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       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).
       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

       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

       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-

       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


       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.


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


       grep(1), regex(3)

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


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