REGEX(7)                   Linux Programmer's Manual                  REGEX(7)



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;  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  deci-
       sions  on these aspects that may not be fully portable to other POSIX.2
       implementations.

       A (modern) RE is one(!) or more  non-empty(!)  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, etc.

       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  fol-
       lowed  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 signifi-
       cance (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 nor-
       mally  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  charac-
       ters between those two (inclusive) in the collating sequence, for exam-
       ple, `[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  rely-
       ing on them.

       To include a literal `]' in the list, make it the first character (fol-
       lowing 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  spe-
       cial  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) 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  multi-character  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  ^  are  the  members  of  an  equivalence  class,  then `[[=o=]]',
       `[[=^=]]', and `[o^]' 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 pri-
       ority  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-com-
       pliant.

       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 them-
       selves ordinary characters.  `^' is an ordinary character except at the
       beginning  of  the RE or(!) the beginning of a parenthesized subexpres-
       sion, `$' is an ordinary character except at the end of  the  RE  or(!)
       the  end of a parenthesized subexpression, and `*' is an ordinary char-
       acter 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 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, 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  effi-
       cient  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 con-
       sensus among implementors as to the right interpretation.

       The syntax for word boundaries is incredibly ugly.

SEE ALSO
       regex(3)

       POSIX.2, section 2.8 (Regular Expression Notation).



                                  1994-02-07                          REGEX(7)
