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           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, 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 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
           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=]]",
           "[[=o<I>^=]]",  and  "[oo<I>^]"  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
           "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
           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 3.35 of the Linux  man-pages  project.   A
           description  of  the project, and information about reporting bugs, can
           be found at
                                      2009-01-12                          REGEX(7)

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