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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 implementa‐
       tions.

       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 (inclu‐
       sive) 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 ex‐
       pression (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 charac‐
       ter(!)  (matching that character taken as an ordinary character, as if the '\' had not been present(!)), or a single character with no other significance (match‐
       ing  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 ex‐
       pression's list.  A bracket expression containing a multicharacter collating element can thus match more than one character, for example, if  the  collating  se‐
       quence 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 el‐
       ements 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 mul‐
       tiple cases appears as an ordinary character outside a bracket expression, it is effectively transformed into a bracket expression containing both cases, for ex‐
       ample, '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 subex‐
       pression, '


 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).

                                                                               2020-08-13                                                                       REGEX(7)