*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->OpenBSD man pages -> re_format (7)              



NAME    [Toc]    [Back]

     re_format - POSIX 1003.2 regular expressions

DESCRIPTION    [Toc]    [Back]

     Regular expressions (``RE''s), as defined in  POSIX  1003.2,
come in two
     forms:  modern  REs (roughly those of egrep(1); 1003.2 calls
these ``extended''
 REs) and obsolete  REs  (roughly  those  of  ed(1);
1003.2 ``basic''
     REs).   Obsolete REs mostly exist for backward compatibility
in some old
     programs; they will be discussed at the end.  1003.2  leaves
some aspects
     of  RE  syntax  and  semantics  open; `-' marks decisions on
these aspects
     that may not be fully portable to other  1003.2  implementations.

     A (modern) RE is one- or more non-empty- 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
     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
     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, e.g., `[0-9]'  in
ASCII matches
     any  decimal  digit.  It is illegal- for two ranges to share
an endpoint,
     e.g., `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
     containing  a  multi-character  collating  element  can thus
match more than
     one character, e.g., if the collating  sequence  includes  a
`ch' collating
     element,  then  the  RE  `[[.ch.]]*c' matches the first five
characters of

     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 ctype(3).
A locale may
     provide others.  A character class may not  be  used  as  an
endpoint of a

     There  are  two  special  cases- of bracket expressions: the
bracket expressions
 `[[:<:]]' and `[[:>:]]' match the null string  at  the
beginning and
     end of a word respectively.  A word is defined as a sequence
of characters
 starting and ending with a word character which is neither preceded
     nor  followed  by  word  characters.  A word character is an
alnum character
     (as defined by ctype(3)) or an underscore.  This is  an  extension, compatible
  with  but not specified by POSIX 1003.2, and should be
used with caution
 in software intended to be portable to other systems.

     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
     subject to the constraint that the whole match be as long as
     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
     when `(.*).*' is matched  against  `abc'  the  parenthesized
     matches  all  three  characters, and when `(a*)*' is matched
against `bc'
     both the whole RE and the parenthesized subexpression  match
the null

     If  case-independent  matching  is  specified, the effect is
much as if all
     case distinctions had vanished from the alphabet.   When  an
     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, e.g., `x' becomes `[xX]'.  When
it appears
     inside a bracket expression, all case counterparts of it are
added to the
     bracket  expression, so that (e.g.) `[x]' becomes `[xX]' and
`[^x]' becomes

     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  POSIXcompliant.

     Obsolete  (``basic'')  regular expressions differ in several
     `|', `+', 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
     (after  a  possible leading `^').  Finally, there is one new
type of atom,
     a back reference: `' followed by a non-zero decimal digit  d
matches the
     same sequence of characters matched by the dth parenthesized
 (numbering subexpressions by  the  positions  of  their
opening parentheses,
 left to right), so that (e.g.) `c]1' matches `bb' or
     but not `bc'.

SEE ALSO    [Toc]    [Back]


     POSIX 1003.2, section 2.8 (Regular Expression Notation).

BUGS    [Toc]    [Back]

     Having two kinds of REs is a botch.

     The current 1003.2 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
     `ab*2*d' match `abbbd'?).  Avoid using them.

     1003.2's  specification  of  case-independent  matching   is
vague.  The ``one
     case  implies  all cases'' definition given above is current
     among implementors as to the right interpretation.

     The syntax for word boundaries is incredibly ugly.

OpenBSD     3.6                          March      20,      1994
[ Back ]
 Similar pages
Name OS Title
pthreads OpenBSD POSIX 1003.1c thread interface
perlre OpenBSD Perl regular expressions
perlre IRIX Perl regular expressions
re_exec Tru64 Handle regular expressions
re_comp Tru64 Handle regular expressions
perlreref OpenBSD Perl Regular Expressions Reference
perlretut OpenBSD Perl regular expressions tutorial
perlrequick OpenBSD Perl regular expressions quick start
perlfaq6 OpenBSD Regular Expressions ($Revision: 1.6 $, $Date: 2003/12/03 03:02:44 $)
regcomp IRIX X/Open regular expressions definition and interface
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service