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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).  Obsolete 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, 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 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 (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 normally 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, 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
       relying on them.

       To include a literal ']' in the list, make it the first character  (following  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  characters,
       including '\', lose their special significance within a bracket expression.

       Within  a  bracket expression, a collating element (a character, a multicharacter 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=]]", "[[=o^=]]",
       and "[oo^]" 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 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 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-compliant.

       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  themselves
       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 nonzero
       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, 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 efficient 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 consensus among implementors as to the right
       interpretation.

AUTHOR

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

SEE ALSO

       grep(1), regex(3)

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

COLOPHON

       This page is part of release 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.

                                            2009-01-12                                   REGEX(7)