regex

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



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, 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 ^ 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 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 5.02 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)