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 pro-
       grams;  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  charac-
       ter 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  charac-
       ters  of all collating elements equivalent to that one, including itself.  (If there are no other equivalent collating elements, the treat-
       ment 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  subexpres-
       sions.

       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  alpha-
       betic  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  counter-
       parts 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 consen-
       sus 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 4.15 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)