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       re_format - POSIX 1003.2 regular expressions


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

       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, 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 collating-
       sequence-dependent, and portable programs should avoid relying 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, 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  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 ctype(3).  A locale
       may provide others.  A character class may not be used as  an  endpoint
       of a range.

       There  are  two  special  cases***  of bracket expressions: the bracket
       expressions '[[:<:]]' and '[[:>:]]' match the null string at the begin-
       ning  and  end of a word respectively.  A word is defined as a sequence
       of word characters which is neither preceded nor followed by word char-
       acters.   A  word  character  is  an  alnum  character  (as  defined by
       ctype(3)) or an underscore.  This is an extension, compatible with  but
       not specified by POSIX 1003.2, and should be used with caution in soft-
       ware 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  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, 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-com-

       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  (e.g.)
       '\([bc]\)\1' matches 'bb' or 'cc' but not 'bc'.



       POSIX 1003.2, section 2.8 (Regular Expression Notation).


       Having two kinds of REs is a botch.

       The  current  1003.2 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.

       1003.2'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.

GNO                             7 October 1997                    RE_FORMAT(7)

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