# regex

## NAME



           regex - POSIX.2 regular expressions
    


## DESCRIPTION



           Regular expressions ("RE"s), as defined in POSIX.2, come in two
           forms: modern REs (roughly those of **egrep**(1); POSIX.2 calls these
           "extended" REs) and obsolete REs (roughly those of **ed**(1); POSIX.2
           "basic" REs).  Obsolete REs mostly exist for backward
           compatibility in some old programs; they will be discussed at the
           end.  POSIX.2 leaves some aspects of RE syntax and semantics open;
           "(!)" marks decisions on these aspects that may not be fully
           portable to other POSIX.2 implementations.
    
           A (modern) RE is one(!) or more nonempty(!)  _branches_ , separated
           by '|'.  It matches anything that matches one of the branches.
    
           A branch is one(!) or more _pieces_ , concatenated.  It matches a
           match for the first, followed by a match for the second, and so
           on.
    
           A piece is an _atom_ possibly followed by a single(!) '*', '+', '?',
           or _bound_.  An atom followed 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 sequence 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 followed by a bound containing
           two integers _i_ and _j_ matches a sequence of _i_ through _j_ (inclusive)
           matches of the atom.
    
           An atom is a regular expression enclosed in "_()_ " (matching a match
           for the regular expression), an empty set of "_()_ " (matching the
           null string)(!), a _bracket expression_ (see below), '.' (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 single 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, for example, "_[0-9]_ " in ASCII matches any
           decimal digit.  It is illegal(!) for two ranges to share an
           endpoint, for example, "_a-c-e_ ".  Ranges are very collating-
           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 combinations 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
           multicharacter sequence that collates as if it were a single
           character, or a collating-sequence name for either) enclosed 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
           multicharacter collating element can thus match more than one
           character, for example, 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 o and ô are the members of an
           equivalence class, then "_[[=o=]]_ ", "_[[=ô=]]_ ", and "_[oô]_ " 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 wctype(3).  A
           locale may provide others.  A character class may not be used as
           an endpoint of a range.
    
           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 substring starting at
           that point, it matches the longest.  Subexpressions also match the
           longest possible substrings, subject to the constraint that the
           whole match be as long as possible, 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 component subexpressions.
    
           Match lengths are measured in characters, not collating elements.
           A null string is considered 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 characters, and when
           "_(a*)*_ " is matched against "bc" both the whole RE and the
           parenthesized subexpression 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, for
           example, 'x' becomes "_[xX]_ ".  When it appears inside a bracket
           expression, all case counterparts of it are added to the bracket
           expression, so that, for example, "_[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.
           '|', '+', and '?' are ordinary characters and there is no
           equivalent for their functionality.  The delimiters for bounds are
           "_\{_ " and "_\}_ ", with '{' and '}' by themselves ordinary characters.
           The parentheses for nested subexpressions are "_\(_ " and "_\)_ ", with
           '(' and ')' by themselves ordinary characters.  '^' is an ordinary
           character except at the beginning of the RE or(!) the beginning 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 (after a possible leading '^').
    
           Finally, there is one new type of atom, a _back reference_ : '\'
           followed by a nonzero decimal digit _d_ matches the same sequence of
           characters matched by the _d_ th parenthesized subexpression
           (numbering subexpressions by the positions of their opening
           parentheses, left to right), so that, for example, "_\([bc]\)\1_ "
           matches "bb" or "cc" but not "bc".
    


## BUGS



           Having two kinds of REs is a botch.
    
           The current 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.
    
           POSIX.2's 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.
    


* * *