path: root/winsup/cygwin/regex/regex.7

RE_FORMAT(7)                   BSD Miscellaneous Information Manual                  RE_FORMAT(7)

NAME
     re_format -- POSIX 1003.2 regular expressions

DESCRIPTION
     Regular expressions ("REs"), as defined in IEEE Std 1003.2 ("POSIX.2"), come in two forms:
     modern REs (roughly those of egrep(1); 1003.2 calls these "extended" REs) and obsolete REs
     (roughly those of ed(1); 1003.2 "basic" REs).  Obsolete REs mostly exist for backward com-
     patibility in some old programs; they will be discussed at the end.  IEEE Std 1003.2
     ("POSIX.2") leaves some aspects of RE syntax and semantics open; `<**>' marks decisions on
     these aspects that may not be fully portable to other IEEE Std 1003.2 ("POSIX.2") implemen-
     tations.

     A (modern) RE is one<**> or more non-empty<**> branches, separated by '|'.  It matches any-
     thing that matches one of the branches.

     A branch is one<**> or more pieces, concatenated.  It matches a match for the first, fol-
     lowed by a match for the second, etc.

     A piece is an atom possibly followed by a single<**> '*', '+', '?', or bound.  An atom fol-
     lowed by '*' matches a sequence of 0 or more matches of the atom.  An atom followed by '+'
     matches a sequence of 1 or more matches of the atom.  An atom followed by '?' matches a se-
     quence of 0 or 1 matches of the atom.

     A bound is '{' followed by an unsigned decimal integer, possibly followed by ',' possibly
     followed by another unsigned decimal integer, always followed by '}'.  The integers must lie
     between 0 and RE_DUP_MAX (255<**>) inclusive, and if there are two of them, the first may
     not exceed the second.  An atom followed by a bound containing one integer i and no comma
     matches a sequence of exactly i matches of the atom.  An atom followed by a bound containing
     one integer i and a comma matches a sequence of i or more matches of the atom.  An atom fol-
     lowed by a bound containing two integers i and j matches a sequence of i through j (inclu-
     sive) matches of the atom.

     An atom is a regular expression enclosed in '()' (matching a match for the regular expres-
     sion), an empty set of '()' (matching the null string)<**>, a bracket expression (see be-
     low), '.' (matching any single character), '^' (matching the null string at the beginning of
     a line), '$' (matching the null string at the end of a line), a '\' followed by one of the
     characters '^.[$()|*+?{\' (matching that character taken as an ordinary character), a '\'
     followed by any other character<**> (matching that character taken as an ordinary character,
     as if the '\' had not been present<**>), or a single character with no other significance
     (matching that character).  A '{' followed by a character other than a digit is an ordinary
     character, not the beginning of a bound<**>.  It is illegal to end an RE with '\'.

     A bracket expression is a list of characters enclosed in '[]'.  It normally matches any sin-
     gle character from the list (but see below).  If the list begins with '^', it matches any
     single character (but see below) not from the rest of the list.  If two characters in the
     list are separated by '-', this is shorthand for the full range of characters between those
     two (inclusive) in the collating sequence, e.g. '[0-9]' in ASCII matches any decimal digit.
     It is illegal<**> for two ranges to share an endpoint, e.g. 'a-c-e'.  Ranges are very col-
     lating-sequence-dependent, and portable programs should avoid relying on them.

     To include a literal ']' in the list, make it the first character (following a possible
     '^').  To include a literal '-', make it the first or last character, or the second endpoint
     of a range.  To use a literal '-' as the first endpoint of a range, enclose it in '[.' and
     '.]' to make it a collating element (see below).  With the exception of these and some com-
     binations using '[' (see next paragraphs), all other special characters, including '\', lose
     their special significance within a bracket expression.

     Within a bracket expression, a collating element (a character, a multi-character sequence
     that collates as if it were a single character, or a collating-sequence name for either) en-
     closed in '[.' and '.]' stands for the sequence of characters of that collating element.
     The sequence is a single element of the bracket expression's list.  A bracket expression
     containing a multi-character collating element can thus match more than one character, e.g.
     if the collating sequence includes a 'ch' collating element, then the RE '[[.ch.]]*c'
     matches the first five characters of 'chchcc'.

     Within a bracket expression, a collating element enclosed in '[=' and '=]' is an equivalence
     class, standing for the sequences of characters of all collating elements equivalent to that
     one, including itself.  (If there are no other equivalent collating elements, the treatment
     is as if the enclosing delimiters were '[.' and '.]'.)  For example, if 'x' and 'y' are the
     members of an equivalence class, then '[[=x=]]', '[[=y=]]', and '[xy]' are all synonymous.
     An equivalence class may not<**> be an endpoint of a range.

     Within a bracket expression, the name of a character class enclosed in '[:' and ':]' stands
     for the list of all characters belonging to that class.  Standard character class names are:

           alnum    digit    punct
           alpha    graph    space
           blank    lower    upper
           cntrl    print    xdigit

     These stand for the character classes defined in ctype(3).  A locale may provide others.  A
     character class may not be used as an endpoint of a range.

     A bracketed expression like '[[:class:]]' can be used to match a single character that be-
     longs to a character class.  The reverse, matching any character that does not belong to a
     specific class, the negation operator of bracket expressions may be used: '[^[:class:]]'.

     There are two special cases<**> of bracket expressions: the bracket expressions '[[:<:]]'
     and '[[:>:]]' match the null string at the beginning and end of a word respectively.  A word
     is defined as a sequence of word characters which is neither preceded nor followed by word
     characters.  A word character is an alnum character (as defined by ctype(3)) or an under-
     score.  This is an extension, compatible with but not specified by IEEE Std 1003.2
     ("POSIX.2"), and should be used with caution in software intended to be portable to other
     systems.

     In the event that an RE could match more than one substring of a given string, the RE
     matches the one starting earliest in the string.  If the RE could match more than one sub-
     string starting at that point, it matches the longest.  Subexpressions also match the long-
     est possible substrings, subject to the constraint that the whole match be as long as possi-
     ble, with subexpressions starting earlier in the RE taking priority over ones starting
     later.  Note that higher-level subexpressions thus take priority over their lower-level com-
     ponent subexpressions.

     Match lengths are measured in characters, not collating elements.  A null string is consid-
     ered longer than no match at all.  For example, 'bb*' matches the three middle characters of
     'abbbc', '(wee|week)(knights|nights)' matches all ten characters of 'weeknights', when
     '(.*).*' is matched against 'abc' the parenthesized subexpression matches all three charac-
     ters, and when '(a*)*' is matched against 'bc' both the whole RE and the parenthesized sub-
     expression match the null string.

     If case-independent matching is specified, the effect is much as if all case distinctions
     had vanished from the alphabet.  When an alphabetic that exists in multiple cases appears as
     an ordinary character outside a bracket expression, it is effectively transformed into a
     bracket expression containing both cases, e.g. 'x' becomes '[xX]'.  When it appears inside a
     bracket expression, all case counterparts of it are added to the bracket expression, so that
     (e.g.)  '[x]' becomes '[xX]' and '[^x]' becomes '[^xX]'.

     No particular limit is imposed on the length of REs<**>.  Programs intended to be portable
     should not employ REs longer than 256 bytes, as an implementation can refuse to accept such
     REs and remain POSIX-compliant.

     Obsolete ("basic") regular expressions differ in several respects.  '|' is an ordinary char-
     acter and there is no equivalent for its functionality.  '+' and '?' are ordinary charac-
     ters, and their functionality can be expressed using bounds ('{1,}' or '{0,1}' respec-
     tively).  Also note that 'x+' in modern REs is equivalent to 'xx*'.  The delimiters for
     bounds are '\{' and '\}', with '{' and '}' by themselves ordinary characters.  The parenthe-
     ses for nested subexpressions are '\(' and '\)', with '(' and ')' by themselves ordinary
     characters.  '^' is an ordinary character except at the beginning of the RE or<**> the be-
     ginning of a parenthesized subexpression, '$' is an ordinary character except at the end of
     the RE or<**> the end of a parenthesized subexpression, and '*' is an ordinary character if
     it appears at the beginning of the RE or the beginning of a parenthesized subexpression (af-
     ter a possible leading '^').  Finally, there is one new type of atom, a back reference: '\'
     followed by a non-zero decimal digit d matches the same sequence of characters matched by
     the dth parenthesized subexpression (numbering subexpressions by the positions of their
     opening parentheses, left to right), so that (e.g.)  '\([bc]\)\1' matches 'bb' or 'cc' but
     not 'bc'.

SEE ALSO
     regex(3)

     Regular Expression Notation, IEEE Std, 1003.2, section 2.8.

BUGS
     Having two kinds of REs is a botch.

     The current IEEE Std 1003.2 ("POSIX.2") spec says that ')' is an ordinary character in the
     absence of an unmatched '('; this was an unintentional result of a wording error, and change
     is likely.  Avoid relying on it.

     Back references are a dreadful botch, posing major problems for efficient implementations.
     They are also somewhat vaguely defined (does 'a\(\(b\)*\2\)*d' match 'abbbd'?).  Avoid using
     them.

     IEEE Std 1003.2 ("POSIX.2") specification of case-independent matching is vague.  The "one
     case implies all cases" definition given above is current consensus among implementors as to
     the right interpretation.

     The syntax for word boundaries is incredibly ugly.

BSD                                       March 20, 1994                                      BSD