regex(7) — Linux manual page

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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 5.10 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/.

                               2020-08-13                       REGEX(7)

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