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RE_FORMAT(7)           Miscellaneous Information Manual           RE_FORMAT(7)



NAME
       re_format - 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; 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-
       tations.

       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 significance (match-
       ing 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 expres-
       sions  `[[::]]' 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
       underscore.  This is an extension, compatible with but not specified by
       POSIX 1003.2, 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  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  `week-
       nights', when `(.*).*' is matched against `abc' the parenthesized  sub-
       expression  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-compli-
       ant.

       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 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  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.



                                March 20, 1994                    RE_FORMAT(7)