path: root/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 char **\fIerrptr\fR, int *\fIerroffset\fR);
.PP
.br
.B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
.ti +5n
.B 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 \fIoptions\fR,
.ti +5n
.B int *\fIovector\fR, int \fIovecsize\fR);
.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 *);
.PP
.br
.B unsigned char *pcre_cbits[128];
.PP
.br
.B unsigned char *pcre_ctypes[256];
.PP
.br
.B unsigned char *pcre_fcc[256];
.PP
.br
.B unsigned char *pcre_lcc[256];



.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 5.004.

PCRE has its own native API, which is described in this man page. There is also
a set of wrapper functions that correspond to the POSIX API. See
\fBpcreposix (3)\fR.

The three functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and
\fBpcre_exec()\fR are used for compiling and matching regular expressions. 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.

The other global variables are character tables. They are initialized when PCRE
is compiled, from source that is generated by reference to the C character type
functions, but which the maintainer of PCRE is free to modify. In principle
they could also be modified at runtime. See PCRE's README file for more
details.


.SH MULTI-THREADING
The PCRE functions can be used in multi-threading applications, with the
proviso that the character tables and the memory management functions pointed
to by \fBpcre_malloc\fR and \fBpcre_free\fR will be 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 the compiled code block
is returned. 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 defined.
.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. Those options that
are compabible with Perl can also be set at compile time from within the
pattern (see the detailed description of regular expressions below) and all
options except PCRE_EXTENDED and PCRE_EXTRA can be set at the time of matching.
.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
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 in any subject string. 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. By default, it 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. By default, 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 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.

  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. 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_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 two extra features currently
provided, and both are in some sense experimental additions that are useful for
influencing the progress of a match.

  (1) The sequence \\X inserts a Prolog-like "cut" into the expression.

  (2) Once a subpattern enclosed in (?>subpat) brackets has matched,
      backtracking never goes back into the pattern.

See below for further details of both of these.



.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. The only one currently supported is
PCRE_CASELESS. It forces the studying to be done in a caseless manner, even if
the original pattern was compiled without PCRE_CASELESS. When the result of
\fBpcre_study()\fR is passed to \fBpcre_exec()\fR, it is used only if its
caseless state is the same as that of the matching process. A pattern that is
compiled without PCRE_CASELESS can be studied with and without PCRE_CASELESS,
and the appropriate data passed to \fBpcre_exec()\fR with and without the
PCRE_CASELESS flag.

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 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 subject string is passed as a pointer in \fIsubject\fR and a length in
\fIlength\fR. Unlike the pattern string, it may contain binary zero characters.

The options PCRE_ANCHORED, PCRE_CASELESS, PCRE_DOLLAR_ENDONLY, PCRE_DOTALL, and
PCRE_MULTILINE can be passed in the \fIoptions\fR argument, whose unused bits
must be zero. However, if a pattern is compiled with any of these options, they
cannot be unset when it is obeyed.

There are also two 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 either compile or match 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. Setting this without PCRE_MULTILINE (at either compile or
match time) causes dollar never to match.

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. This should always be an even number, because the
elements are used in pairs. If an odd number is passed, it is rounded down.

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.

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, 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.

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_BADREF         (-2)

There was a back-reference in the pattern to a capturing subpattern that had
not previously been set.

  PCRE_ERROR_NULL           (-3)

Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
NULL and \fIovecsize\fR was not zero.

  PCRE_ERROR_BADOPTION      (-4)

An unrecognized bit was set in the \fIoptions\fR argument.

  PCRE_ERROR_BADMAGIC       (-5)

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   (-6)

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       (-7)

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 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.

If the \fIfirstcharptr\fR argument is not NULL, is 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 the
pattern was compiled with the PCRE_MULTILINE option, and every branch started
with "^", then -1 is returned, indicating that the pattern will match at the
start of a subject string or after any "\\n" within the string. Otherwise -2 is
returned.


.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 and assertions, 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 5.004.

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 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. If a backreference can never be matched, PCRE diagnoses an error. In a case
like

  /(123)\\2/

the error occurs at compile time. Perl gives no compile time error; version
5.004 either always fails to match, or gives a segmentation fault at runtime.
In more complicated cases such as

  /(1)(2)(3)(4)(5)(6)(7)(8)(9)(10\\10)/

PCRE returns PCRE_ERROR_BADREF at run time. Perl always fails to match.

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

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

(b) If PCRE_EXTRA is set, the \\X assertion (a Prolog-like "cut") is
recognized, and a backslash followed by a letter with no special meaning is
faulted. There is also a new kind of parenthesized subpattern starting with (?>
which has a block on backtracking into it once it has matched.


.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 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 if the character that follows could otherwise
be taken as another 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". 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 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 backslashed assertions
are

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

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. More complicated
assertions are also supported (see below).

The "\\A" and "\\Z" assertions differ from the traditional "^" and "$"
(described below) in that they only ever match at the very start and end of the
subject string, respectively, whatever options are set.

When the PCRE_EXTRA flag is set on a call to \fBpcre_compile()\fR, the
additional assertion \\X, which has no equivalent in Perl, is recognized.
This operates like the "cut" operation in Prolog: it prevents the matching
operation from backtracking past it. For example, if the expression

  .*/foo

is matched against the string "/foo/this/is/not" then after the greedy .*
has swallowed the whole string, PCRE keeps backtracking all the way to the
beginning before failing. If, on the other hand, the expression is

  .*/\\Xfoo

then once it has discovered that "/not" is not "/foo", backtracking ceases, and
the match fails. See also the section on "once-only" subpatterns below.



.SH CIRCUMFLEX AND DOLLAR
Outside a character class, the circumflex character is an assertion which is
true only if the current matching point is at the start of the subject string,
in the default matching mode. 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.

The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set at compile or matching time. 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. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.

Note that the sequences "\\A" 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.


.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 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 \\.

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.

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
\\ 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 character "]" as the end character of a range, since a
sequence such as [w-] is interpreted as a class of two characters. The octal or
hexadecimal representation of "]" can, however, 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 such as
[W-c] is used when PCRE_CASELESS is set, it matches the letters involved in
either case.

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, the class [^\\W_] matches any letter or digit.

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. The matching
process tries all the alternatives in turn. For example, the pattern

  gilbert|sullivan

matches either "gilbert" or "sullivan". Any number of alternatives can be used,
and an empty alternative is permitted (matching the empty string).


.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.


.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 if there have not been that many previous capturing left parentheses.
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".

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.


.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
  a parenthesized subpattern

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}

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.

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 .* then it is implicitly anchored, since whatever
follows will be tried against every character position in the subject string.
PCRE treats this as though it were preceded by \\A.

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

   (\s*tweedle[dume]{3})+\\1

matches "tweedledum tweedledee tweedledee" but not "tweedledum tweedledee
tweedledum".


.SH ASSERTIONS
An assertion is a test on the characters following the current matching point
that does not actually consume any of those characters. The simple assertions
coded as \\b, \\B, \\A, \\Z, ^ and $ are described above. More complicated
assertions are coded as subpatterns starting 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".

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

Assertions count towards the maximum of 200 parenthesized subpatterns.


.SH ONCE-ONLY SUBPATTERNS
The facility described in this section is available only when the PCRE_EXTRA
option is set at compile time. It is an extension to Perl regular expressions.

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.

For simple cases such as the above example, this feature can be though 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. Contrast with the \\X assertion, which is a Prolog-like
"cut".


.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 INTERNAL FLAG SETTING
If the sequence (?i) occurs anywhere in a pattern, it has the effect of setting
the PCRE_CASELESS option, that is, all letters are matched in a
case-independent manner. The option applies to the whole pattern, not just to
the portion that follows it.

If the sequence (?m) occurs anywhere in a pattern, it has the effect of setting
the PCRE_MULTILINE option, that is, subject strings matched by this pattern are
treated as consisting of multiple lines.

If the sequence (?s) occurs anywhere in a pattern, it has the effect of setting
the PCRE_DOTALL option, so that dot metacharacters match newlines as well as
all other characters.

If the sequence (?x) occurs anywhere in a pattern, it has the effect of setting
the PCRE_EXTENDED option, that is, whitespace is ignored and # introduces a
comment that lasts till the next newline. The option applies to the whole
pattern, not just to the portion that follows it.

If more than one option is required, they can be specified jointly, for example
as (?ix) or (?mi).


.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.

The use of PCRE_MULTILINE causes additional processing and should be avoided
when it is not necessary. Caseless matching of character classes is more
efficient if PCRE_CASELESS is set when the pattern is compiled.


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

Copyright (c) 1997 University of Cambridge.