, \\Z, and \\z can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\\A is it always anchored, whether PCRE_MULTILINE is set or not.


.SH FULL STOP (PERIOD, DOT)
Outside a character class, a dot in the pattern matches any one character in
the subject, including a non-printing character, but not (by default) newline.
If the PCRE_DOTALL option is set, then dots match newlines as well. The
handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newline characters.
Dot has no special meaning in a character class.


.SH SQUARE BRACKETS
An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special. If a
closing square bracket is required as a member of the class, it should be the
first data character in the class (after an initial circumflex, if present) or
escaped with a backslash.

A character class matches a single character in the subject; the character must
be in the set of characters defined by the class, unless the first character in
the class is a circumflex, in which case the subject character must not be in
the set defined by the class. If a circumflex is actually required as a member
of the class, ensure it is not the first character, or escape it with a
backslash.

For example, the character class [aeiou] matches any lower case vowel, while
[^aeiou] matches any character that is not a lower case vowel. Note that a
circumflex is just a convenient notation for specifying the characters which
are in the class by enumerating those that are not. It is not an assertion: it
still consumes a character from the subject string, and fails if the current
pointer is at the end of the string.

When caseless matching is set, any letters in a class represent both their
upper case and lower case versions, so for example, a caseless [aeiou] matches
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
caseful version would.

The newline character is never treated in any special way in character classes,
whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
such as [^a] will always match a newline.

The minus (hyphen) character can be used to specify a range of characters in a
character class. For example, [d-m] matches any letter between d and m,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class.

It is not possible to have the literal character "]" as the end character of a
range. A pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\\]46] is interpreted as a single class containing a
range followed by two separate characters. The octal or hexadecimal
representation of "]" can also be used to end a range.

Ranges operate in ASCII collating sequence. They can also be used for
characters specified numerically, for example [\\000-\\037]. If a range that
includes letters is used when caseless matching is set, it matches the letters
in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
caselessly, and if character tables for the "fr" locale are in use,
[\\xc8-\\xcb] matches accented E characters in both cases.

The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
character class, and add the characters that they match to the class. For
example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
conveniently be used with the upper case character types to specify a more
restricted set of characters than the matching lower case type. For example,
the class [^\\W_] matches any letter or digit, but not underscore.

All non-alphameric characters other than \\, -, ^ (at the start) and the
terminating ] are non-special in character classes, but it does no harm if they
are escaped.


.SH VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For example,
the pattern

  gilbert|sullivan

matches either "gilbert" or "sullivan". Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string).
The matching process tries each alternative in turn, from left to right,
and the first one that succeeds is used. If the alternatives are within a
subpattern (defined below), "succeeds" means matching the rest of the main
pattern as well as the alternative in the subpattern.


.SH INTERNAL OPTION SETTING
The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
can be changed from within the pattern by a sequence of Perl option letters
enclosed between "(?" and ")". The option letters are

  i  for PCRE_CASELESS
  m  for PCRE_MULTILINE
  s  for PCRE_DOTALL
  x  for PCRE_EXTENDED

For example, (?im) sets caseless, multiline matching. It is also possible to
unset these options by preceding the letter with a hyphen, and a combined
setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
permitted. If a letter appears both before and after the hyphen, the option is
unset.

The scope of these option changes depends on where in the pattern the setting
occurs. For settings that are outside any subpattern (defined below), the
effect is the same as if the options were set or unset at the start of
matching. The following patterns all behave in exactly the same way:

  (?i)abc
  a(?i)bc
  ab(?i)c
  abc(?i)

which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
In other words, such "top level" settings apply to the whole pattern (unless
there are other changes inside subpatterns). If there is more than one setting
of the same option at top level, the rightmost setting is used.

If an option change occurs inside a subpattern, the effect is different. This
is a change of behaviour in Perl 5.005. An option change inside a subpattern
affects only that part of the subpattern that follows it, so

  (a(?i)b)c

matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
By this means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same subpattern. For example,

  (a(?i)b|c)

matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.

The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
same way as the Perl-compatible options by using the characters U and X
respectively. The (?X) flag setting is special in that it must always occur
earlier in the pattern than any of the additional features it turns on, even
when it is at top level. It is best put at the start.


.SH SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be nested.
Marking part of a pattern as a subpattern does two things:

1. It localizes a set of alternatives. For example, the pattern

  cat(aract|erpillar|)

matches one of the words "cat", "cataract", or "caterpillar". Without the
parentheses, it would match "cataract", "erpillar" or the empty string.

2. It sets up the subpattern as a capturing subpattern (as defined above).
When the whole pattern matches, that portion of the subject string that matched
the subpattern is passed back to the caller via the \fIovector\fR argument of
\fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
from 1) to obtain the numbers of the capturing subpatterns.

For example, if the string "the red king" is matched against the pattern

  the ((red|white) (king|queen))

the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3.

The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by "?:", the
subpattern does not do any capturing, and is not counted when computing the
number of any subsequent capturing subpatterns. For example, if the string "the
white queen" is matched against the pattern

  the ((?:red|white) (king|queen))

the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of captured substrings is 99, and the maximum number of
all subpatterns, both capturing and non-capturing, is 200.

As a convenient shorthand, if any option settings are required at the start of
a non-capturing subpattern, the option letters may appear between the "?" and
the ":". Thus the two patterns

  (?i:saturday|sunday)
  (?:(?i)saturday|sunday)

match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the subpattern
is reached, an option setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".


.SH REPETITION
Repetition is specified by quantifiers, which can follow any of the following
items:

  a single character, possibly escaped
  the . metacharacter
  a character class
  a back reference (see next section)
  a parenthesized subpattern (unless it is an assertion - see below)

The general repetition quantifier specifies a minimum and maximum number of
permitted matches, by giving the two numbers in curly brackets (braces),
separated by a comma. The numbers must be less than 65536, and the first must
be less than or equal to the second. For example:

  z{2,4}

matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus

  [aeiou]{3,}

matches at least 3 successive vowels, but may match many more, while

  \\d{8}

matches exactly 8 digits. An opening curly bracket that appears in a position
where a quantifier is not allowed, or one that does not match the syntax of a
quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.

The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present.

For convenience (and historical compatibility) the three most common
quantifiers have single-character abbreviations:

  *    is equivalent to {0,}
  +    is equivalent to {1,}
  ?    is equivalent to {0,1}

It is possible to construct infinite loops by following a subpattern that can
match no characters with a quantifier that has no upper limit, for example:

  (a?)*

Earlier versions of Perl and PCRE used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but if any repetition of the subpattern does in fact
match no characters, the loop is forcibly broken.

By default, the quantifiers are "greedy", that is, they match as much as
possible (up to the maximum number of permitted times), without causing the
rest of the pattern to fail. The classic example of where this gives problems
is in trying to match comments in C programs. These appear between the
sequences /* and */ and within the sequence, individual * and / characters may
appear. An attempt to match C comments by applying the pattern

  /\\*.*\\*/

to the string

  /* first command */  not comment  /* second comment */

fails, because it matches the entire string due to the greediness of the .*
item.

However, if a quantifier is followed by a question mark, then it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern

  /\\*.*?\\*/

does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of matches.
Do not confuse this use of question mark with its use as a quantifier in its
own right. Because it has two uses, it can sometimes appear doubled, as in

  \\d??\\d

which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.

If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
then the quantifiers are not greedy by default, but individual ones can be made
greedy by following them with a question mark. In other words, it inverts the
default behaviour.

When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more store is required for the
compiled pattern, in proportion to the size of the minimum or maximum.

If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
to Perl's /s) is set, thus allowing the . to match newlines, then the pattern
is implicitly anchored, because whatever follows will be tried against every
character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE treats such a pattern as
though it were preceded by \\A. In cases where it is known that the subject
string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
begins with .* in order to obtain this optimization, or alternatively using ^
to indicate anchoring explicitly.

When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after

  (tweedle[dume]{3}\\s*)+

has matched "tweedledum tweedledee" the value of the captured substring is
"tweedledee". However, if there are nested capturing subpatterns, the
corresponding captured values may have been set in previous iterations. For
example, after

  /(a|(b))+/

matches "aba" the value of the second captured substring is "b".


.SH BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a back reference to a capturing subpattern earlier
(i.e. to its left) in the pattern, provided there have been that many previous
capturing left parentheses.

However, if the decimal number following the backslash is less than 10, it is
always taken as a back reference, and causes an error only if there are not
that many capturing left parentheses in the entire pattern. In other words, the
parentheses that are referenced need not be to the left of the reference for
numbers less than 10. See the section entitled "Backslash" above for further
details of the handling of digits following a backslash.

A back reference matches whatever actually matched the capturing subpattern in
the current subject string, rather than anything matching the subpattern
itself. So the pattern

  (sens|respons)e and \\1ibility

matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If caseful matching is in force at the time of the
back reference, then the case of letters is relevant. For example,

  ((?i)rah)\\s+\\1

matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly.

There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, then any back
references to it always fail. For example, the pattern

  (a|(bc))\\2

always fails if it starts to match "a" rather than "bc". Because there may be
up to 99 back references, all digits following the backslash are taken
as part of a potential back reference number. If the pattern continues with a
digit character, then some delimiter must be used to terminate the back
reference. If the PCRE_EXTENDED option is set, this can be whitespace.
Otherwise an empty comment can be used.

A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\\1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern

  (a|b\\1)+

matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
the subpattern, the back reference matches the character string corresponding
to the previous iteration. In order for this to work, the pattern must be such
that the first iteration does not need to match the back reference. This can be
done using alternation, as in the example above, or by a quantifier with a
minimum of zero.


.SH ASSERTIONS
An assertion is a test on the characters following or preceding the current
matching point that does not actually consume any characters. The simple
assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
complicated assertions are coded as subpatterns. There are two kinds: those
that look ahead of the current position in the subject string, and those that
look behind it.

An assertion subpattern is matched in the normal way, except that it does not
cause the current matching position to be changed. Lookahead assertions start
with (?= for positive assertions and (?! for negative assertions. For example,

  \\w+(?=;)

matches a word followed by a semicolon, but does not include the semicolon in
the match, and

  foo(?!bar)

matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern

  (?!foo)bar

does not find an occurrence of "bar" that is preceded by something other than
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
(?!foo) is always true when the next three characters are "bar". A
lookbehind assertion is needed to achieve this effect.

Lookbehind assertions start with (?<= for positive assertions and (?<! for
negative assertions. For example,

  (?<!foo)bar

does find an occurrence of "bar" that is not preceded by "foo". The contents of
a lookbehind assertion are restricted such that all the strings it matches must
have a fixed length. However, if there are several alternatives, they do not
all have to have the same fixed length. Thus

  (?<=bullock|donkey)

is permitted, but

  (?<!dogs?|cats?)

causes an error at compile time. Branches that match different length strings
are permitted only at the top level of a lookbehind assertion. This is an
extension compared with Perl 5.005, which requires all branches to match the
same length of string. An assertion such as

  (?<=ab(c|de))

is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable if rewritten to use two top-level branches:

  (?<=abc|abde)

The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed width and then try to
match. If there are insufficient characters before the current position, the
match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
can be particularly useful for matching at the ends of strings; an example is
given at the end of the section on once-only subpatterns.

Several assertions (of any sort) may occur in succession. For example,

  (?<=\\d{3})(?<!999)foo

matches "foo" preceded by three digits that are not "999". Notice that each of
the assertions is applied independently at the same point in the subject
string. First there is a check that the previous three characters are all
digits, then there is a check that the same three characters are not "999".
This pattern does \fInot\fR match "foo" preceded by six characters, the first
of which are digits and the last three of which are not "999". For example, it
doesn't match "123abcfoo". A pattern to do that is

  (?<=\\d{3}...)(?<!999)foo

This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not "999".

Assertions can be nested in any combination. For example,

  (?<=(?<!foo)bar)baz

matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo", while

  (?<=\\d{3}(?!999)...)foo

is another pattern which matches "foo" preceded by three digits and any three
characters that are not "999".

Assertion subpatterns are not capturing subpatterns, and may not be repeated,
because it makes no sense to assert the same thing several times. If any kind
of assertion contains capturing subpatterns within it, these are counted for
the purposes of numbering the capturing subpatterns in the whole pattern.
However, substring capturing is carried out only for positive assertions,
because it does not make sense for negative assertions.

Assertions count towards the maximum of 200 parenthesized subpatterns.


.SH ONCE-ONLY SUBPATTERNS
With both maximizing and minimizing repetition, failure of what follows
normally causes the repeated item to be re-evaluated to see if a different
number of repeats allows the rest of the pattern to match. Sometimes it is
useful to prevent this, either to change the nature of the match, or to cause
it fail earlier than it otherwise might, when the author of the pattern knows
there is no point in carrying on.

Consider, for example, the pattern \\d+foo when applied to the subject line

  123456bar

After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \\d+
item, and then with 4, and so on, before ultimately failing. Once-only
subpatterns provide the means for specifying that once a portion of the pattern
has matched, it is not to be re-evaluated in this way, so the matcher would
give up immediately on failing to match "foo" the first time. The notation is
another kind of special parenthesis, starting with (?> as in this example:

  (?>\\d+)bar

This kind of parenthesis "locks up" the  part of the pattern it contains once
it has matched, and a failure further into the pattern is prevented from
backtracking into it. Backtracking past it to previous items, however, works as
normal.

An alternative description is that a subpattern of this type matches the string
of characters that an identical standalone pattern would match, if anchored at
the current point in the subject string.

Once-only subpatterns are not capturing subpatterns. Simple cases such as the
above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\\d+) can only match an entire sequence of digits.

This construction can of course contain arbitrarily complicated subpatterns,
and it can be nested.

Once-only subpatterns can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of the subject string. Consider a simple
pattern such as

  abcd$

when applied to a long string which does not match it. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject and
then see if what follows matches the rest of the pattern. If the pattern is
specified as

  ^.*abcd$

then the initial .* matches the entire string at first, but when this fails, it
backtracks to match all but the last character, then all but the last two
characters, and so on. Once again the search for "a" covers the entire string,
from right to left, so we are no better off. However, if the pattern is written
as

  ^(?>.*)(?<=abcd)

then there can be no backtracking for the .* item; it can match only the entire
string. The subsequent lookbehind assertion does a single test on the last four
characters. If it fails, the match fails immediately. For long strings, this
approach makes a significant difference to the processing time.


.SH CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns, depending on
the result of an assertion, or whether a previous capturing subpattern matched
or not. The two possible forms of conditional subpattern are

  (?(condition)yes-pattern)
  (?(condition)yes-pattern|no-pattern)

If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alternatives in the
subpattern, a compile-time error occurs.

There are two kinds of condition. If the text between the parentheses consists
of a sequence of digits, then the condition is satisfied if the capturing
subpattern of that number has previously matched. Consider the following
pattern, which contains non-significant white space to make it more readable
(assume the PCRE_EXTENDED option) and to divide it into three parts for ease
of discussion:

  ( \\( )?    [^()]+    (?(1) \\) )

The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second part
matches one or more characters that are not parentheses. The third part is a
conditional subpattern that tests whether the first set of parentheses matched
or not. If they did, that is, if subject started with an opening parenthesis,
the condition is true, and so the yes-pattern is executed and a closing
parenthesis is required. Otherwise, since no-pattern is not present, the
subpattern matches nothing. In other words, this pattern matches a sequence of
non-parentheses, optionally enclosed in parentheses.

If the condition is not a sequence of digits, it must be an assertion. This may
be a positive or negative lookahead or lookbehind assertion. Consider this
pattern, again containing non-significant white space, and with the two
alternatives on the second line:

  (?(?=[^a-z]*[a-z])
  \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )

The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.


.SH COMMENTS
The sequence (?# marks the start of a comment which continues up to the next
closing parenthesis. Nested parentheses are not permitted. The characters
that make up a comment play no part in the pattern matching at all.

If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues up to the next newline
character in the pattern.


.SH PERFORMANCE
Certain items that may appear in patterns are more efficient than others. It is
more efficient to use a character class like [aeiou] than a set of alternatives
such as (a|e|i|o|u). In general, the simplest construction that provides the
required behaviour is usually the most efficient. Jeffrey Friedl's book
contains a lot of discussion about optimizing regular expressions for efficient
performance.

When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
implicitly anchored by PCRE, since it can match only at the start of a subject
string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
because the . metacharacter does not then match a newline, and if the subject
string contains newlines, the pattern may match from the character immediately
following one of them instead of from the very start. For example, the pattern

  (.*) second

matches the subject "first\\nand second" (where \\n stands for a newline
character) with the first captured substring being "and". In order to do this,
PCRE has to retry the match starting after every newline in the subject.

If you are using such a pattern with subject strings that do not contain
newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
having to scan along the subject looking for a newline to restart at.

Beware of patterns that contain nested indefinite repeats. These can take a
long time to run when applied to a string that does not match. Consider the
pattern fragment

  (a+)*

This can match "aaaa" in 33 different ways, and this number increases very
rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
times, and for each of those cases other than 0, the + repeats can match
different numbers of times.) When the remainder of the pattern is such that the
entire match is going to fail, PCRE has in principle to try every possible
variation, and this can take an extremely long time.

An optimization catches some of the more simple cases such as

  (a+)*b

where a literal character follows. Before embarking on the standard matching
procedure, PCRE checks that there is a "b" later in the subject string, and if
there is not, it fails the match immediately. However, when there is no
following literal this optimization cannot be used. You can see the difference
by comparing the behaviour of

  (a+)*\\d

with the pattern above. The former gives a failure almost instantly when
applied to a whole line of "a" characters, whereas the latter takes an
appreciable time with strings longer than about 20 characters.

.SH AUTHOR
Philip Hazel <ph10@cam.ac.uk>
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University Computing Service,
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New Museums Site,
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Cambridge CB2 3QG, England.
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Phone: +44 1223 334714

Last updated: 29 July 1999
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Copyright (c) 1997-1999 University of Cambridge.