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RE_FORMAT(7)	     BSD Miscellaneous Information Manual	  RE_FORMAT(7)

     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 semantics	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 end-
     point, 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 special charac-
     ters, including `\', lose their special significance within a bracket ex-

     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
     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

     There are two special cases<**> of	bracket	expressions: the bracket ex-
     pressions `[[:<:]]' 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 un-
     derscore.	This is	an extension, compatible with but not specified	by
     IEEE Std 1003.2 ("POSIX.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 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

     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 expres-
     sion containing both cases, e.g. `x' becomes `[xX]'.  When	it appears in-
     side a bracket expression,	all case counterparts of it are	added to the
     bracket expression, so that (e.g.)	 `[x]' becomes `[xX]' and `[^x]' be-
     comes `[^xX]'.

     No	particular limit is imposed on the length of REs<**>.  Programs	in-
     tended 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.	 `|'
     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 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.

     IEEE Std 1003.2 ("POSIX.2") specification of case-independent matching is
     vague.  The "one case implies all cases" definition given above is	cur-
     rent consensus among implementors as to the right interpretation.

     The syntax	for word boundaries is incredibly ugly.

BSD				March 20, 1994				   BSD


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