REGEX(7)                  (2020-08-13)                   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, sepa-
          rated by aq|aq.  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(!) aq*aq,
          aq+aq, aq?aq, or bound.  An atom followed by aq*aq matches a
          sequence of 0 or more matches of the atom.  An atom followed
          by aq+aq matches a sequence of 1 or more matches of the atom.
          An atom followed by aq?aq matches a sequence of 0 or 1 matches
          of the atom.

          A bound is aq{aq followed by an unsigned decimal integer, pos-
          sibly followed by aq,aq possibly followed by another unsigned
          decimal integer, always followed by aq}aq.  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), aq.aq (matching any single character), aqhaaq (matching
          the null string at the beginning of a line), aq$aq (matching
          the null string at the end of a line), a aq\aq followed by one
          of the characters "ha.[$()|*+?{\" (matching that character
          taken as an ordinary character), a aq\aq followed by any other

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          character(!)  (matching that character taken as an ordinary
          character, as if the aq\aq had not been present(!)), or a sin-
          gle character with no other significance (matching that
          character).  A aq{aq 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 aq\aq.

          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 aqhaaq, it
          matches any single character (but see below) not from the
          rest of the list.  If two characters in the list are sepa-
          rated by aq-aq, this is shorthand for the full range of char-
          acters 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 end-
          point, for example, "a-c-e".  Ranges are very collating-
          sequence-dependent, and portable programs should avoid rely-
          ing on them.

          To include a literal aq]aq in the list, make it the first
          character (following a possible aqhaaq).  To include a literal
          aq-aq, make it the first or last character, or the second end-
          point of a range.  To use a literal aq-aq as the first end-
          point of a range, enclose it in "[." and ".]"  to make it a
          collating element (see below).  With the exception of these
          and some combinations using aq[aq (see next paragraphs), all
          other special characters, including aq\aq, lose their special
          significance within a bracket expression.

          Within a bracket expression, a collating element (a charac-
          ter, 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 charac-
          ters of that collating element.  The sequence is a single
          element of the bracket expression's list.  A bracket expres-
          sion 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 oha are the members of an equivalence class, then
          "[[=o=]]", "[[=oha=]]", and "[ooha]" are all synonymous.  An
          equivalence class may not(!) be an endpoint of a range.

          Within a bracket expression, the name of a character class

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          enclosed in "[:" and ":]" stands for the list of all charac-
          ters belonging to that class.  Standard character class
          names are:

               l l l.  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 sub-
          string 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 ele-
          ments.  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 expres-
          sion, it is effectively transformed into a bracket expres-
          sion containing both cases, for example, aqxaq becomes "[xX]".
          When it appears inside a bracket expression, all case coun-
          terparts of it are added to the bracket expression, so that,
          for example, "[x]" becomes "[xX]" and "[hax]" becomes
          "[haxX]".

          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.  aq|aq, aq+aq, and aq?aq are ordinary characters and
          there is no equivalent for their functionality.  The

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          delimiters for bounds are "\{" and "\}", with aq{aq and aq}aq by
          themselves ordinary characters.  The parentheses for nested
          subexpressions are "\(" and "\)", with aq(aq and aq)aq by them-
          selves ordinary characters.  aqhaaq is an ordinary character
          except at the beginning of the RE or(!) the beginning of a
          parenthesized subexpression, aq$aq is an ordinary character
          except at the end of the RE or(!) the end of a parenthesized
          subexpression, and aq*aq is an ordinary character if it
          appears at the beginning of the RE or the beginning of a
          parenthesized subexpression (after a possible leading aqhaaq).

          Finally, there is one new type of atom, a back reference:
          aq\aq 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 aq)aq is an ordinary char-
          acter in the absence of an unmatched aq(aq; this was an unin-
          tentional 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 5.10 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/.

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