NAME | DESCRIPTION | BUGS | AUTHOR | SEE ALSO | COLOPHON

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

NAME         top

       regex - POSIX.2 regular expressions

DESCRIPTION         top

       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, 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
       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 o^ 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         top

       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         top

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

SEE ALSO         top

       grep(1), regex(3)

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

COLOPHON         top

       This page is part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

                                 2009-01-12                         REGEX(7)