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source checkout too large, added a README file pointing to the reference
copy of the docs in the PCRE vendor branch


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-.TH PCRE 3
-.SH NAME
-PCRE - Perl-compatible regular expressions
-.SH "PCRE REGULAR EXPRESSION DETAILS"
-.rs
-.sp
-The syntax and semantics of the regular expressions supported by PCRE are
-described below. Regular expressions are also described in the Perl
-documentation and in a number of 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, you must
-build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
-the PCRE_UTF8 option. How this 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
-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. 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 in 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 character 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 usually 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        newline (hex 0A)
-  \er        carriage return (hex 0D)
-  \et        tab (hex 09)
-  \eddd      character with octal code ddd, or backreference
-  \exhh      character with hex code hh
-  \ex{hhh..} character with hex code hhh... (UTF-8 mode only)
-.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). In UTF-8 mode, any number of hexadecimal digits may
-appear between \ex{ and }, but the value of the character code must be less
-than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). 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 when PCRE is in UTF-8 mode. 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. In both cases, 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 generates a single byte from the least
-significant 8 bits of the value. Any subsequent digits stand for themselves.
-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 byte value or a single UTF-8 character
-(in UTF-8 mode) 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), and the sequence \eX is interpreted as the
-character "X". Outside a character class, these sequences have different
-meanings
-.\" HTML <a href="#uniextseq">
-.\" </a>
-(see below).
-.\"
-.
-.
-.SS "Generic character types"
-.rs
-.sp
-The third use of backslash is for specifying generic character types. The
-following are always recognized:
-.sp
-  \ed     any decimal digit
-  \eD     any character that is not a decimal digit
-  \es     any whitespace character
-  \eS     any character that is not a whitespace character
-  \ew     any "word" character
-  \eW     any "non-word" character
-.sp
-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.
-.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).
-.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 the "fr_FR" (French) locale, some character codes
-greater than 128 are used for accented letters, and these are matched by \ew.
-.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.
-.
-.
-.\" HTML <a name="uniextseq"></a>
-.SS Unicode character properties
-.rs
-.sp
-When PCRE is built with Unicode character property support, three additional
-escape sequences to match generic character types are available when UTF-8 mode
-is selected. They 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 general category properties. Each character has exactly one such
-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 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 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
-Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
-PCRE.
-.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.
-.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="smallassertions"></a>
-.SS "Simple assertions"
-.rs
-.sp
-The fourth use of backslash is for certain simple assertions. An assertion
-specifies a condition that has to be met at a particular point in a match,
-without consuming any characters from the subject string. The use of
-subpatterns for more complicated assertions is described
-.\" 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 start of subject
-  \eZ     matches at end of subject or before newline at end
-  \ez     matches at end of subject
-  \eG     matches at first matching position in subject
-.sp
-These assertions may not appear in character classes (but note that \eb has a
-different meaning, namely the backspace character, inside a character class).
-.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.
-.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 that is the
-last character of the string as well as at the end of the string, 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
-character that is the last character in the string (by default). Dollar need
-not be the last character of the pattern if a number of alternatives are
-involved, but it should be the last item in any branch in which it appears.
-Dollar has no special meaning in a character class.
-.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, they match immediately
-after and immediately before an internal newline character, respectively, in
-addition to matching at the start and end of the subject string. For example,
-the pattern /^abc$/ matches the subject string "def\enabc" (where \en
-represents a newline character) 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 PCRE_MULTILINE is set or not.
-.
-.
-.SH "FULL STOP (PERIOD, DOT)"
-.rs
-.sp
-Outside a character class, a dot in the pattern matches any one character in
-the subject, including a non-printing character, but not (by default) newline.
-In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
-byte long, except (by default) newline. If the PCRE_DOTALL option is set,
-dots match newlines as well. The handling of dot is entirely independent of the
-handling of circumflex and dollar, the only relationship being that they both
-involve newline characters. Dot has no special meaning in a character class.
-.
-.
-.SH "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 can match a newline. 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. 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 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.
-.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. When running 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 newline character is never treated in any special way in character classes,
-whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
-such as [^a] will always match a newline.
-.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 the "fr_FR" 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 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
-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.
-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 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
-The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
-same way as the Perl-compatible options by using the characters U and X
-respectively. The (?X) flag setting is special in that it must always occur
-earlier in the pattern than any of the additional features it turns on, even
-when it is at top level. It is best to put it at the start.
-.
-.
-.\" 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 the 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, and the maximum depth
-of nesting of all subpatterns, both capturing and non-capturing, is 200.
-.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".
-.
-.
-.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, something that Perl does
-not provide. The Python syntax (?P<name>...) is used. Names consist of
-alphanumeric characters and underscores, and must be unique within a pattern.
-.P
-Named capturing parentheses are still allocated numbers as well as names. 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. For further details see the
-.\" HREF
-\fBpcreapi\fP
-.\"
-documentation.
-.
-.
-.SH REPETITION
-.rs
-.sp
-Repetition is specified by quantifiers, which can follow any of the following
-items:
-.sp
-  a literal data character
-  the . metacharacter
-  the \eC escape sequence
-  the \eX escape sequence (in UTF-8 mode with Unicode properties)
-  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)
-.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.
-.P
-For convenience (and historical compatibility) 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 which 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 . 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 backreference
-elsewhere in the pattern, a match at the start may fail, and a later one
-succeed. 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 and minimizing repetition, failure of what follows
-normally causes the repeated item to be re-evaluated to see if a different
-number of repeats allows the rest of the pattern to match. Sometimes it is
-useful to prevent this, either to change the nature of the match, or to cause
-it fail earlier than it otherwise might, when the author of the pattern knows
-there is no point in carrying on.
-.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 would give 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
-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 or processing of a
-possessive quantifier and the equivalent atomic group.
-.P
-The possessive quantifier syntax is an extension to the Perl syntax. It
-originates in Sun's Java package.
-.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. See the subsection entitled "Non-printing characters"
-.\" HTML <a href="#digitsafterbackslash">
-.\" </a>
-above
-.\"
-for further details of the handling of digits following a backslash.
-.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
-Back references to named subpatterns use the Python syntax (?P=name). We could
-rewrite the above example as follows:
-.sp
-  (?<p1>(?i)rah)\es+(?P=p1)
-.sp
-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
-.sp
-  (a|(bc))\e2
-.sp
-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 PCRE_EXTENDED option is set, this can be whitespace.
-Otherwise an empty comment (see
-.\" HTML <a href="#comments">
-.\" </a>
-"Comments"
-.\"
-below) can be used.
-.P
-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.
-.
-.
-.\" 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.
-.
-.
-.\" 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 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 (at least for 5.8), 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 if rewritten to use two top-level branches:
-.sp
-  (?<=abc|abde)
-.sp
-The implementation of lookbehind assertions is, for each alternative, to
-temporarily move the current position back by the fixed width and then try to
-match. If there are insufficient characters before the current position, the
-match is deemed to fail.
-.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 escape, which can match different numbers
-of bytes, is also not permitted.
-.P
-Atomic groups 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
-.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
-or, equivalently, using the possessive quantifier syntax,
-.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".
-.
-.
-.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 previous capturing subpattern matched
-or not. 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 three kinds of condition. If the text between the parentheses
-consists of a sequence of digits, the condition is satisfied if the capturing
-subpattern of that number has previously matched. The number must be greater
-than zero. 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 the condition is the string (R), it is satisfied if a recursive call to the
-pattern or subpattern has been made. At "top level", the condition is false.
-This is a PCRE extension. Recursive patterns are described in the next section.
-.P
-If the condition is not a sequence of digits or (R), 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 up to the next newline
-character in the pattern.
-.
-.
-.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. Perl provides 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 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. Obviously, PCRE cannot support
-the interpolation of Perl code. Instead, it supports some special syntax for
-recursion of the entire pattern, and also for individual subpattern recursion.
-.P
-The 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) is a recursive call of the entire regular expression.
-.P
-For example, 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.
-.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. In a larger pattern, keeping track of
-parenthesis numbers can be tricky. It may be more convenient to use named
-parentheses instead. For this, PCRE uses (?P>name), which is an extension to
-the Python syntax that PCRE uses for named parentheses (Perl does not provide
-named parentheses). We could rewrite the above example as follows:
-.sp
-  (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
-.sp
-This particular example pattern 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
-.sp
-  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
-.sp
-it yields "no match" 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.
-.P
-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 you want to obtain intermediate values, a callout function can be used (see
-the next section and the
-.\" HREF
-\fBpcrecallout\fP
-.\"
-documentation). If the pattern above is matched against
-.sp
-  (ab(cd)ef)
-.sp
-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
-.sp
-  \e( ( ( (?>[^()]+) | (?R) )* ) \e)
-     ^                        ^
-     ^                        ^
-.sp
-the string they capture is "ab(cd)ef", the contents of the top level
-parentheses. 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="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. 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. Such references must, however, follow the subpattern to which they
-refer.
-.
-.
-.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)\dabc(?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.
-.P
-.in 0
-Last updated: 09 September 2004
-.br
-Copyright (c) 1997-2004 University of Cambridge.