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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 pro‐
       grams; they will be discussed at the end.  POSIX.2 leaves some aspects of RE syntax  and  se‐
       mantics  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  fol‐
       lowed  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  se‐
       quence 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 in‐
       teger 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  expres‐
       sion),  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  sin‐
       gle  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 combina‐
       tions 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 se‐
       quence 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  mem‐
       bers  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 possi‐
       ble  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 subex‐
       pressions.

       Match lengths are measured in characters, not collating elements.  A null string  is  consid‐
       ered  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 charac‐
       ters, and when "(a*)*" is matched against "bc" both the whole RE and the parenthesized subex‐
       pression 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 or‐
       dinary  characters  and  there  is no equivalent for their functionality.  The delimiters for
       bounds are "\{" and "\}", with '{' and '}' by themselves ordinary characters.  The  parenthe‐
       ses  for  nested  subexpressions  are  "\(" and "\)", with '(' and ')' by themselves ordinary
       characters.  '^' is an ordinary character except at the beginning of the RE or(!) the  begin‐
       ning  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 ap‐
       pears  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 subexpres‐
       sion (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 un‐
       matched '('; 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  inter‐
       pretation.

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/.



                                             2020-08-13                                     REGEX(7)