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     re_format -- POSIX 1003.2 regular expressions


     Regular expressions (``REs''), as defined in IEEE Std 1003.2
     (``POSIX.2''), come in two forms: modern REs (roughly those of egrep(1);
     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.
     IEEE Std 1003.2 (``POSIX.2'') leaves some aspects of RE syntax and seman-
     tics open; `=' marks decisions on these aspects that may not be fully
     portable to other IEEE Std 1003.2 (``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 followed
     by `,' possibly followed by another unsigned decimal integer, always fol-
     lowed 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 fol-
     lowed 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 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 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
     ters, including `\', lose their special significance within a bracket

     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

     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 `x' and
     `y' are the members of an equivalence class, then `[[=x=]]', `[[=y=]]',
     and `[xy]' are all synonymous.  An equivalence class may not= be an end-
     point 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

     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
     IEEE Std 1003.2 (``POSIX.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 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
     sion 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 imple-
     mentation can refuse to accept such REs and remain POSIX-compliant.

     Obsolete (``basic'') regular expressions differ in several respects.  `|'
     is an ordinary character and there is no equivalent for its functional-
     ity.  `+' and `?' are ordinary characters, and their functionality can be
     expressed using bounds (`{1,}' or `{0,1}' respectively).  Also note that
     `x+' in modern REs is equivalent to `xx*'.  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 subex-
     pression, `$' is an ordinary character except at the end of the RE or=
     the end of a parenthesized subexpression, and `*' is an ordinary charac-
     ter if it appears at the beginning of the RE or the beginning of a paren-
     thesized subexpression (after a possible leading `^').  Finally, there is
     one new type of atom, a back reference: `\' followed by a non-zero deci-
     mal 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'.



     Regular Expression Notation, IEEE Std, 1003.2, section 2.8.


     Having two kinds of REs is a botch.

     The current IEEE Std 1003.2 (``POSIX.2'') spec says that `)' is an ordi-
     nary character in the absence of an unmatched `('; this was an uninten-
     tional result of a wording error, and change is likely.  Avoid relying on

     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.

     IEEE Std 1003.2 (``POSIX.2'') 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.

FreeBSD 5.4			March 20, 1994			   FreeBSD 5.4


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