preg.txt

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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. 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 which are in the class by enumerating those that are
   not. It is not an assertion: it still consumes a character from the
   subject string, and fails if the current pointer is at the end of the
   string.

   When caseless matching is set, any letters in a class represent both
   their upper case and lower case versions, so for example, a caseless
   [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
   match "A", whereas a caseful version would. PCRE does not support the
   concept of case for characters with values greater than 255. A class
   such as [^a] will always match a newline.

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

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

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

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

    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 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

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

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

   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 pcre_fullinfo() function).

   An option change within a subpattern affects only that part of the
   current pattern that follows it, so

   (a(?i)b)c

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

   (a(?i)b|c)

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

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

    SUBPATTERNS

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

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

   cat(aract|erpillar|)

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

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

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

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

   the captured substrings are "red king", "red", and "king", and are
   numbered 1, 2, and 3, 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, and the maximum depth of nesting of all subpatterns, both
   capturing and non-capturing, is 200.

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

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

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

    REPETITION

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

       a literal data character
       the . meta-character
       the \C escape sequence
       escapes such as \d that match single characters
       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.

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

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

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

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

   (a?)*

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

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

   /\*.*\*/

   to the string

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

   fails, because it matches the entire string 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 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.