Regexp Details
--------------

The syntax and semantics of PCRE regular expressions, as used in Monotone, are described in detail below.  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 is intended as reference material.

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

         The quick brown fox

matches a portion of a subject string that is identical to itself. When caseless matching is specified, letters are matched independently of case.

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 metacharacters, which do not stand for themselves but instead are interpreted in some special way.

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:

\
     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

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

\
     general escape character
^
     negate the class, but only if the first character
-
     indicates character range
[
     POSIX character class (only if followed by POSIX syntax)
]
     terminates the character class

The following sections describe the use of each of the metacharacters.

Backslash
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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.

For example, if you want to match a * character, you write \* 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 \\.

If a pattern is compiled with the (?x) 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.

If you want to remove the special meaning from a sequence of characters, you can do so by putting them between \Q and \E.  The \Q...\E sequence is recognized both inside and outside character classes.

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

\a
     alarm, that is, the BEL character (hex 07)
\cX
     "control-X", where X is any character
\e
     escape (hex 1B)
\f
     formfeed (hex 0C)
\n
     linefeed (hex 0A)
\r
     carriage return (hex 0D)
\t
     tab (hex 09)
\DDD
     character with octal code DDD, or backreference
\xHH
     character with hex code HH
\x{HHH...}
     character with hex code HHH...

The precise effect of \cX is as follows: if X is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  Thus \cz becomes hex 1A (the SUB control character, in ASCII), but \c{ becomes hex 3B (;), and \c; becomes hex 7B ({).

After \x, from zero to two hexadecimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \x{ 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.

If characters other than hexadecimal digits appear between \x{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \x will be interpreted as a basic hexadecimal escape, with no following digits, giving a character whose value is zero.

Characters whose value is less than 256 can be defined by either of the two syntaxes for \x. There is no difference in the way they are handled. For example, \xdc is exactly the same as \x{dc}.

After \0 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 \0\x\07 specifies two binary zeros followed by a BEL character (octal 007). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit.

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

Inside a character class, or if the decimal number is greater than 9 and there have not been that many capturing subpatterns, PCRE re-reads up to three octal digits following the backslash, and 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 \400. In UTF-8 mode, values up to \777 are permitted. For example:

\040
     is another way of writing a space
\40
     is the same, provided there are fewer than 40 previous capturing subpatterns
\7
     is always a back reference
\11
     might be a back reference, or another way of writing a tab
\011
     is always a tab
\0113
     is a tab followed by the character 3
\113
     might be a back reference, otherwise the character with octal code 113
\377
     might be a back reference, otherwise the byte consisting entirely of 1 bits
\81
     is either a back reference, or a binary zero followed by the two characters 8 and 1

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

All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, the sequence \b is interpreted as the BS character (hex 08), and the sequences \R and \X are interpreted as the characters R and X, respectively. Outside a character class, these sequences have different meanings (see below).

Absolute and Relative Back References
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The sequence \g 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 \g{name}. Back references are discussed later, following the discussion of parenthesized subpatterns.

Generic character types
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Another use of backslash is for specifying generic character types. The following are always recognized:

\d
     any decimal digit
\D
     any character that is not a decimal digit
\h
     any horizontal whitespace character
\H
     any character that is not a horizontal whitespace character
\s
     any whitespace character
\S
     any character that is not a whitespace character
\v
     any vertical whitespace character
\V
     any character that is not a vertical whitespace character
\w
     any ``word'' character
\W
     any ``non-word'' character

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

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.

For compatibility with Perl, \s does not match the VT character (code 11).  This makes it different from the the POSIX ``space'' class. The \s characters are TAB (9), LF (10), FF (12), CR (13), and SPACE (32).

In UTF-8 mode, characters with values greater than 128 never match \d, \s, or \w, and always match \D, \S, and \W.  These sequences retain their original meanings from before UTF-8 support was available, mainly for efficiency reasons.

The sequences \h, \H, \v, and \V 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:

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

The vertical space characters are:

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

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 that used for the ``C'' locale.

Newline Conventions
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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 default is to match any Unicode newline sequence.  It is possible to override the default newline convention by starting a pattern string with one of the following five sequences:

(*CR)
     carriage return
(*LF)
     linefeed
(*CRLF)
     carriage return, followed by linefeed
(*ANYCRLF)
     any of the three above
(*ANY)
     all Unicode newline sequences

For example, the pattern

         (*CR)a.b

changes the convention to CR. That pattern matches a\nb 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.

The newline convention does not affect what the \R escape sequence matches. By default, this is any Unicode newline sequence, for Perl compatibility. However, this can be changed; see the description of \R below.  A change of \R setting can be combined with a change of newline convention.

Newline Sequences
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Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is equivalent to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

This is an example of an "atomic group", details of which are given 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.  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).

It is possible to change the meaning of \R by starting a pattern string with one of the following sequences:

(*BSR_ANYCRLF)
     CR, LF, or CRLF only
(*BSR_UNICODE)
     any Unicode newline sequence (the default)

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:

         (*ANY)(*BSR_ANYCRLF)

Inside a character class, \R matches the letter R.

Unicode Character Properties
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Three additional escape sequences match characters with specific Unicode properties.  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:

\p{XX}
     a character with the XX property
\P{XX}
     a character without the XX property
\X
     an extended Unicode sequence

The property names represented by XX 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 \P{Any} does not match any characters, so always causes a match failure.

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:

         \p{Greek}
         \P{Han}

Those that are not part of an identified script are lumped together as ``Common.'' The current list of scripts is:

Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.

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, \p{^Lu} is the same as \P{Lu}.

If only one letter is specified with \p or \P, 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:

         \p{L}
         \pL

The following general category property codes are supported:

C
     Other
Cc
     Control
Cf
     Format
Cn
     Unassigned
Co
     Private use
Cs
     Surrogate

L
     Letter
Ll
     Lower case letter
Lm
     Modifier letter
Lo
     Other letter
Lt
     Title case letter
Lu
     Upper case letter

M
     Mark
Mc
     Spacing mark
Me
     Enclosing mark
Mn
     Non-spacing mark

N
     Number
Nd
     Decimal number
Nl
     Letter number
No
     Other number

P
     Punctuation
Pc
     Connector punctuation
Pd
     Dash punctuation
Pe
     Close punctuation
Pf
     Final punctuation
Pi
     Initial punctuation
Po
     Other punctuation
Ps
     Open punctuation

S
     Symbol
Sc
     Currency symbol
Sk
     Modifier symbol
Sm
     Mathematical symbol
So
     Other symbol

Z
     Separator
Zl
     Line separator
Zp
     Paragraph separator
Zs
     Space separator

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.''

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.

The long synonyms for these properties that Perl supports (such as \p{Letter}) are not supported by PCRE, nor is it permitted to prefix any of these properties with Is.

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.

Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters.

The \X escape matches any number of Unicode characters that form an extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

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 (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 \X matches any one character.

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 \d and \w do not use Unicode properties in PCRE.

Resetting the Match Start
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The escape sequence \K, 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:

         foo\Kbar

matches foobar, but reports that it has matched bar. This feature is similar to a lookbehind assertion (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 \K does not interfere with the setting of captured substrings.  For example, when the pattern

         (foo)\Kbar

matches foobar, the first substring is still set to foo.

Simple assertions
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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 below.  The backslashed assertions are:

\b
     matches at a word boundary
\B
     matches when not at a word boundary
\A
     matches at the start of the subject
\Z
     matches at the end of the subject also matches before a newline at the end of the subject
\z
     matches only at the end of the subject
\G
     matches at the first matching position in the subject

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

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

The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described 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. The difference between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, whereas \z matches only at the end.

The \G assertion is true only when the current matching position is at the start point of the match.  As used in Monotone, \G is always equal to \A.

Circumflex and Dollar
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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. Inside a character class, circumflex has an entirely different meaning (see below).

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

A dollar character, $, is an assertion 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.

The meanings of the circumflex and dollar characters are changed if the (?m) 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 (?m) is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines.

For example, the pattern ^abc$ matches the subject string def\nabc (where \n 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.

Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored.

Full Stop (Period, Dot)
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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.

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.

The behaviour of dot with regard to newlines can be changed. If the (?s) 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.

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.

Matching a Single Byte
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Outside a character class, the escape sequence \C 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 \C escape sequence is best avoided.

PCRE does not allow \C to appear in lookbehind assertions (described below), because in UTF-8 mode this would make it impossible to calculate the length of the lookbehind.

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

A character class matches a single character in the subject. In UTF-8 mode, the character may occupy more than one byte. 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.

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.

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 \x{ escaping mechanism.

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 for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 support.

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 (?s) and (?m) options is used. A class such as [^a] always matches one of these characters.

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

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

Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example [\000-\037]. In UTF-8 mode, ranges can include characters whose values are greater than 255, for example [\x{100}-\x{2ff}].

If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly.

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

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.

POSIX Character Classes
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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,

         [01[:alpha:]%]

matches 0, 1, any alphabetic character, or %. The supported class names are

alnum
     letters and digits
alpha
     letters
ascii
     character codes 0 -- 127
blank
     space or tab only
cntrl
     control characters
digit
     decimal digits (same as \d)
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 \s)
upper
     upper case letters
word
     ``word'' characters (same as \w)
xdigit
     hexadecimal digits

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 \s, which does not include VT (for Perl compatibility).

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,

         [12[:^digit:]]

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.

In UTF-8 mode, characters with values greater than 128 do not match any of the POSIX character classes.

Vertical Bar
............

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

         gilbert|sullivan

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

Internal Option Setting
.......................

The behavior of the matching engine can be adjusted from within the pattern by a sequence of option letters enclosed between (? and ).  The option letters are

i
     Caseless: characters in one case match the corresponding characters in other cases as well.
m
     Multiline: ^ and $ match at newlines as well as at beginning and end of string.
s
     Dotall: dot matches any character, including newline characters.
x
     Extended syntax: unescaped white space is ignored and embedded comments are possible.
J
     Dupnames: names for capturing subpattern need not be unique.
U
     Ungreedy: quantifiers match as few times as possible by default.
X
     Extra: for forward compatibility, give an error if any escape sequence with no defined meaning appears.

For example, (?im) sets caseless, multiline matching. It is also possible to unset these options by preceding the letters with a hyphen, and a combined setting and unsetting such as (?im-sx) is also permitted.  (This would set the caseless and multiline options while unsetting the dotall and extended-syntax options.)  If a letter appears both before and after the hyphen, the option is unset.  The lowercase option letters are Perl-compatible; the uppercase ones are PCRE only.

When an option change occurs at top level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows.  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

         (a(?i)b)c

matches abc and aBc and no other strings.  By this means, options can be made to have different settings in different parts of the pattern. Any changes made in one alternative do carry on into subsequent branches within the same subpattern. For example,

         (a(?i)b|c)

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

Note: Unlike these options, the similar, PCRE-specific option sequences that start with (* may appear only at the very beginning of the pattern.  Details of these sequences are given in the section entitled ``Newline sequences,'' above.

Subpatterns
...........

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

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

              cat(aract|erpillar|)

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

 2. It sets up the subpattern as a capturing subpattern.  As used in Monotone this only means that during matching, the portion of the subject string that matched the subpattern is available for back references.  Captured subpatterns are, for instance, not available to callers of regex.search.  Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns.

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

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

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

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

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

the captured substrings are white queen and queen, and are numbered 1 and 2. The maximum number of capturing subpatterns is 65535.

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

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

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

Duplicate Subpattern Numbers
............................

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:

         (?|(Sat)ur|(Sun))day

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.

  # before  ---------------branch-reset----------- after
  / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
  # 1            2         2  3        2     3     4

A backreference or a recursive call to a numbered subpattern always refers to the first one in the pattern with the given number.

An alternative approach to using this ``branch reset'' feature is to use duplicate named subpatterns, as described in the next section.

Named Subpatterns
.................

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.

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 backreferences, recursion, and conditions, can be made by name as well as by number.

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.

By default, a name must be unique within a pattern, but it is possible to relax this constraint by setting the (?J) option. This 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:

         (?Jx)
         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

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.)

Repetition
..........

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

  * a literal data character

  * the dot metacharacter

  * the \C escape sequence

  * the \X escape sequence (in UTF-8 mode with Unicode properties)

  * the \R escape sequence

  * an escape such as \d that matches a single character

  * a character class

  * a back reference (see next section)

  * a parenthesized subpattern (unless it is an assertion)

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

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of which is represented by a two-byte sequence. Similarly, \X{3} matches three Unicode extended sequences, each of which may be several bytes long (and they may be of different lengths).

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

For convenience, the three most common quantifiers have single-character abbreviations:

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

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

         (a?)*

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

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

         /\*.*\*/

to the string

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

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

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

         /\*.*?\*/

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

         \d??\d

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

If the (?U) 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.

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.

If a pattern starts with .* or .{0,} and the (?s) option 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 \A.

In cases where it is known that the subject string contains no newlines, it is worth setting (?s) in order to obtain this optimization, or alternatively using ^ or \A to indicate anchoring explicitly.

However, there is one situation where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a backreference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example:

         (.*)abc\1

If the subject is xyz123abc123 the match point is the fourth character. For this reason, such a pattern is not implicitly anchored.

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

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

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

         (a|(b))+

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

Atomic Grouping and Possessive Quantifiers
..........................................

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.

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

         123456bar

After matching all 6 digits and then failing to match foo, the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. 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.

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:

         (?>\d+)foo

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.  Atomic grouping subpatterns are not capturing subpatterns.

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.

Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are prepared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits.

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

         \d++foo

Note that a possessive quantifier can be used with an entire group, for example:

         (abc|xyz){2,3}+

Possessive quantifiers are always greedy; the setting of the (?U) 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.

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.

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 As when B must follow.

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

         (\D+|<\d+>)*[!?]

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

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ 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:

         ((?>\D+)|<\d+>)*[!?]

sequences of non-digits cannot be broken, and failure happens quickly.

Back References
...............

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

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

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 \50 is interpreted as a character defined in octal. See the subsection entitled ``Non-printing characters'' 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).

Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g 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:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

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:

         (abc(def)ghi)\g{-1}

The sequence \g{-1} is a reference to the most recently started capturing subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2} would be equivalent to \1. 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.

A back reference matches whatever actually matched the capturing subpattern in the current subject string, rather than anything matching the subpattern itself (see ``Subpatterns as subroutines'' below for a way of doing that). So the pattern

         (sens|respons)e and \1ibility

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

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

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

There are several different ways of writing back references to named subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified back reference syntax, in which \g can be used for both numeric and named references, is also supported. We could rewrite the above example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

A subpattern that is referenced by name may appear in the pattern before or after the reference.

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. For example, the pattern

         (a|(bc))\2

always fails if it starts to match a rather than bc. Because there may be many capturing parentheses in a pattern, all digits following the 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 (?x) option is set, this can be whitespace. Otherwise an empty comment (see ``Comments'' below) can be used.

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

         (a|b\1)+

matches any number of as 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.

Assertions
..........

An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above.

More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed.

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.

Lookahead Assertions
....................

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

         \w+(?=;)

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

         foo(?!bar)

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

         (?!foo)bar

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

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.

Lookbehind Assertions
.....................

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

         (?<!foo)bar

matches 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

         (?<=bullock|donkey)

is permitted, but

         (?<!dogs?|cats?)

causes an error at compile time. Branches that match different length strings are permitted only at the top level of a lookbehind assertion. This is an extension compared with Perl (at least for 5.8), which requires all branches to match the same length of string. An assertion such as

         (?<=ab(c|de))

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

         (?<=abc|abde)

In some cases, the Perl 5.10 escape sequence \K (see above) can be used instead of a lookbehind assertion; this is not restricted to a fixed-length.

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.

PCRE does not allow the \C 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 \X and \R escapes, which can match different numbers of bytes, are also not permitted.

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

         abcd$

when applied to a long string 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

         ^.*abcd$

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

         ^.*+(?<=abcd)

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.

Using multiple assertions
.........................

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

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

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

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

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

Assertions can be nested in any combination. For example,

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

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

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

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

Conditional Subpatterns
.......................

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

  * (?(CONDITION)YES-PATTERN)

  * (?(CONDITION)YES-PATTERN|NO-PATTERN)

If the CONDITION is satisfied, the YES-PATTERN is used; otherwise the NO-PATTERN (if present) is used. If there are more than two alternatives in the subpattern, a compile-time error occurs.

There are four kinds of condition: references to subpatterns, references to recursion, a pseudo-condition called DEFINE, and assertions.

Checking for a used subpattern by number
........................................

If the text between the parentheses consists of a sequence of digits, the condition is true if the capturing subpattern of that number has previously matched. 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).

Consider the following pattern, which contains non-significant white space to make it more readable and to divide it into three parts for ease of discussion (assume a preceding (?x)):

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

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

If you were embedding this pattern in a larger one, you could use a relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

This makes the fragment independent of the parentheses in the larger pattern.

Checking for a used subpattern by name
......................................

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.

Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

Checking for pattern recursion
..............................

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:

         (?(R3)...) or (?(R&name)...)

the condition is true if the most recent recursion is into the subpattern whose number or name is given. This condition does not check the entire recursion stack.

At ``top level,'' all these recursion test conditions are false. Recursive patterns are described below.

Defining subpatterns for use by reference only
..............................................

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 subroutines is described below.) For example, a pattern to match an IPv4 address could be written like this (ignore whitespace and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

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.

Assertion conditions
....................

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:

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

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

Comments
........

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.

If the (?x) option is set, an unescaped # character outside a character class introduces a comment that continues to immediately after the next newline in the pattern.

Recursive Patterns
..................

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.

PCRE 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 introduced into Perl at release 5.10.

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 subroutine call, which is described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression.

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 PCRE pattern solves the nested parentheses problem (whitespace is insignificant):

         \( ( (?>[^()]+) | (?R) )* \)

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.

If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this:

         ( \( ( (?>[^()]+) | (?1) )* \) )

We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern.

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.

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 subroutine calls, as described in the next section.

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:

         (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )

If there is more than one subpattern with the same name, the earliest one is used.

This particular example pattern that we have been looking at contains nested unlimited repeats, and so the use of atomic grouping 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

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

it fails quickly. However, if atomic grouping 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.

At the end of a match, the values set for any capturing subpatterns are those from the outermost level of the recursion at which the subpattern value is set.  If the pattern above is matched against

         (ab(cd)ef)

the value for the capturing parentheses is ef, which is the last value taken on at the top level. If additional parentheses are added, giving

         \( ( ( (?>[^()]+) | (?R) )* ) \)
            ^                        ^
@end example

@noindent
the string they capture is @samp{ab(cd)ef}, the contents of the top
level parentheses.

Do not confuse the @samp{(?R)} item with the condition @samp{(?(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.

@verbatim
         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

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.

Subpatterns as Subroutines
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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:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

matches sense and sensibility and response and responsibility, but not sense and responsibility. If instead the pattern

         (sens|respons)e and (?1)ibility

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.

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.

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:

         (abc)(?i:(?-1))

It matches abcabc. It does not match abcABC because the change of processing option does not affect the called subpattern.

Backtracking Control
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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.

The new verbs make use of what was previously invalid syntax: an opening parenthesis followed by an asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does not support the use of arguments, so its general form is just (*VERB). Any number of these verbs may occur in a pattern. There are two kinds:

Verbs that act immediately
..........................

The following verbs act as soon as they are encountered:

(*ACCEPT)

     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. PCRE differs from Perl in what happens if the (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is captured: in PCRE no data is captured. For example:

              A(A|B(*ACCEPT)|C)D

     This matches AB, AAD, or ACD, but when it matches AB, no data is captured.

(*FAIL) or (*F)

     This verb causes the match to fail, forcing backtracking to occur. It is equivalent to (?!) but easier to read.  It is not clear whether there is any use for this without the ability to execute code in the middle of the pattern (which Perl has but PCRE in Monotone does not).

Verbs that act after backtracking
.................................

The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is no subsequent match, a failure is forced.  The verbs differ in exactly what kind of failure occurs.

(*COMMIT)

     This verb 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 start point take place. Once (*COMMIT) has been passed, the regular expression engine is committed to finding a match at the current starting point, or not at all. For example:

              a+(*COMMIT)b

     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.''

(*PRUNE)

     This verb causes the match to fail at the current position if the rest of the pattern does not match. If the pattern is unanchored, the normal ``bump-along'' advance to the next starting character then happens. Backtracking can occur as usual to the left of (*PRUNE), 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.

(*SKIP)

     This verb 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:

              a+(*SKIP)b

     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 in 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.

(*THEN)

     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:

              ( COND1 (*THEN) FOO
              | COND2 (*THEN) BAR
              | COND3 (*THEN) BAZ ) ...

     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. If (*THEN) is used outside of any alternation, it acts exactly like (*PRUNE).