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REGEX(7) Linux Programmer's Manual REGEX(7)

NAME

     regex - POSIX.2 regular expressions

DESCRIPTION

     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,
     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  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
     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, for exam-
     ple,  "[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
     expression containing a multicharacter collating element can thus match
     more  than  one  character,  for  example,  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 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
     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,  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
     intended to be portable should not employ REs longer than 256 bytes, as
     an  implementation  can refuse to accept such REs and remain POSIX-com-
     pliant.

     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".

BUGS

     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.

AUTHOR

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