Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
REGEX(7)		   Linux Programmer's Manual		      REGEX(7)

       regex - POSIX.2 regular expressions

       Regular	expressions ("RE"s), as	defined	in POSIX.2, come in two	forms:
       modern REs (roughly those of egrep; POSIX.2 calls these "extended" REs)
       and  obsolete REs (roughly those	of ed(1); POSIX.2 "basic" REs).	 Obso-
       lete REs	mostly exist for backward compatibility	in some	old  programs;
       they  will  be discussed	at the end.  POSIX.2 leaves some aspects of RE
       syntax and semantics open; "(!)"	marks decisions	on these aspects  that
       may not be fully	portable to other POSIX.2 implementations.

       A (modern) RE is	one(!) or more nonempty(!) 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, and so on.

       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, al-
       ways  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 ex-
       ceed  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 (in-
       clusive)	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 be-
       low) 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,	 for  example,
       "[0-9]"	in  ASCII matches any decimal digit.  It is illegal(!) for two
       ranges to share an endpoint, for	example,  "a-c-e".   Ranges  are  very
       collating-sequence-dependent,  and portable programs should avoid rely-
       ing 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 ex-
       pression	containing a multicharacter collating element can  thus	 match
       more  than  one	character,  for	example, if the	collating sequence in-
       cludes 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 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 ex-
       pression	containing both	cases, for example, 'x'	becomes	"[xX]".	  When
       it appears inside a bracket expression, all case	counterparts of	it are
       added to	the bracket expression,	so that, for  example,	"[x]"  becomes
       "[xX]" and "[^x]" becomes "[^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-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  nonzero  decimal digit d matches the same	sequence of characters
       matched by the dth parenthesized	 subexpression	(numbering  subexpres-
       sions by	the positions of their opening parentheses, left to right), so
       that, for example, "\([bc]\)\1" matches "bb" or "cc" but	not "bc".

       Having two kinds	of REs is a botch.

       The current 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	 effi-
       cient  implementations.	 They  are also	somewhat vaguely defined (does
       "a\(\(b\)*\2\)*d" match "abbbd"?).  Avoid using them.

       POSIX.2's specification of case-independent  matching  is  vague.   The
       "one  case implies all cases" definition	given above is current consen-
       sus among implementors as to the	right interpretation.

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

       grep(1),	regex(3)

       POSIX.2,	section	2.8 (Regular Expression	Notation).

       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at

				  2009-01-12			      REGEX(7)


Want to link to this manual page? Use this URL:

home | help