trunk/doc/pcrepattern.3

.TH PCREPATTERN 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE REGULAR EXPRESSION DETAILS"
.rs
.sp
The syntax and semantics of the regular expressions that are supported by PCRE
are described in detail below. There is a quick-reference syntax summary in the
.\" HREF
\fBpcresyntax\fP
.\"
page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
also supports some alternative regular expression syntax (which does not
conflict with the Perl syntax) in order to provide some compatibility with
regular expressions in Python, .NET, and Oniguruma.
.P
Perl's regular expressions are described in its own documentation, and
regular expressions in general are covered in a number of books, some of which
have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
published by O'Reilly, covers regular expressions in great detail. This
description of PCRE's regular expressions is intended as reference material.
.P
The original operation of PCRE was on strings of one-byte characters. However,
there is now also support for UTF-8 character strings. To use this,
PCRE must be built to include UTF-8 support, and you must call
\fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
is also a special sequence that can be given at the start of a pattern:
.sp
  (*UTF8)
.sp
Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
option. This feature is not Perl-compatible. How setting UTF-8 mode affects
pattern matching is mentioned in several places below. There is also a summary
of UTF-8 features in the
.\" HTML <a href="pcre.html#utf8support">
.\" </a>
section on UTF-8 support
.\"
in the main
.\" HREF
\fBpcre\fP
.\"
page.
.P
The remainder of this document discusses the patterns that are supported by
PCRE when its main matching function, \fBpcre_exec()\fP, is used.
From release 6.0, PCRE offers a second matching function,
\fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
Perl-compatible. Some of the features discussed below are not available when
\fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
alternative function, and how it differs from the normal function, are
discussed in the
.\" HREF
\fBpcrematching\fP
.\"
page.
.
.
.SH "NEWLINE CONVENTIONS"
.rs
.sp
PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (linefeed)
character, the two-character sequence CRLF, any of the three preceding, or any
Unicode newline sequence. The
.\" HREF
\fBpcreapi\fP
.\"
page has
.\" HTML <a href="pcreapi.html#newlines">
.\" </a>
further discussion
.\"
about newlines, and shows how to set the newline convention in the
\fIoptions\fP arguments for the compiling and matching functions.
.P
It is also possible to specify a newline convention by starting a pattern
string with one of the following five sequences:
.sp
  (*CR)        carriage return
  (*LF)        linefeed
  (*CRLF)      carriage return, followed by linefeed
  (*ANYCRLF)   any of the three above
  (*ANY)       all Unicode newline sequences
.sp
These override the default and the options given to \fBpcre_compile()\fP or
\fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
newline sequence, the pattern
.sp
  (*CR)a.b
.sp
changes the convention to CR. That pattern matches "a\enb" because LF is no
longer a newline. Note that these special settings, which are not
Perl-compatible, are recognized only at the very start of a pattern, and that
they must be in upper case. If more than one of them is present, the last one
is used.
.P
The newline convention affects the interpretation of the dot metacharacter when
PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
affect what the \eR escape sequence matches. By default, this is any Unicode
newline sequence, for Perl compatibility. However, this can be changed; see the
description of \eR in the section entitled
.\" HTML <a href="#newlineseq">
.\" </a>
"Newline sequences"
.\"
below. A change of \eR setting can be combined with a change of newline
convention.
.
.
.SH "CHARACTERS AND METACHARACTERS"
.rs
.sp
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
.sp
  The quick brown fox
.sp
matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the PCRE_CASELESS option), letters are matched
independently of case. In UTF-8 mode, PCRE always understands the concept of
case for characters whose values are less than 128, so caseless matching is
always possible. For characters with higher values, the concept of case is
supported if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching for characters 128 and above, you must
ensure that PCRE is compiled with Unicode property support as well as with
UTF-8 support.
.P
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
\fImetacharacters\fP, which do not stand for themselves but instead are
interpreted in some special way.
.P
There are two different sets of metacharacters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized within square brackets. Outside square brackets, the metacharacters
are as follows:
.sp
  \e      general escape character with several uses
  ^      assert start of string (or line, in multiline mode)
  $      assert end of string (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
         also "possessive quantifier"
  {      start min/max quantifier
.sp
Part of a pattern that is in square brackets is called a "character class". In
a character class the only metacharacters are:
.sp
  \e      general escape character
  ^      negate the class, but only if the first character
  -      indicates character range
.\" JOIN
  [      POSIX character class (only if followed by POSIX
           syntax)
  ]      terminates the character class
.sp
The following sections describe the use of each of the metacharacters.
.
.
.SH BACKSLASH
.rs
.sp
The backslash character has several uses. Firstly, if it is followed by a
non-alphanumeric 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.
.P
For example, if you want to match a * character, you write \e* in the pattern.
This escaping action applies whether or not the following character would
otherwise be interpreted as a metacharacter, so it is always safe to precede a
non-alphanumeric with backslash to specify that it stands for itself. In
particular, if you want to match a backslash, you write \e\e.
.P
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 are ignored. An escaping backslash can
be used to include a whitespace or # character as part of the pattern.
.P
If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \eQ and \eE. This is different from Perl in
that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
Perl, $ and @ cause variable interpolation. Note the following examples:
.sp
  Pattern            PCRE matches   Perl matches
.sp
.\" JOIN
  \eQabc$xyz\eE        abc$xyz        abc followed by the
                                      contents of $xyz
  \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
  \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
.sp
The \eQ...\eE sequence is recognized both inside and outside character classes.
.
.
.\" HTML <a name="digitsafterbackslash"></a>
.SS "Non-printing characters"
.rs
.sp
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 often easier to use
one of the following escape sequences than the binary character it represents:
.sp
  \ea        alarm, that is, the BEL character (hex 07)
  \ecx       "control-x", where x is any character
  \ee        escape (hex 1B)
  \ef        formfeed (hex 0C)
  \en        linefeed (hex 0A)
  \er        carriage return (hex 0D)
  \et        tab (hex 09)
  \eddd      character with octal code ddd, or back reference
  \exhh      character with hex code hh
  \ex{hhh..} character with hex code hhh..
.sp
The precise effect of \ecx 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 \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
7B.
.P
After \ex, from zero to two hexadecimal digits are read (letters can be in
upper or lower case). Any number of hexadecimal digits may appear between \ex{
and }, but the value of the character code must be less than 256 in non-UTF-8
mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
point, which is 10FFFF.
.P
If characters other than hexadecimal digits appear between \ex{ and }, or if
there is no terminating }, this form of escape is not recognized. Instead, the
initial \ex will be interpreted as a basic hexadecimal escape, with no
following digits, giving a character whose value is zero.
.P
Characters whose value is less than 256 can be defined by either of the two
syntaxes for \ex. There is no difference in the way they are handled. For
example, \exdc is exactly the same as \ex{dc}.
.P
After \e0 up to two further octal digits are read. If there are fewer than two
digits, just those that are present are used. Thus the sequence \e0\ex\e07
specifies two binary zeros followed by a BEL character (code value 7). Make
sure you supply two digits after the initial zero if the pattern character that
follows is itself an octal digit.
.P
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\fP. A description of how this works is given
.\" HTML <a href="#backreferences">
.\" </a>
later,
.\"
following the discussion of
.\" HTML <a href="#subpattern">
.\" </a>
parenthesized subpatterns.
.\"
.P
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 uses them to generate a data character. Any
subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
character specified in octal must be less than \e400. In UTF-8 mode, values up
to \e777 are permitted. For example:
.sp
  \e040   is another way of writing a space
.\" JOIN
  \e40    is the same, provided there are fewer than 40
            previous capturing subpatterns
  \e7     is always a back reference
.\" JOIN
  \e11    might be a back reference, or another way of
            writing a tab
  \e011   is always a tab
  \e0113  is a tab followed by the character "3"
.\" JOIN
  \e113   might be a back reference, otherwise the
            character with octal code 113
.\" JOIN
  \e377   might be a back reference, otherwise
            the byte consisting entirely of 1 bits
.\" JOIN
  \e81    is either a back reference, or a binary zero
            followed by the two characters "8" and "1"
.sp
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.
.P
All the sequences that define a single character value can be used both inside
and outside character classes. In addition, inside a character class, the
sequence \eb is interpreted as the backspace character (hex 08). The sequences
\eB, \eN, \eR, and \eX are not special inside a character class. Like any other
unrecognized escape sequences, they are treated as the literal characters "B",
"N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
set. Outside a character class, these sequences have different meanings.
.
.
.SS "Absolute and relative back references"
.rs
.sp
The sequence \eg followed by an unsigned or a negative number, optionally
enclosed in braces, is an absolute or relative back reference. A named back
reference can be coded as \eg{name}. Back references are discussed
.\" HTML <a href="#backreferences">
.\" </a>
later,
.\"
following the discussion of
.\" HTML <a href="#subpattern">
.\" </a>
parenthesized subpatterns.
.\"
.
.
.SS "Absolute and relative subroutine calls"
.rs
.sp
For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for referencing a subpattern as a "subroutine". Details are discussed
.\" HTML <a href="#onigurumasubroutines">
.\" </a>
later.
.\"
Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
synonymous. The former is a back reference; the latter is a
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
subroutine
.\"
call.
.
.
.SS "Generic character types"
.rs
.sp
Another use of backslash is for specifying generic character types:
.sp
  \ed     any decimal digit
  \eD     any character that is not a decimal digit
  \eh     any horizontal whitespace character
  \eH     any character that is not a horizontal whitespace character
  \es     any whitespace character
  \eS     any character that is not a whitespace character
  \ev     any vertical whitespace character
  \eV     any character that is not a vertical whitespace character
  \ew     any "word" character
  \eW     any "non-word" character
.sp
There is also the single sequence \eN, which matches a non-newline character. 
This is the same as 
.\" HTML <a href="#fullstopdot">
.\" </a>
the "." metacharacter 
.\"
when PCRE_DOTALL is not set.
.P
Each pair of lower and upper case escape sequences partitions the complete set
of characters into two disjoint sets. Any given character matches one, and only
one, of each pair.
.P
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.
.P
For compatibility with Perl, \es does not match the VT character (code 11).
This makes it different from the the POSIX "space" class. The \es characters
are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
included in a Perl script, \es may match the VT character. In PCRE, it never
does.
.P
In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
\ew, and always match \eD, \eS, and \eW. This is true even when Unicode
character property support is available. These sequences retain their original
meanings from before UTF-8 support was available, mainly for efficiency
reasons. Note that this also affects \eb, because it is defined in terms of \ew
and \eW.
.P
The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
other sequences, these do match certain high-valued codepoints in UTF-8 mode.
The horizontal space characters are:
.sp
  U+0009     Horizontal tab
  U+0020     Space
  U+00A0     Non-break space
  U+1680     Ogham space mark
  U+180E     Mongolian vowel separator
  U+2000     En quad
  U+2001     Em quad
  U+2002     En space
  U+2003     Em space
  U+2004     Three-per-em space
  U+2005     Four-per-em space
  U+2006     Six-per-em space
  U+2007     Figure space
  U+2008     Punctuation space
  U+2009     Thin space
  U+200A     Hair space
  U+202F     Narrow no-break space
  U+205F     Medium mathematical space
  U+3000     Ideographic space
.sp
The vertical space characters are:
.sp
  U+000A     Linefeed
  U+000B     Vertical tab
  U+000C     Formfeed
  U+000D     Carriage return
  U+0085     Next line
  U+2028     Line separator
  U+2029     Paragraph separator
.P
A "word" character is an underscore or any character less than 256 that is a
letter or digit. The definition of letters and digits is controlled by PCRE's
low-valued character tables, and may vary if locale-specific matching is taking
place (see
.\" HTML <a href="pcreapi.html#localesupport">
.\" </a>
"Locale support"
.\"
in the
.\" HREF
\fBpcreapi\fP
.\"
page). For example, in a French locale such as "fr_FR" in Unix-like systems,
or "french" in Windows, some character codes greater than 128 are used for
accented letters, and these are matched by \ew. The use of locales with Unicode
is discouraged.
.
.
.\" HTML <a name="newlineseq"></a>
.SS "Newline sequences"
.rs
.sp
Outside a character class, by default, the escape sequence \eR matches any
Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
equivalent to the following:
.sp
  (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
.sp
This is an example of an "atomic group", details of which are given
.\" HTML <a href="#atomicgroup">
.\" </a>
below.
.\"
This particular group matches either the two-character sequence CR followed by
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
line, U+0085). The two-character sequence is treated as a single unit that
cannot be split.
.P
In UTF-8 mode, two additional characters whose codepoints are greater than 255
are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
Unicode character property support is not needed for these characters to be
recognized.
.P
It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
either at compile time or when the pattern is matched. (BSR is an abbrevation
for "backslash R".) This can be made the default when PCRE is built; if this is
the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
It is also possible to specify these settings by starting a pattern string with
one of the following sequences:
.sp
  (*BSR_ANYCRLF)   CR, LF, or CRLF only
  (*BSR_UNICODE)   any Unicode newline sequence
.sp
These override the default and the options given to \fBpcre_compile()\fP or
\fBpcre_compile2()\fP, but they can be overridden by options given to
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
which are not Perl-compatible, are recognized only at the very start of a
pattern, and that they must be in upper case. If more than one of them is
present, the last one is used. They can be combined with a change of newline
convention, for example, a pattern can start with:
.sp
  (*ANY)(*BSR_ANYCRLF)
.sp
Inside a character class, \eR is treated as an unrecognized escape sequence, 
and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is
set.
.
.
.\" HTML <a name="uniextseq"></a>
.SS Unicode character properties
.rs
.sp
When PCRE is built with Unicode character property support, three additional
escape sequences that match characters with specific properties are available.
When not in UTF-8 mode, these sequences are of course limited to testing
characters whose codepoints are less than 256, but they do work in this mode.
The extra escape sequences are:
.sp
  \ep{\fIxx\fP}   a character with the \fIxx\fP property
  \eP{\fIxx\fP}   a character without the \fIxx\fP property
  \eX       an extended Unicode sequence
.sp
The property names represented by \fIxx\fP above are limited to the Unicode
script names, the general category properties, and "Any", which matches any
character (including newline). Other properties such as "InMusicalSymbols" are
not currently supported by PCRE. Note that \eP{Any} does not match any
characters, so always causes a match failure.
.P
Sets of Unicode characters are defined as belonging to certain scripts. A
character from one of these sets can be matched using a script name. For
example:
.sp
  \ep{Greek}
  \eP{Han}
.sp
Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
.P
Arabic,
Armenian,
Avestan,
Balinese,
Bamum,
Bengali,
Bopomofo,
Braille,
Buginese,
Buhid,
Canadian_Aboriginal,
Carian,
Cham,
Cherokee,
Common,
Coptic,
Cuneiform,
Cypriot,
Cyrillic,
Deseret,
Devanagari,
Egyptian_Hieroglyphs,
Ethiopic,
Georgian,
Glagolitic,
Gothic,
Greek,
Gujarati,
Gurmukhi,
Han,
Hangul,
Hanunoo,
Hebrew,
Hiragana,
Imperial_Aramaic,
Inherited,
Inscriptional_Pahlavi,
Inscriptional_Parthian,
Javanese,
Kaithi,
Kannada,
Katakana,
Kayah_Li,
Kharoshthi,
Khmer,
Lao,
Latin,
Lepcha,
Limbu,
Linear_B,
Lisu,
Lycian,
Lydian,
Malayalam,
Meetei_Mayek,
Mongolian,
Myanmar,
New_Tai_Lue,
Nko,
Ogham,
Old_Italic,
Old_Persian,
Old_South_Arabian,
Old_Turkic,
Ol_Chiki,
Oriya,
Osmanya,
Phags_Pa,
Phoenician,
Rejang,
Runic,
Samaritan,
Saurashtra,
Shavian,
Sinhala,
Sundanese,
Syloti_Nagri,
Syriac,
Tagalog,
Tagbanwa,
Tai_Le,
Tai_Tham,
Tai_Viet,
Tamil,
Telugu,
Thaana,
Thai,
Tibetan,
Tifinagh,
Ugaritic,
Vai,
Yi.
.P
Each character has exactly one general category property, specified by a
two-letter abbreviation. For compatibility with Perl, negation can be specified
by including a circumflex between the opening brace and the property name. For
example, \ep{^Lu} is the same as \eP{Lu}.
.P
If only one letter is specified with \ep or \eP, it includes all the general
category properties that start with that letter. In this case, in the absence
of negation, the curly brackets in the escape sequence are optional; these two
examples have the same effect:
.sp
  \ep{L}
  \epL
.sp
The following general category property codes are supported:
.sp
  C     Other
  Cc    Control
  Cf    Format
  Cn    Unassigned
  Co    Private use
  Cs    Surrogate
.sp
  L     Letter
  Ll    Lower case letter
  Lm    Modifier letter
  Lo    Other letter
  Lt    Title case letter
  Lu    Upper case letter
.sp
  M     Mark
  Mc    Spacing mark
  Me    Enclosing mark
  Mn    Non-spacing mark
.sp
  N     Number
  Nd    Decimal number
  Nl    Letter number
  No    Other number
.sp
  P     Punctuation
  Pc    Connector punctuation
  Pd    Dash punctuation
  Pe    Close punctuation
  Pf    Final punctuation
  Pi    Initial punctuation
  Po    Other punctuation
  Ps    Open punctuation
.sp
  S     Symbol
  Sc    Currency symbol
  Sk    Modifier symbol
  Sm    Mathematical symbol
  So    Other symbol
.sp
  Z     Separator
  Zl    Line separator
  Zp    Paragraph separator
  Zs    Space separator
.sp
The special property L& is also supported: it matches a character that has
the Lu, Ll, or Lt property, in other words, a letter that is not classified as
a modifier or "other".
.P
The Cs (Surrogate) property applies only to characters in the range U+D800 to
U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
(see the discussion of PCRE_NO_UTF8_CHECK in the
.\" HREF
\fBpcreapi\fP
.\"
page). Perl does not support the Cs property.
.P
The long synonyms for property names that Perl supports (such as \ep{Letter})
are not supported by PCRE, nor is it permitted to prefix any of these
properties with "Is".
.P
No character that is in the Unicode table has the Cn (unassigned) property.
Instead, this property is assumed for any code point that is not in the
Unicode table.
.P
Specifying caseless matching does not affect these escape sequences. For
example, \ep{Lu} always matches only upper case letters.
.P
The \eX escape matches any number of Unicode characters that form an extended
Unicode sequence. \eX is equivalent to
.sp
  (?>\ePM\epM*)
.sp
That is, it matches a character without the "mark" property, followed by zero
or more characters with the "mark" property, and treats the sequence as an
atomic group
.\" HTML <a href="#atomicgroup">
.\" </a>
(see below).
.\"
Characters with the "mark" property are typically accents that affect the
preceding character. None of them have codepoints less than 256, so in
non-UTF-8 mode \eX matches any one character.
.P
Matching characters by Unicode property is not fast, because PCRE has to search
a structure that contains data for over fifteen thousand characters. That is
why the traditional escape sequences such as \ed and \ew do not use Unicode
properties in PCRE.
.
.
.\" HTML <a name="resetmatchstart"></a>
.SS "Resetting the match start"
.rs
.sp
The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
matched characters not to be included in the final matched sequence. For
example, the pattern:
.sp
  foo\eKbar
.sp
matches "foobar", but reports that it has matched "bar". This feature is
similar to a lookbehind assertion
.\" HTML <a href="#lookbehind">
.\" </a>
(described below).
.\"
However, in this case, the part of the subject before the real match does not
have to be of fixed length, as lookbehind assertions do. The use of \eK does
not interfere with the setting of
.\" HTML <a href="#subpattern">
.\" </a>
captured substrings.
.\"
For example, when the pattern
.sp
  (foo)\eKbar
.sp
matches "foobar", the first substring is still set to "foo".
.P
Perl documents that the use of \eK within assertions is "not well defined". In
PCRE, \eK is acted upon when it occurs inside positive assertions, but is
ignored in negative assertions.
.
.
.\" HTML <a name="smallassertions"></a>
.SS "Simple assertions"
.rs
.sp
The final 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
.\" HTML <a href="#bigassertions">
.\" </a>
below.
.\"
The backslashed assertions are:
.sp
  \eb     matches at a word boundary
  \eB     matches when not at a word boundary
  \eA     matches at the start of the subject
  \eZ     matches at the end of the subject
          also matches before a newline at the end of the subject
  \ez     matches only at the end of the subject
  \eG     matches at the first matching position in the subject
.sp
Inside a character class, \eb has a different meaning; it matches the backspace
character. If any other of these assertions appears in a character class, by 
default it matches the corresponding literal character (for example, \eB
matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
escape sequence" error is generated instead.
.P
A word boundary is a position in the subject string where the current character
and the previous character do not both match \ew or \eW (i.e. one matches
\ew and the other matches \eW), or the start or end of the string if the
first or last character matches \ew, respectively. Neither PCRE nor Perl has a
separte "start of word" or "end of word" metasequence. However, whatever
follows \eb normally determines which it is. For example, the fragment
\eba matches "a" at the start of a word.
.P
The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
dollar (described in the next section) in that they only ever match at the very
start and end of the subject string, whatever options are set. Thus, they are
independent of multiline mode. These three assertions are not affected by the
PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
at a point other than the beginning of the subject, \eA can never match. The
difference between \eZ and \ez is that \eZ matches before a newline at the end
of the string as well as at the very end, whereas \ez matches only at the end.
.P
The \eG assertion is true only when the current matching position is at the
start point of the match, as specified by the \fIstartoffset\fP argument of
\fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
arguments, you can mimic Perl's /g option, and it is in this kind of
implementation where \eG can be useful.
.P
Note, however, that PCRE's interpretation of \eG, as the start of the current
match, is subtly different from Perl's, which defines it as the end of the
previous match. In Perl, these can be different when the previously matched
string was empty. Because PCRE does just one match at a time, it cannot
reproduce this behaviour.
.P
If all the alternatives of a pattern begin with \eG, the expression is anchored
to the starting match position, and the "anchored" flag is set in the compiled
regular expression.
.
.
.SH "CIRCUMFLEX AND DOLLAR"
.rs
.sp
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point is
at the start of the subject string. If the \fIstartoffset\fP argument of
\fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
option is unset. Inside a character class, circumflex has an entirely different
meaning
.\" HTML <a href="#characterclass">
.\" </a>
(see below).
.\"
.P
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.)
.P
A dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline
at the end of 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.
.P
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 time. This
does not affect the \eZ assertion.
.P
The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, a circumflex matches
immediately after internal newlines as well as at the start of the subject
string. It does not match after a newline that ends the string. A dollar
matches before any newlines in the string, as well as at the very end, when
PCRE_MULTILINE is set. When newline is specified as the two-character
sequence CRLF, isolated CR and LF characters do not indicate newlines.
.P
For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
\en represents a newline) 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\fP argument of \fBpcre_exec()\fP is non-zero. The
PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
.P
Note that the sequences \eA, \eZ, and \ez can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\eA it is always anchored, whether or not PCRE_MULTILINE is set.
.
.
.\" HTML <a name="fullstopdot"></a>
.SH "FULL STOP (PERIOD, DOT) AND \eN"
.rs
.sp
Outside a character class, a dot in the pattern matches any one character in
the subject string except (by default) a character that signifies the end of a
line. In UTF-8 mode, the matched character may be more than one byte long.
.P
When a line ending is defined as a single character, dot never matches that
character; when the two-character sequence CRLF is used, dot does not match CR
if it is immediately followed by LF, but otherwise it matches all characters
(including isolated CRs and LFs). When any Unicode line endings are being
recognized, dot does not match CR or LF or any of the other line ending
characters.
.P
The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
option is set, a dot matches any one character, without exception. If the
two-character sequence CRLF is present in the subject string, it takes two dots
to match it.
.P
The handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newlines. Dot has no
special meaning in a character class.
.P
The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not 
set. In other words, it matches any one character except one that signifies the 
end of a line.
.
.
.SH "MATCHING A SINGLE BYTE"
.rs
.sp
Outside a character class, the escape sequence \eC matches any one byte, both
in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
characters. The feature is provided in Perl in order to match individual bytes
in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
what remains in the string may be a malformed UTF-8 string. For this reason,
the \eC escape sequence is best avoided.
.P
PCRE does not allow \eC to appear in lookbehind assertions
.\" HTML <a href="#lookbehind">
.\" </a>
(described below),
.\"
because in UTF-8 mode this would make it impossible to calculate the length of
the lookbehind.
.
.
.\" HTML <a name="characterclass"></a>
.SH "SQUARE BRACKETS AND CHARACTER CLASSES"
.rs
.sp
An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special by default.
However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
bracket causes a compile-time error. 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.
.P
A character class matches a single character in the subject. In UTF-8 mode, the
character may be more than one byte long. A matched character must be in the
set of characters defined by the class, unless the first character in the class
definition 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.
.P
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 that
are in the class by enumerating those that are not. A class that starts with a
circumflex is not an assertion; it still consumes a character from the subject
string, and therefore it fails if the current pointer is at the end of the
string.
.P
In UTF-8 mode, characters with values greater than 255 can be included in a
class as a literal string of bytes, or by using the \ex{ escaping mechanism.
.P
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. In UTF-8 mode, PCRE always understands the concept of
case for characters whose values are less than 128, so caseless matching is
always possible. For characters with higher values, the concept of case is
supported if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching in UTF8-mode for characters 128 and above,
you must ensure that PCRE is compiled with Unicode property support as well as
with UTF-8 support.
.P
Characters that might indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending sequence is in use, and
whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
such as [^a] always matches one of these characters.
.P
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.
.P
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-\e]46] is interpreted as a class containing a range
followed by two other characters. The octal or hexadecimal representation of
"]" can also be used to end a range.
.P
Ranges operate in the collating sequence of character values. They can also be
used for characters specified numerically, for example [\e000-\e037]. In UTF-8
mode, ranges can include characters whose values are greater than 255, for
example [\ex{100}-\ex{2ff}].
.P
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
[][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
tables for a French locale are in use, [\exc8-\excb] matches accented E
characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
characters with values greater than 128 only when it is compiled with Unicode
property support.
.P
The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
in a character class, and add the characters that they match to the class. For
example, [\edABCDEF] 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 [^\eW_] matches any letter or digit, but not underscore.
.P
The only metacharacters that are recognized in character classes are backslash,
hyphen (only where it can be interpreted as specifying a range), circumflex
(only at the start), opening square bracket (only when it can be interpreted as
introducing a POSIX class name - see the next section), and the terminating
closing square bracket. However, escaping other non-alphanumeric characters
does no harm.
.
.
.SH "POSIX CHARACTER CLASSES"
.rs
.sp
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also supports
this notation. For example,
.sp
  [01[:alpha:]%]
.sp
matches "0", "1", any alphabetic character, or "%". The supported class names
are
.sp
  alnum    letters and digits
  alpha    letters
  ascii    character codes 0 - 127
  blank    space or tab only
  cntrl    control characters
  digit    decimal digits (same as \ed)
  graph    printing characters, excluding space
  lower    lower case letters
  print    printing characters, including space
  punct    printing characters, excluding letters and digits
  space    white space (not quite the same as \es)
  upper    upper case letters
  word     "word" characters (same as \ew)
  xdigit   hexadecimal digits
.sp
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32). Notice that this list includes the VT character (code 11). This
makes "space" different to \es, which does not include VT (for Perl
compatibility).
.P
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
5.8. Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,
.sp
  [12[:^digit:]]
.sp
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
supported, and an error is given if they are encountered.
.P
In UTF-8 mode, characters with values greater than 128 do not match any of
the POSIX character classes.
.
.
.SH "VERTICAL BAR"
.rs
.sp
Vertical bar characters are used to separate alternative patterns. For example,
the pattern
.sp
  gilbert|sullivan
.sp
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
.\" HTML <a href="#subpattern">
.\" </a>
(defined below),
.\"
"succeeds" means matching the rest of the main pattern as well as the
alternative in the subpattern.
.
.
.SH "INTERNAL OPTION SETTING"
.rs
.sp
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
The option letters are
.sp
  i  for PCRE_CASELESS
  m  for PCRE_MULTILINE
  s  for PCRE_DOTALL
  x  for PCRE_EXTENDED
.sp
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.
.P
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
changed in the same way as the Perl-compatible options by using the characters
J, U and X respectively.
.P
When one of these option changes occurs at top level (that is, not inside
subpattern parentheses), the change applies to the remainder of the pattern
that follows. If the change is placed right at the start of a pattern, PCRE
extracts it into the global options (and it will therefore show up in data
extracted by the \fBpcre_fullinfo()\fP function).
.P
An option change within a subpattern (see below for a description of
subpatterns) affects only that part of the current pattern that follows it, so
.sp
  (a(?i)b)c
.sp
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,
.sp
  (a(?i)b|c)
.sp
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.
.P
\fBNote:\fP There are other PCRE-specific options that can be set by the
application when the compile or match functions are called. In some cases the
pattern can contain special leading sequences such as (*CRLF) to override what
the application has set or what has been defaulted. Details are given in the
section entitled
.\" HTML <a href="#newlineseq">
.\" </a>
"Newline sequences"
.\"
above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
mode; this is equivalent to setting the PCRE_UTF8 option.
.
.
.\" HTML <a name="subpattern"></a>
.SH SUBPATTERNS
.rs
.sp
Subpatterns are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into a subpattern does two things:
.sp
1. It localizes a set of alternatives. For example, the pattern
.sp
  cat(aract|erpillar|)
.sp
matches one of the words "cat", "cataract", or "caterpillar". Without the
parentheses, it would match "cataract", "erpillar" or an empty string.
.sp
2. It sets up the subpattern as a capturing subpattern. This means that, when
the whole pattern matches, that portion of the subject string that matched the
subpattern is passed back to the caller via the \fIovector\fP argument of
\fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
from 1) to obtain numbers for the capturing subpatterns.
.P
For example, if the string "the red king" is matched against the pattern
.sp
  the ((red|white) (king|queen))
.sp
the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3, respectively.
.P
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 a question mark
and a colon, 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
.sp
  the ((?:red|white) (king|queen))
.sp
the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of capturing subpatterns is 65535.
.P
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
.sp
  (?i:saturday|sunday)
  (?:(?i)saturday|sunday)
.sp
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".
.
.
.\" HTML <a name="dupsubpatternnumber"></a>
.SH "DUPLICATE SUBPATTERN NUMBERS"
.rs
.sp
Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
the same numbers for its capturing parentheses. Such a subpattern starts with
(?| and is itself a non-capturing subpattern. For example, consider this
pattern:
.sp
  (?|(Sat)ur|(Sun))day
.sp
Because the two alternatives are inside a (?| group, both sets of capturing
parentheses are numbered one. Thus, when the pattern matches, you can look
at captured substring number one, whichever alternative matched. This construct
is useful when you want to capture part, but not all, of one of a number of
alternatives. Inside a (?| group, parentheses are numbered as usual, but the
number is reset at the start of each branch. The numbers of any capturing
buffers that follow the subpattern start after the highest number used in any
branch. The following example is taken from the Perl documentation.
The numbers underneath show in which buffer the captured content will be
stored.
.sp
  # before  ---------------branch-reset----------- after
  / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
  # 1            2         2  3        2     3     4
.sp
A back reference to a numbered subpattern uses the most recent value that is
set for that number by any subpattern. The following pattern matches "abcabc"
or "defdef":
.sp
  /(?|(abc)|(def))\e1/
.sp
In contrast, a recursive or "subroutine" call to a numbered subpattern always
refers to the first one in the pattern with the given number. The following
pattern matches "abcabc" or "defabc":
.sp
  /(?|(abc)|(def))(?1)/
.sp
If a
.\" HTML <a href="#conditions">
.\" </a>
condition test
.\"
for a subpattern's having matched refers to a non-unique number, the test is
true if any of the subpatterns of that number have matched.
.P
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
.
.
.SH "NAMED SUBPATTERNS"
.rs
.sp
Identifying capturing parentheses by number is simple, but it can be very hard
to keep track of the numbers in complicated regular expressions. Furthermore,
if an expression is modified, the numbers may change. To help with this
difficulty, PCRE supports the naming of subpatterns. This feature was not
added to Perl until release 5.10. Python had the feature earlier, and PCRE
introduced it at release 4.0, using the Python syntax. PCRE now supports both
the Perl and the Python syntax. Perl allows identically numbered subpatterns to
have different names, but PCRE does not.
.P
In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
parentheses from other parts of the pattern, such as
.\" HTML <a href="#backreferences">
.\" </a>
back references,
.\"
.\" HTML <a href="#recursion">
.\" </a>
recursion,
.\"
and
.\" HTML <a href="#conditions">
.\" </a>
conditions,
.\"
can be made by name as well as by number.
.P
Names consist of up to 32 alphanumeric characters and underscores. Named
capturing parentheses are still allocated numbers as well as names, exactly as
if the names were not present. The PCRE API provides function calls for
extracting the name-to-number translation table from a compiled pattern. There
is also a convenience function for extracting a captured substring by name.
.P
By default, a name must be unique within a pattern, but it is possible to relax
this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
names are also always permitted for subpatterns with the same number, set up as
described in the previous section.) Duplicate names can be useful for patterns
where only one instance of the named parentheses can match. Suppose you want to
match the name of a weekday, either as a 3-letter abbreviation or as the full
name, and in both cases you want to extract the abbreviation. This pattern
(ignoring the line breaks) does the job:
.sp
  (?<DN>Mon|Fri|Sun)(?:day)?|
  (?<DN>Tue)(?:sday)?|
  (?<DN>Wed)(?:nesday)?|
  (?<DN>Thu)(?:rsday)?|
  (?<DN>Sat)(?:urday)?
.sp
There are five capturing substrings, but only one is ever set after a match.
(An alternative way of solving this problem is to use a "branch reset"
subpattern, as described in the previous section.)
.P
The convenience function for extracting the data by name returns the substring
for the first (and in this example, the only) subpattern of that name that
matched. This saves searching to find which numbered subpattern it was.
.P
If you make a back reference to a non-unique named subpattern from elsewhere in
the pattern, the one that corresponds to the first occurrence of the name is
used. In the absence of duplicate numbers (see the previous section) this is
the one with the lowest number. If you use a named reference in a condition
test (see the
.\"
.\" HTML <a href="#conditions">
.\" </a>
section about conditions
.\"
below), either to check whether a subpattern has matched, or to check for
recursion, all subpatterns with the same name are tested. If the condition is
true for any one of them, the overall condition is true. This is the same
behaviour as testing by number. For further details of the interfaces for
handling named subpatterns, see the
.\" HREF
\fBpcreapi\fP
.\"
documentation.
.P
\fBWarning:\fP You cannot use different names to distinguish between two
subpatterns with the same number because PCRE uses only the numbers when
matching. For this reason, an error is given at compile time if different names
are given to subpatterns with the same number. However, you can give the same
name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
.
.
.SH REPETITION
.rs
.sp
Repetition is specified by quantifiers, which can follow any of the following
items:
.sp
  a literal data character
  the dot metacharacter
  the \eC escape sequence
  the \eX escape sequence (in UTF-8 mode with Unicode properties)
  the \eR escape sequence
  an escape such as \ed that matches a single character
  a character class
  a back reference (see next section)
  a parenthesized subpattern (unless it is an assertion)
  a recursive or "subroutine" call to a subpattern
.sp
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:
.sp
  z{2,4}
.sp
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
.sp
  [aeiou]{3,}
.sp
matches at least 3 successive vowels, but may match many more, while
.sp
  \ed{8}
.sp
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.
.P
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
which is represented by a two-byte sequence. Similarly, when Unicode property
support is available, \eX{3} matches three Unicode extended sequences, each of
which may be several bytes long (and they may be of different lengths).
.P
The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present. This may be useful for
subpatterns that are referenced as
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
subroutines
.\"
from elsewhere in the pattern. Items other than subpatterns that have a {0}
quantifier are omitted from the compiled pattern.
.P
For convenience, the three most common quantifiers have single-character
abbreviations:
.sp
  *    is equivalent to {0,}
  +    is equivalent to {1,}
  ?    is equivalent to {0,1}
.sp
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:
.sp
  (a?)*
.sp
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.
.P
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 /* and */
and within the comment, individual * and / characters may appear. An attempt to
match C comments by applying the pattern
.sp
  /\e*.*\e*/
.sp
to the string
.sp
  /* first comment */  not comment  /* second comment */
.sp
fails, because it matches the entire string owing to the greediness of the .*
item.
.P
However, if a quantifier is followed by a question mark, it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern
.sp
  /\e*.*?\e*/
.sp
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
.sp
  \ed??\ed
.sp
which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.
.P
If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
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.
.P
When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.
.P
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
to Perl's /s) is set, thus allowing the dot to match newlines, 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 normally treats such a
pattern as though it were preceded by \eA.
.P
In cases where it is known that the subject string contains no newlines, it is
worth setting PCRE_DOTALL in order to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.
.P
However, there is one situation where the optimization cannot be used. When .*
is inside capturing parentheses that are the subject of a back reference
elsewhere in the pattern, a match at the start may fail where a later one
succeeds. Consider, for example:
.sp
  (.*)abc\e1
.sp
If the subject is "xyz123abc123" the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.
.P
When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after
.sp
  (tweedle[dume]{3}\es*)+
.sp
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
.sp
  /(a|(b))+/
.sp
matches "aba" the value of the second captured substring is "b".
.
.
.\" HTML <a name="atomicgroup"></a>
.SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
.rs
.sp
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
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.
.P
Consider, for example, the pattern \ed+foo when applied to the subject line
.sp
  123456bar
.sp
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 \ed+
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(a term taken from Jeffrey Friedl's book) provides the means for specifying
that once a subpattern has matched, it is not to be re-evaluated in this way.
.P
If we use atomic grouping for the previous example, the matcher gives up
immediately on failing to match "foo" the first time. The notation is a kind of
special parenthesis, starting with (?> as in this example:
.sp
  (?>\ed+)foo
.sp
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.
.P
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.
.P
Atomic grouping 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 \ed+ and \ed+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\ed+) can only match an entire sequence of digits.
.P
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional + character following a quantifier. Using this notation, the
previous example can be rewritten as
.sp
  \ed++foo
.sp
Note that a possessive quantifier can be used with an entire group, for
example:
.sp
  (abc|xyz){2,3}+
.sp
Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
option is ignored. They are a convenient notation for the simpler forms of
atomic group. However, there is no difference in the meaning of a possessive
quantifier and the equivalent atomic group, though there may be a performance
difference; possessive quantifiers should be slightly faster.
.P
The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
Jeffrey Friedl originated the idea (and the name) in the first edition of his
book. Mike McCloskey liked it, so implemented it when he built Sun's Java
package, and PCRE copied it from there. It ultimately found its way into Perl
at release 5.10.
.P
PCRE has an optimization that automatically "possessifies" certain simple
pattern constructs. For example, the sequence A+B is treated as A++B because
there is no point in backtracking into a sequence of A's when B must follow.
.P
When a pattern contains an unlimited repeat inside a subpattern that can itself
be repeated an unlimited number of times, the use of an atomic group is the
only way to avoid some failing matches taking a very long time indeed. The
pattern
.sp
  (\eD+|<\ed+>)*[!?]
.sp
matches an unlimited number of substrings that either consist of non-digits, or
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to
.sp
  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
.sp
it takes a long time before reporting failure. This is because the string can
be divided between the internal \eD+ repeat and the external * repeat in a
large number of ways, and all have to be tried. (The example uses [!?] rather
than a single character at the end, because both PCRE and Perl have an
optimization that allows for fast failure when a single character is used. They
remember the last single character that is required for a match, and fail early
if it is not present in the string.) If the pattern is changed so that it uses
an atomic group, like this:
.sp
  ((?>\eD+)|<\ed+>)*[!?]
.sp
sequences of non-digits cannot be broken, and failure happens quickly.
.
.
.\" HTML <a name="backreferences"></a>
.SH "BACK REFERENCES"
.rs
.sp
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
(that is, to its left) in the pattern, provided there have been that many
previous capturing left parentheses.
.P
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. A "forward back reference" of this type can make sense
when a repetition is involved and the subpattern to the right has participated
in an earlier iteration.
.P
It is not possible to have a numerical "forward back reference" to a subpattern
whose number is 10 or more using this syntax because a sequence such as \e50 is
interpreted as a character defined in octal. See the subsection entitled
"Non-printing characters"
.\" HTML <a href="#digitsafterbackslash">
.\" </a>
above
.\"
for further details of the handling of digits following a backslash. There is
no such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
.P
Another way of avoiding the ambiguity inherent in the use of digits following a
backslash is to use the \eg escape sequence, which is a feature introduced in
Perl 5.10. This escape must be followed by an unsigned number or a negative
number, optionally enclosed in braces. These examples are all identical:
.sp
  (ring), \e1
  (ring), \eg1
  (ring), \eg{1}
.sp
An unsigned number specifies an absolute reference without the ambiguity that
is present in the older syntax. It is also useful when literal digits follow
the reference. A negative number is a relative reference. Consider this
example:
.sp
  (abc(def)ghi)\eg{-1}
.sp
The sequence \eg{-1} is a reference to the most recently started capturing
subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
would be equivalent to \e1. The use of relative references can be helpful in
long patterns, and also in patterns that are created by joining together
fragments that contain references within themselves.
.P
A back reference matches whatever actually matched the capturing subpattern in
the current subject string, rather than anything matching the subpattern
itself (see
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"Subpatterns as subroutines"
.\"
below for a way of doing that). So the pattern
.sp
  (sens|respons)e and \e1ibility
.sp
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, the case of letters is relevant. For example,
.sp
  ((?i)rah)\es+\e1
.sp
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly.
.P
There are several different ways of writing back references to named
subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
\ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
back reference syntax, in which \eg can be used for both numeric and named
references, is also supported. We could rewrite the above example in any of
the following ways:
.sp
  (?<p1>(?i)rah)\es+\ek<p1>
  (?'p1'(?i)rah)\es+\ek{p1}
  (?P<p1>(?i)rah)\es+(?P=p1)
  (?<p1>(?i)rah)\es+\eg{p1}
.sp
A subpattern that is referenced by name may appear in the pattern before or
after the reference.
.P
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail by default. For example, the pattern
.sp
  (a|(bc))\e2
.sp
always fails if it starts to match "a" rather than "bc". However, if the
PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
unset value matches an empty string.
.P
Because there may be many capturing parentheses in a pattern, all digits
following a backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be used to
terminate the back reference. If the PCRE_EXTENDED option is set, this can be
whitespace. Otherwise, the \eg{ syntax or an empty comment (see
.\" HTML <a href="#comments">
.\" </a>
"Comments"
.\"
below) can be used.
.
.SS "Recursive back references"
.rs
.sp
A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\e1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern
.sp
  (a|b\e1)+
.sp
matches any number of "a"s and also "aba", "ababbaa" 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.
.P
Back references of this type cause the group that they reference to be treated
as an
.\" HTML <a href="#atomicgroup">
.\" </a>
atomic group.
.\"
Once the whole group has been matched, a subsequent matching failure cannot
cause backtracking into the middle of the group.
.
.
.\" HTML <a name="bigassertions"></a>
.SH ASSERTIONS
.rs
.sp
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 \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
.\" HTML <a href="#smallassertions">
.\" </a>
above.
.\"
.P
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.
.P
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.
.
.
.SS "Lookahead assertions"
.rs
.sp
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
.sp
  \ew+(?=;)
.sp
matches a word followed by a semicolon, but does not include the semicolon in
the match, and
.sp
  foo(?!bar)
.sp
matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern
.sp
  (?!foo)bar
.sp
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 the other effect.
.P
If you want to force a matching failure at some point in a pattern, the most
convenient way to do it is with (?!) because an empty string always matches, so
an assertion that requires there not to be an empty string must always fail.
The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
synonym for (?!).
.
.
.\" HTML <a name="lookbehind"></a>
.SS "Lookbehind assertions"
.rs
.sp
Lookbehind assertions start with (?<= for positive assertions and (?<! for
negative assertions. For example,
.sp
  (?<!foo)bar
.sp
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 top-level alternatives, they
do not all have to have the same fixed length. Thus
.sp
  (?<=bullock|donkey)
.sp
is permitted, but
.sp
  (?<!dogs?|cats?)
.sp
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 (5.8 and 5.10), which requires all branches to
match the same length of string. An assertion such as
.sp
  (?<=ab(c|de))
.sp
is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable to PCRE if rewritten to use two top-level
branches:
.sp
  (?<=abc|abde)
.sp
In some cases, the Perl 5.10 escape sequence \eK
.\" HTML <a href="#resetmatchstart">
.\" </a>
(see above)
.\"
can be used instead of a lookbehind assertion to get round the fixed-length
restriction.
.P
The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed length and then try to
match. If there are insufficient characters before the current position, the
assertion fails.
.P
PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
to appear in lookbehind assertions, because it makes it impossible to calculate
the length of the lookbehind. The \eX and \eR escapes, which can match
different numbers of bytes, are also not permitted.
.P
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"Subroutine"
.\"
calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
as the subpattern matches a fixed-length string.
.\" HTML <a href="#recursion">
.\" </a>
Recursion,
.\"
however, is not supported.
.P
Possessive quantifiers can be used in conjunction with lookbehind assertions to
specify efficient matching of fixed-length strings at the end of subject
strings. Consider a simple pattern such as
.sp
  abcd$
.sp
when applied to a long string that does not match. 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
.sp
  ^.*abcd$
.sp
the initial .* matches the entire string at first, but when this fails (because
there is no following "a"), 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
.sp
  ^.*+(?<=abcd)
.sp
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.
.
.
.SS "Using multiple assertions"
.rs
.sp
Several assertions (of any sort) may occur in succession. For example,
.sp
  (?<=\ed{3})(?<!999)foo
.sp
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, and then there is a check that the same three characters are not "999".
This pattern does \fInot\fP 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
.sp
  (?<=\ed{3}...)(?<!999)foo
.sp
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".
.P
Assertions can be nested in any combination. For example,
.sp
  (?<=(?<!foo)bar)baz
.sp
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo", while
.sp
  (?<=\ed{3}(?!999)...)foo
.sp
is another pattern that matches "foo" preceded by three digits and any three
characters that are not "999".
.
.
.\" HTML <a name="conditions"></a>
.SH "CONDITIONAL SUBPATTERNS"
.rs
.sp
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 specific capturing subpattern has
already been matched. The two possible forms of conditional subpattern are:
.sp
  (?(condition)yes-pattern)
  (?(condition)yes-pattern|no-pattern)
.sp
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.
.P
There are four kinds of condition: references to subpatterns, references to
recursion, a pseudo-condition called DEFINE, and assertions.
.
.SS "Checking for a used subpattern by number"
.rs
.sp
If the text between the parentheses consists of a sequence of digits, the
condition is true if a capturing subpattern of that number has previously
matched. If there is more than one capturing subpattern with the same number
(see the earlier
.\"
.\" HTML <a href="#recursion">
.\" </a>
section about duplicate subpattern numbers),
.\"
the condition is true if any of them have been set. An alternative notation is
to precede the digits with a plus or minus sign. In this case, the subpattern
number is relative rather than absolute. The most recently opened parentheses
can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
looping constructs it can also make sense to refer to subsequent groups with
constructs such as (?(+2).
.P
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:
.sp
  ( \e( )?    [^()]+    (?(1) \e) )
.sp
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.
.P
If you were embedding this pattern in a larger one, you could use a relative
reference:
.sp
  ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
.sp
This makes the fragment independent of the parentheses in the larger pattern.
.
.SS "Checking for a used subpattern by name"
.rs
.sp
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
subpattern by name. For compatibility with earlier versions of PCRE, which had
this facility before Perl, the syntax (?(name)...) is also recognized. However,
there is a possible ambiguity with this syntax, because subpattern names may
consist entirely of digits. PCRE looks first for a named subpattern; if it
cannot find one and the name consists entirely of digits, PCRE looks for a
subpattern of that number, which must be greater than zero. Using subpattern
names that consist entirely of digits is not recommended.
.P
Rewriting the above example to use a named subpattern gives this:
.sp
  (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
.sp
If the name used in a condition of this kind is a duplicate, the test is
applied to all subpatterns of the same name, and is true if any one of them has
matched.
.
.SS "Checking for pattern recursion"
.rs
.sp
If the condition is the string (R), and there is no subpattern with the name R,
the condition is true if a recursive call to the whole pattern or any
subpattern has been made. If digits or a name preceded by ampersand follow the
letter R, for example:
.sp
  (?(R3)...) or (?(R&name)...)
.sp
the condition is true if the most recent recursion is into a subpattern whose
number or name is given. This condition does not check the entire recursion
stack. If the name used in a condition of this kind is a duplicate, the test is
applied to all subpatterns of the same name, and is true if any one of them is
the most recent recursion.
.P
At "top level", all these recursion test conditions are false.
.\" HTML <a href="#recursion">
.\" </a>
The syntax for recursive patterns
.\"
is described below.
.
.SS "Defining subpatterns for use by reference only"
.rs
.sp
If the condition is the string (DEFINE), and there is no subpattern with the
name DEFINE, the condition is always false. In this case, there may be only one
alternative in the subpattern. It is always skipped if control reaches this
point in the pattern; the idea of DEFINE is that it can be used to define
"subroutines" that can be referenced from elsewhere. (The use of
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"subroutines"
.\"
is described below.) For example, a pattern to match an IPv4 address could be
written like this (ignore whitespace and line breaks):
.sp
  (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
  \eb (?&byte) (\e.(?&byte)){3} \eb
.sp
The first part of the pattern is a DEFINE group inside which a another group
named "byte" is defined. This matches an individual component of an IPv4
address (a number less than 256). When matching takes place, this part of the
pattern is skipped because DEFINE acts like a false condition. The rest of the
pattern uses references to the named group to match the four dot-separated
components of an IPv4 address, insisting on a word boundary at each end.
.
.SS "Assertion conditions"
.rs
.sp
If the condition is not in any of the above formats, 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:
.sp
  (?(?=[^a-z]*[a-z])
  \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
.sp
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.
.
.
.\" HTML <a name="comments"></a>
.SH COMMENTS
.rs
.sp
The sequence (?# marks the start of a comment that 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.
.P
If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues to immediately after the
next newline in the pattern.
.
.
.\" HTML <a name="recursion"></a>
.SH "RECURSIVE PATTERNS"
.rs
.sp
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best that can
be done is to use a pattern that matches up to some fixed depth of nesting. It
is not possible to handle an arbitrary nesting depth.
.P
For some time, Perl has provided a facility that allows regular expressions to
recurse (amongst other things). It does this by interpolating Perl code in the
expression at run time, and the code can refer to the expression itself. A Perl
pattern using code interpolation to solve the parentheses problem can be
created like this:
.sp
  $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
.sp
The (?p{...}) item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears.
.P
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual subpattern recursion. After its introduction in PCRE and Python,
this kind of recursion was subsequently introduced into Perl at release 5.10.
.P
A special item that consists of (? followed by a number greater than zero and a
closing parenthesis is a recursive call of the subpattern of the given number,
provided that it occurs inside that subpattern. (If not, it is a
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"subroutine"
.\"
call, which is described in the next section.) The special item (?R) or (?0) is
a recursive call of the entire regular expression.
.P
This PCRE pattern solves the nested parentheses problem (assume the
PCRE_EXTENDED option is set so that white space is ignored):
.sp
  \e( ( [^()]++ | (?R) )* \e)
.sp
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a recursive
match of the pattern itself (that is, a correctly parenthesized substring).
Finally there is a closing parenthesis. Note the use of a possessive quantifier
to avoid backtracking into sequences of non-parentheses.
.P
If this were part of a larger pattern, you would not want to recurse the entire
pattern, so instead you could use this:
.sp
  ( \e( ( [^()]++ | (?1) )* \e) )
.sp
We have put the pattern into parentheses, and caused the recursion to refer to
them instead of the whole pattern.
.P
In a larger pattern, keeping track of parenthesis numbers can be tricky. This
is made easier by the use of relative references (a Perl 5.10 feature).
Instead of (?1) in the pattern above you can write (?-2) to refer to the second
most recently opened parentheses preceding the recursion. In other words, a
negative number counts capturing parentheses leftwards from the point at which
it is encountered.
.P
It is also possible to refer to subsequently opened parentheses, by writing
references such as (?+2). However, these cannot be recursive because the
reference is not inside the parentheses that are referenced. They are always
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"subroutine"
.\"
calls, as described in the next section.
.P
An alternative approach is to use named parentheses instead. The Perl syntax
for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
could rewrite the above example as follows:
.sp
  (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
.sp
If there is more than one subpattern with the same name, the earliest one is
used.
.P
This particular example pattern that we have been looking at contains nested
unlimited repeats, and so the use of a possessive quantifier for matching
strings of non-parentheses is important when applying the pattern to strings
that do not match. For example, when this pattern is applied to
.sp
  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
.sp
it yields "no match" quickly. However, if a possessive quantifier is not used,
the match runs for a very long time indeed because there are so many different
ways the + and * repeats can carve up the subject, and all have to be tested
before failure can be reported.
.P
At the end of a match, the values of capturing parentheses are those from
the outermost level. If you want to obtain intermediate values, a callout
function can be used (see below and the
.\" HREF
\fBpcrecallout\fP
.\"
documentation). If the pattern above is matched against
.sp
  (ab(cd)ef)
.sp
the value for the inner capturing parentheses (numbered 2) is "ef", which is
the last value taken on at the top level. If a capturing subpattern is not
matched at the top level, its final value is unset, even if it is (temporarily)
set at a deeper level.
.P
If there are more than 15 capturing parentheses in a pattern, PCRE has to
obtain extra memory to store data during a recursion, which it does by using
\fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
.P
Do not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.
.sp
  < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
.sp
In this pattern, (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases. The (?R) item
is the actual recursive call.
.
.
.\" HTML <a name="recursiondifference"></a>
.SS "Recursion difference from Perl"
.rs
.sp
In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
treated as an atomic group. That is, once it has matched some of the subject
string, it is never re-entered, even if it contains untried alternatives and
there is a subsequent matching failure. This can be illustrated by the
following pattern, which purports to match a palindromic string that contains
an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
.sp
  ^(.|(.)(?1)\e2)$
.sp
The idea is that it either matches a single character, or two identical
characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
it does not if the pattern is longer than three characters. Consider the
subject string "abcba":
.P
At the top level, the first character is matched, but as it is not at the end
of the string, the first alternative fails; the second alternative is taken
and the recursion kicks in. The recursive call to subpattern 1 successfully
matches the next character ("b"). (Note that the beginning and end of line
tests are not part of the recursion).
.P
Back at the top level, the next character ("c") is compared with what
subpattern 2 matched, which was "a". This fails. Because the recursion is
treated as an atomic group, there are now no backtracking points, and so the
entire match fails. (Perl is able, at this point, to re-enter the recursion and
try the second alternative.) However, if the pattern is written with the
alternatives in the other order, things are different:
.sp
  ^((.)(?1)\e2|.)$
.sp
This time, the recursing alternative is tried first, and continues to recurse
until it runs out of characters, at which point the recursion fails. But this
time we do have another alternative to try at the higher level. That is the big
difference: in the previous case the remaining alternative is at a deeper
recursion level, which PCRE cannot use.
.P
To change the pattern so that matches all palindromic strings, not just those
with an odd number of characters, it is tempting to change the pattern to this:
.sp
  ^((.)(?1)\e2|.?)$
.sp
Again, this works in Perl, but not in PCRE, and for the same reason. When a
deeper recursion has matched a single character, it cannot be entered again in
order to match an empty string. The solution is to separate the two cases, and
write out the odd and even cases as alternatives at the higher level:
.sp
  ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
.sp
If you want to match typical palindromic phrases, the pattern has to ignore all
non-word characters, which can be done like this:
.sp
  ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
.sp
If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
the use of the possessive quantifier *+ to avoid backtracking into sequences of
non-word characters. Without this, PCRE takes a great deal longer (ten times or
more) to match typical phrases, and Perl takes so long that you think it has
gone into a loop.
.P
\fBWARNING\fP: The palindrome-matching patterns above work only if the subject
string does not start with a palindrome that is shorter than the entire string.
For example, although "abcba" is correctly matched, if the subject is "ababa",
PCRE finds the palindrome "aba" at the start, then fails at top level because
the end of the string does not follow. Once again, it cannot jump back into the
recursion to try other alternatives, so the entire match fails.
.
.
.\" HTML <a name="subpatternsassubroutines"></a>
.SH "SUBPATTERNS AS SUBROUTINES"
.rs
.sp
If the syntax for a recursive subpattern reference (either by number or by
name) is used outside the parentheses to which it refers, it operates like a
subroutine in a programming language. The "called" subpattern may be defined
before or after the reference. A numbered reference can be absolute or
relative, as in these examples:
.sp
  (...(absolute)...)...(?2)...
  (...(relative)...)...(?-1)...
  (...(?+1)...(relative)...
.sp
An earlier example pointed out that the pattern
.sp
  (sens|respons)e and \e1ibility
.sp
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If instead the pattern
.sp
  (sens|respons)e and (?1)ibility
.sp
is used, it does match "sense and responsibility" as well as the other two
strings. Another example is given in the discussion of DEFINE above.
.P
Like recursive subpatterns, a subroutine call is always treated as an atomic
group. That is, once it has matched some of the subject string, it is never
re-entered, even if it contains untried alternatives and there is a subsequent
matching failure. Any capturing parentheses that are set during the subroutine
call revert to their previous values afterwards.
.P
When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They cannot be
changed for different calls. For example, consider this pattern:
.sp
  (abc)(?i:(?-1))
.sp
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
.
.
.\" HTML <a name="onigurumasubroutines"></a>
.SH "ONIGURUMA SUBROUTINE SYNTAX"
.rs
.sp
For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for referencing a subpattern as a subroutine, possibly recursively. Here
are two of the examples used above, rewritten using this syntax:
.sp
  (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
  (sens|respons)e and \eg'1'ibility
.sp
PCRE supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:
.sp
  (abc)(?i:\eg<-1>)
.sp
Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
synonymous. The former is a back reference; the latter is a subroutine call.
.
.
.SH CALLOUTS
.rs
.sp
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
code to be obeyed in the middle of matching a regular expression. This makes it
possible, amongst other things, to extract different substrings that match the
same pair of parentheses when there is a repetition.
.P
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE provides an external
function by putting its entry point in the global variable \fIpcre_callout\fP.
By default, this variable contains NULL, which disables all calling out.
.P
Within a regular expression, (?C) indicates the points at which the external
function is to be called. If you want to identify different callout points, you
can put a number less than 256 after the letter C. The default value is zero.
For example, this pattern has two callout points:
.sp
  (?C1)abc(?C2)def
.sp
If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
automatically installed before each item in the pattern. They are all numbered
255.
.P
During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
set), the external function is called. It is provided with the number of the
callout, the position in the pattern, and, optionally, one item of data
originally supplied by the caller of \fBpcre_exec()\fP. The callout function
may cause matching to proceed, to backtrack, or to fail altogether. A complete
description of the interface to the callout function is given in the
.\" HREF
\fBpcrecallout\fP
.\"
documentation.
.
.
.\" HTML <a name="backtrackcontrol"></a>
.SH "BACKTRACKING CONTROL"
.rs
.sp
Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
are described in the Perl documentation as "experimental and subject to change
or removal in a future version of Perl". It goes on to say: "Their usage in
production code should be noted to avoid problems during upgrades." The same
remarks apply to the PCRE features described in this section.
.P
Since these verbs are specifically related to backtracking, most of them can be
used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
a backtracking algorithm. With the exception of (*FAIL), which behaves like a
failing negative assertion, they cause an error if encountered by
\fBpcre_dfa_exec()\fP.
.P
If any of these verbs are used in an assertion or subroutine subpattern
(including recursive subpatterns), their effect is confined to that subpattern;
it does not extend to the surrounding pattern. Note that such subpatterns are
processed as anchored at the point where they are tested.
.P
The new verbs make use of what was previously invalid syntax: an opening
parenthesis followed by an asterisk. They are generally of the form
(*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
depending on whether or not an argument is present. An name is a sequence of
letters, digits, and underscores. If the name is empty, that is, if the closing
parenthesis immediately follows the colon, the effect is as if the colon were
not there. Any number of these verbs may occur in a pattern.
.P
PCRE contains some optimizations that are used to speed up matching by running
some checks at the start of each match attempt. For example, it may know the
minimum length of matching subject, or that a particular character must be
present. When one of these optimizations suppresses the running of a match, any
included backtracking verbs will not, of course, be processed. You can suppress
the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
when calling \fBpcre_exec()\fP.
.
.
.SS "Verbs that act immediately"
.rs
.sp
The following verbs act as soon as they are encountered. They may not be
followed by a name.
.sp
   (*ACCEPT)
.sp
This verb causes the match to end successfully, skipping the remainder of the
pattern. When inside a recursion, only the innermost pattern is ended
immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
captured. (This feature was added to PCRE at release 8.00.) For example:
.sp
  A((?:A|B(*ACCEPT)|C)D)
.sp
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
the outer parentheses.
.sp
  (*FAIL) or (*F)
.sp
This verb causes the match to fail, forcing backtracking to occur. It is
equivalent to (?!) but easier to read. The Perl documentation notes that it is
probably useful only when combined with (?{}) or (??{}). Those are, of course,
Perl features that are not present in PCRE. The nearest equivalent is the
callout feature, as for example in this pattern:
.sp
  a+(?C)(*FAIL)
.sp
A match with the string "aaaa" always fails, but the callout is taken before
each backtrack happens (in this example, 10 times).
.
.
.SS "Recording which path was taken"
.rs
.sp
There is one verb whose main purpose is to track how a match was arrived at,
though it also has a secondary use in conjunction with advancing the match
starting point (see (*SKIP) below).
.sp
  (*MARK:NAME) or (*:NAME)
.sp
A name is always required with this verb. There may be as many instances of
(*MARK) as you like in a pattern, and their names do not have to be unique.
.P
When a match succeeds, the name of the last-encountered (*MARK) is passed back
to the caller via the \fIpcre_extra\fP data structure, as described in the
.\" HTML <a href="pcreapi.html#extradata">
.\" </a>
section on \fIpcre_extra\fP
.\"
in the
.\" HREF
\fBpcreapi\fP
.\"
documentation. No data is returned for a partial match. Here is an example of
\fBpcretest\fP output, where the /K modifier requests the retrieval and
outputting of (*MARK) data:
.sp
  /X(*MARK:A)Y|X(*MARK:B)Z/K
  XY
   0: XY
  MK: A
  XZ
   0: XZ
  MK: B
.sp
The (*MARK) name is tagged with "MK:" in this output, and in this example it
indicates which of the two alternatives matched. This is a more efficient way
of obtaining this information than putting each alternative in its own
capturing parentheses.
.P
A name may also be returned after a failed match if the final path through the
pattern involves (*MARK). However, unless (*MARK) used in conjunction with
(*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
starting point for matching is advanced, the final check is often with an empty
string, causing a failure before (*MARK) is reached. For example:
.sp
  /X(*MARK:A)Y|X(*MARK:B)Z/K
  XP
  No match
.sp
There are three potential starting points for this match (starting with X,
starting with P, and with an empty string). If the pattern is anchored, the
result is different:
.sp
  /^X(*MARK:A)Y|^X(*MARK:B)Z/K
  XP
  No match, mark = B
.sp
PCRE's start-of-match optimizations can also interfere with this. For example,
if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
subject length for a match, a shorter subject will not be scanned at all.
.P
Note that similar anomalies (though different in detail) exist in Perl, no
doubt for the same reasons. The use of (*MARK) data after a failed match of an
unanchored pattern is not recommended, unless (*COMMIT) is involved.
.
.
.SS "Verbs that act after backtracking"
.rs
.sp
The following verbs do nothing when they are encountered. Matching continues
with what follows, but if there is no subsequent match, causing a backtrack to
the verb, a failure is forced. That is, backtracking cannot pass to the left of
the verb. However, when one of these verbs appears inside an atomic group, its
effect is confined to that group, because once the group has been matched,
there is never any backtracking into it. In this situation, backtracking can
"jump back" to the left of the entire atomic group. (Remember also, as stated
above, that this localization also applies in subroutine calls and assertions.)
.P
These verbs differ in exactly what kind of failure occurs when backtracking
reaches them.
.sp
  (*COMMIT)
.sp
This verb, which may not be followed by a name, causes the whole match to fail
outright if the rest of the pattern does not match. Even if the pattern is
unanchored, no further attempts to find a match by advancing the starting point
take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
finding a match at the current starting point, or not at all. For example:
.sp
  a+(*COMMIT)b
.sp
This matches "xxaab" but not "aacaab". It can be thought of as a kind of
dynamic anchor, or "I've started, so I must finish." The name of the most
recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
match failure.
.P
Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
unless PCRE's start-of-match optimizations are turned off, as shown in this
\fBpcretest\fP example:
.sp
  /(*COMMIT)abc/
  xyzabc
   0: abc
  xyzabc\eY
  No match
.sp
PCRE knows that any match must start with "a", so the optimization skips along
the subject to "a" before running the first match attempt, which succeeds. When
the optimization is disabled by the \eY escape in the second subject, the match
starts at "x" and so the (*COMMIT) causes it to fail without trying any other
starting points.
.sp
  (*PRUNE) or (*PRUNE:NAME)
.sp
This verb causes the match to fail at the current starting position in the
subject if the rest of the pattern does not match. If the pattern is
unanchored, the normal "bumpalong" advance to the next starting character then
happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
reached, or when matching to the right of (*PRUNE), but if there is no match to
the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
(*PRUNE) is just an alternative to an atomic group or possessive quantifier,
but there are some uses of (*PRUNE) that cannot be expressed in any other way.
The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
match fails completely; the name is passed back if this is the final attempt.
(*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
pattern (*PRUNE) has the same effect as (*COMMIT).
.sp
  (*SKIP)
.sp
This verb, when given without a name, is like (*PRUNE), except that if the
pattern is unanchored, the "bumpalong" advance is not to the next character,
but to the position in the subject where (*SKIP) was encountered. (*SKIP)
signifies that whatever text was matched leading up to it cannot be part of a
successful match. Consider:
.sp
  a+(*SKIP)b
.sp
If the subject is "aaaac...", after the first match attempt fails (starting at
the first character in the string), the starting point skips on to start the
next attempt at "c". Note that a possessive quantifer does not have the same
effect as this example; although it would suppress backtracking during the
first match attempt, the second attempt would start at the second character
instead of skipping on to "c".
.sp
  (*SKIP:NAME)
.sp
When (*SKIP) has an associated name, its behaviour is modified. If the
following pattern fails to match, the previous path through the pattern is
searched for the most recent (*MARK) that has the same name. If one is found,
the "bumpalong" advance is to the subject position that corresponds to that
(*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
matching name is found, normal "bumpalong" of one character happens (the
(*SKIP) is ignored).
.sp
  (*THEN) or (*THEN:NAME)
.sp
This verb causes a skip to the next alternation if the rest of the pattern does
not match. That is, it cancels pending backtracking, but only within the
current alternation. Its name comes from the observation that it can be used
for a pattern-based if-then-else block:
.sp
  ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
.sp
If the COND1 pattern matches, FOO is tried (and possibly further items after
the end of the group if FOO succeeds); on failure the matcher skips to the
second alternative and tries COND2, without backtracking into COND1. The
behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
overall match fails. If (*THEN) is not directly inside an alternation, it acts
like (*PRUNE).
.
.
.SH "SEE ALSO"
.rs
.sp
\fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
\fBpcresyntax\fP(3), \fBpcre\fP(3).
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 03 May 2010
Copyright (c) 1997-2010 University of Cambridge.
.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"