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NAME

       regex - POSIX 1003.2 regular expressions

DESCRIPTION

       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(1); 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
       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
       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,
       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
       (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 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 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, 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.
       ‘|’,  ‘+’,  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 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)

       POSIX 1003.2, section 2.8 (Regular Expression Notation).

BUGS

       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
       efficient 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
       consensus among implementors as to the right interpretation.

       The syntax for word boundaries is incredibly ugly.

AUTHOR

       This page was taken from Henry Spencer’s regex package.

                                  1994-02-07                          REGEX(7)