NAME
       re_syntax - Syntax of Tcl regular expressions.


DESCRIPTION
       A  regular  expression describes strings of characters.  It's a pattern
       that matches certain strings and doesn't match others.


DIFFERENT FLAVORS OF REs
       Regular expressions (``RE''s), as defined by POSIX, come  in  two  fla-
       vors:  extended  REs  (``EREs'')  and  basic  REs (``BREs'').  EREs are
       roughly those of the traditional egrep, while BREs are roughly those of
       the  traditional ed.  This implementation adds a third flavor, advanced
       REs (``AREs''), basically EREs with some significant extensions.

       This manual page primarily describes AREs.  BREs mostly exist for back-
       ward  compatibility in some old programs; they will be discussed at the
       end.  POSIX EREs are almost an exact subset of AREs.  Features of  AREs
       that are not present in EREs will be indicated.


REGULAR EXPRESSION SYNTAX
       Tcl  regular  expressions  are implemented using the package written by
       Henry Spencer, based on the 1003.2 spec and some (not quite all) of the
       Perl5  extensions (thanks, Henry!).  Much of the description of regular
       expressions below is copied verbatim from his manual entry.

       An ARE is one or more branches, separated  by  `|',  matching  anything
       that matches any of the branches.

       A branch is zero or more constraints or quantified atoms, concatenated.
       It matches a match for the first, followed by a match for  the  second,
       etc; an empty branch matches the empty string.

       A  quantified atom is an atom possibly followed by a single quantifier.
       Without a quantifier, it matches a match for  the  atom.   The  quanti-
       fiers, and what a so-quantified atom matches, are:

         *     a sequence of 0 or more matches of the atom

         +     a sequence of 1 or more matches of the atom

         ?     a sequence of 0 or 1 matches of the atom

         {m}   a sequence of exactly m matches of the atom

         {m,}  a sequence of m or more matches of the atom

         {m,n} a  sequence  of  m through n (inclusive) matches of the atom; m
               may not exceed n

         *?  +?  ??  {m}?  {m,}?  {m,n}?
               non-greedy quantifiers, which match the same possibilities, but
               prefer  the  smallest  number rather than the largest number of
               matches (see MATCHING)

       The forms using { and } are known as bounds.  The numbers m and  n  are
       unsigned  decimal integers with permissible values from 0 to 255 inclu-
       sive.

       An atom is one of:

         (re)  (where re is any regular expression) matches a  match  for  re,
               with the match noted for possible reporting

         (?:re)
               as  previous, but does no reporting (a ``non-capturing'' set of
               parentheses)

         ()    matches an empty string, noted for possible reporting

         (?:)  matches an empty string, without reporting

         [chars]
               a bracket expression,  matching  any  one  of  the  chars  (see
               BRACKET EXPRESSIONS for more detail)

          .    matches any single character

         \k    (where  k is a non-alphanumeric character) matches that charac-
               ter taken as an ordinary character, e.g. \\ matches a backslash
               character

         \c    where  c  is  alphanumeric  (possibly followed by other charac-
               ters), an escape (AREs only), see ESCAPES below

         {     when followed by a character other than a  digit,  matches  the
               left-brace  character  `{'; when followed by a digit, it is the
               beginning of a bound (see above)

         x     where x is a  single  character  with  no  other  significance,
               matches that character.

       A  constraint matches an empty string when specific conditions are met.
       A constraint may not be followed by  a  quantifier.   The  simple  con-
       straints  are  as  follows;  some more constraints are described later,
       under ESCAPES.

         ^       matches at the beginning of a line

         $       matches at the end of a line

         (?=re)  positive lookahead (AREs only), matches at any point where  a
                 substring matching re begins

         (?!re)  negative lookahead (AREs only), matches at any point where no
                 substring matching re begins

       The lookahead constraints may not contain back references (see  later),
       and all parentheses within them are considered non-capturing.

       An RE may not end with `\'.


BRACKET EXPRESSIONS
       A bracket expression is a list of characters enclosed in `[]'.  It nor-
       mally 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, e.g.  [0-9] in ASCII  matches  any  decimal  digit.
       Two  ranges  may  not  share  an  endpoint,  so e.g.  a-c-e is illegal.
       Ranges are very  collating-sequence-dependent,  and  portable  programs
       should avoid relying on them.

       To  include  a  literal  ]  or - in the list, the simplest method is to
       enclose it in [. and .]  to make it a collating  element  (see  below).
       Alternatively,  make it the first character (following a possible `^'),
       or (AREs only) precede it with `\'.  Alternatively, for  `-',  make  it
       the  last  character, or the second endpoint of a range.  To use a lit-
       eral - as the first endpoint of a range, make it a collating element or
       (AREs  only)  precede  it  with `\'.  With the exception of these, some
       combinations using [ (see next paragraphs), and escapes, all other spe-
       cial  characters  lose  their  special  significance  within  a bracket
       expression.

       Within a bracket expression, a collating element (a character, a multi-
       character 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  expres-
       sion  in  a locale that has multi-character collating elements can thus
       match more than one character.  So (insidiously), a bracket  expression
       that starts with ^ can match multi-character collating elements even if
       none of them appear in the bracket expression!   (Note:  Tcl  currently
       has  no  multi-character  collating elements.  This information is only
       for illustration.)

       For example, assume the collating sequence includes a ch  multi-charac-
       ter  collating element.  Then the RE [[.ch.]]*c (zero or more ch's fol-
       lowed by c) matches the first five characters of `chchcc'.   Also,  the
       RE [^c]b matches all of `chb' (because [^c] matches the multi-character
       ch).

       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 members of an equivalence class, then `[[=o=]]', `[[=^=]]',
       and `[o^]' are all synonymous.  An equivalence class may not be an end-
       point  of  a  range.   (Note: Tcl currently implements only the Unicode
       locale.  It doesn't define any equivalence classes.  The examples above
       are just illustrations.)

       Within  a bracket expression, the name of a character class enclosed in
       [: and :] stands for the list of all characters (not all collating ele-
       ments!)  belonging to that class.  Standard character classes are:

              alpha       A letter.
              upper       An upper-case letter.
              lower       A lower-case letter.
              digit       A decimal digit.
              xdigit      A hexadecimal digit.
              alnum       An alphanumeric (letter or digit).
              print       An alphanumeric (same as alnum).
              blank       A space or tab character.
              space       A character producing white space in displayed text.
              punct       A punctuation character.
              graph       A character with a visible representation.
              cntrl       A control character.

       A locale may provide others.  (Note that the current Tcl implementation
       has only one locale: the Unicode locale.)  A character class may not be
       used as an endpoint of a range.

       There are two special cases of bracket expressions: the bracket expres-
       sions [[:<:]] and [[:>:]] are constraints, matching  empty  strings  at
       the  beginning  and end of a word respectively.  A word is defined as a
       sequence of word characters that is neither preceded  nor  followed  by
       word  characters.   A word character is an alnum character or an under-
       score (_).  These special bracket expressions are deprecated; users  of
       AREs should use constraint escapes instead (see below).

ESCAPES
       Escapes  (AREs  only), which begin with a \ followed by an alphanumeric
       character, come in several varieties:  character  entry,  class  short-
       hands,  constraint  escapes,  and  back references.  A \ followed by an
       alphanumeric character but not constituting a valid escape  is  illegal
       in  AREs.  In EREs, there are no escapes: outside a bracket expression,
       a \ followed by an alphanumeric character merely stands for that  char-
       acter  as  an ordinary character, and inside a bracket expression, \ is
       an ordinary character.  (The latter is the one  actual  incompatibility
       between EREs and AREs.)

       Character-entry  escapes (AREs only) exist to make it easier to specify
       non-printing and otherwise inconvenient characters in REs:

         \a   alert (bell) character, as in C

         \b   backspace, as in C

         \B   synonym for \ to help reduce backslash doubling in some applica-
              tions where there are multiple levels of backslash processing

         \cX  (where  X is any character) the character whose low-order 5 bits
              are the same as those of X, and whose other bits are all zero

         \e   the character whose collating-sequence name is `ESC', or failing
              that, the character with octal value 033

         \f   formfeed, as in C

         \n   newline, as in C

         \r   carriage return, as in C

         \t   horizontal tab, as in C

         \uwxyz
              (where  wxyz  is  exactly  four  hexadecimal digits) the Unicode
              character U+wxyz in the local byte ordering

         \Ustuvwxyz
              (where stuvwxyz is exactly eight  hexadecimal  digits)  reserved
              for a somewhat-hypothetical Unicode extension to 32 bits

         \v   vertical tab, as in C are all available.

         \xhhh
              (where  hhh is any sequence of hexadecimal digits) the character
              whose hexadecimal value is 0xhhh (a single character  no  matter
              how many hexadecimal digits are used).

         \0   the character whose value is 0

         \xy  (where  xy is exactly two octal digits, and is not a back refer-
              ence (see below)) the character whose octal value is 0xy

         \xyz (where xyz is exactly three octal digits, and is not a back ref-
              erence (see below)) the character whose octal value is 0xyz

       Hexadecimal digits are `0'-`9', `a'-`f', and `A'-`F'.  Octal digits are
       `0'-`7'.

       The character-entry escapes are always taken  as  ordinary  characters.
       For  example, \135 is ] in ASCII, but \135 does not terminate a bracket
       expression.  Beware, however, that some applications (e.g.,  C  compil-
       ers)  interpret such sequences themselves before the regular-expression
       package gets to see them,  which  may  require  doubling  (quadrupling,
       etc.) the `\'.

       Class-shorthand escapes (AREs only) provide shorthands for certain com-
       monly-used character classes:

         \d        [[:digit:]]

         \s        [[:space:]]

         \w        [[:alnum:]_] (note underscore)

         \D        [^[:digit:]]

         \S        [^[:space:]]

         \W        [^[:alnum:]_] (note underscore)

       Within bracket expressions, `\d',  `\s',  and  `\w'  lose  their  outer
       brackets,  and `\D', `\S', and `\W' are illegal.  (So, for example, [a-
       c\d] is equivalent to [a-c[:digit:]].  Also, [a-c\D], which is  equiva-
       lent to [a-c^[:digit:]], is illegal.)

       A  constraint  escape  (AREs  only) is a constraint, matching the empty
       string if specific conditions are met, written as an escape:

         \A    matches only at the beginning  of  the  string  (see  MATCHING,
               below, for how this differs from `^')

         \m    matches only at the beginning of a word

         \M    matches only at the end of a word

         \y    matches only at the beginning or end of a word

         \Y    matches  only  at a point that is not the beginning or end of a
               word

         \Z    matches only at the end of the string (see MATCHING, below, for
               how this differs from `$')

         \m    (where m is a nonzero digit) a back reference, see below

         \mnn  (where  m  is  a nonzero digit, and nn is some more digits, and
               the decimal value mnn is not greater than the number of closing
               capturing parentheses seen so far) a back reference, see below

       A word is defined as in the specification of [[:<:]] and [[:>:]] above.
       Constraint escapes are illegal within bracket expressions.

       A back reference (AREs only) matches the same  string  matched  by  the
       parenthesized  subexpression  specified  by  the number, so that (e.g.)
       ([bc])\1 matches bb  or  cc  but  not  `bc'.   The  subexpression  must
       entirely precede the back reference in the RE.  Subexpressions are num-
       bered in the order of their leading parentheses.  Non-capturing  paren-
       theses do not define subexpressions.

       There is an inherent historical ambiguity between octal character-entry
       escapes and back references, which is resolved by heuristics, as hinted
       at  above.   A leading zero always indicates an octal escape.  A single
       non-zero digit, not followed by another digit, is  always  taken  as  a
       back  reference.   A  multi-digit  sequence not starting with a zero is
       taken as a back reference if it comes after  a  suitable  subexpression
       (i.e.  the number is in the legal range for a back reference), and oth-
       erwise is taken as octal.

METASYNTAX
       In addition to the main syntax described above, there are some  special
       forms and miscellaneous syntactic facilities available.

       Normally the flavor of RE being used is specified by application-depen-
       dent means.  However, this can be overridden by a director.  If  an  RE
       of  any flavor begins with `***:', the rest of the RE is an ARE.  If an
       RE of any flavor begins with `***=', the rest of the RE is taken to  be
       a literal string, with all characters considered ordinary characters.

       An ARE may begin with embedded options: a sequence (?xyz) (where xyz is
       one or more alphabetic characters) specifies options affecting the rest
       of  the  RE.  These supplement, and can override, any options specified
       by the application.  The available option letters are:

         b  rest of RE is a BRE

         c  case-sensitive matching (usual default)

         e  rest of RE is an ERE

         i  case-insensitive matching (see MATCHING, below)

         m  historical synonym for n

         n  newline-sensitive matching (see MATCHING, below)

         p  partial newline-sensitive matching (see MATCHING, below)

         q  rest of RE is a literal (``quoted'') string, all ordinary  charac-
            ters

         s  non-newline-sensitive matching (usual default)

         t  tight syntax (usual default; see below)

         w  inverse partial newline-sensitive (``weird'') matching (see MATCH-
            ING, below)

         x  expanded syntax (see below)

       Embedded options take effect at the ) terminating the  sequence.   They
       are  available  only  at the start of an ARE, and may not be used later
       within it.

       In addition to the usual (tight) RE syntax, in which all characters are
       significant,  there  is an expanded syntax, available in all flavors of
       RE with the -expanded switch, or in AREs with the  embedded  x  option.
       In  the  expanded  syntax,  white-space  characters are ignored and all
       characters between a # and the following newline (or the end of the RE)
       are  ignored,  permitting  paragraphing  and  commenting  a complex RE.
       There are three exceptions to that basic rule:

         a white-space character or `#' preceded by `\' is retained

         white space or `#' within a bracket expression is retained

         white space and comments are illegal within  multi-character  symbols
         like the ARE `(?:' or the BRE `\('

       Expanded-syntax white-space characters are blank, tab, newline, and any
       character that belongs to the space character class.

       Finally, in an ARE, outside bracket expressions, the sequence `(?#ttt)'
       (where  ttt  is any text not containing a `)') is a comment, completely
       ignored.  Again, this is not allowed between the characters  of  multi-
       character  symbols  like  `(?:'.   Such  comments are more a historical
       artifact than a useful facility, and their use is deprecated;  use  the
       expanded syntax instead.

       None of these metasyntax extensions is available if the application (or
       an initial ***= director)  has  specified  that  the  user's  input  be
       treated as a literal string rather than as an RE.

MATCHING
       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, its
       choice is determined by its preference: either the  longest  substring,
       or the shortest.

       Most  atoms,  and all constraints, have no preference.  A parenthesized
       RE has the same preference (possibly none) as  the  RE.   A  quantified
       atom  with  quantifier  {m}  or {m}?  has the same preference (possibly
       none) as the atom itself.  A quantified atom with other normal  quanti-
       fiers  (including  {m,n}  with  m equal to n) prefers longest match.  A
       quantified atom with other  non-greedy  quantifiers  (including  {m,n}?
       with m equal to n) prefers shortest match.  A branch has the same pref-
       erence as the first quantified atom in it which has a  preference.   An
       RE  consisting  of  two  or  more  branches connected by the | operator
       prefers longest match.

       Subject to the constraints imposed by the rules for matching the  whole
       RE,  subexpressions  also  match  the longest or shortest possible sub-
       strings, based on their preferences, with subexpressions starting  ear-
       lier  in  the  RE  taking priority over ones starting later.  Note that
       outer subexpressions thus take priority over their component subexpres-
       sions.

       Note  that the quantifiers {1,1} and {1,1}?  can be used to force long-
       est and shortest preference, respectively,  on  a  subexpression  or  a
       whole RE.

       Match  lengths  are measured in characters, not collating elements.  An
       empty string is considered longer than no match at all.   For  example,
       bb*    matches    the    three    middle    characters    of   `abbbc',
       (week|wee)(night|knights) 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 an empty 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, so that x becomes  `[xX]'.   When  it
       appears  inside  a  bracket expression, all case counterparts of it are
       added to the bracket expression, so that  [x]  becomes  [xX]  and  [^x]
       becomes `[^xX]'.

       If  newline-sensitive matching is specified, .  and bracket expressions
       using ^ will never match the newline character (so  that  matches  will
       never  cross newlines unless the RE explicitly arranges it) and ^ and $
       will match the empty string after and before a newline respectively, in
       addition  to matching at beginning and end of string respectively.  ARE
       \A and \Z continue to match beginning or end of string only.

       If partial newline-sensitive matching is specified, this affects .  and
       bracket  expressions  as with newline-sensitive matching, but not ^ and
       `$'.

       If  inverse  partial  newline-sensitive  matching  is  specified,  this
       affects  ^  and  $  as  with newline-sensitive matching, but not .  and
       bracket expressions.  This isn't very useful but is provided for symme-
       try.

LIMITS AND COMPATIBILITY
       No particular limit is imposed on the length of REs.  Programs intended
       to be highly portable should not employ REs longer than 256 bytes, as a
       POSIX-compliant implementation can refuse to accept such REs.

       The  only feature of AREs that is actually incompatible with POSIX EREs
       is that \ does not lose its special significance inside bracket expres-
       sions.  All other ARE features use syntax which is illegal or has unde-
       fined or unspecified effects in POSIX EREs; the *** syntax of directors
       likewise is outside the POSIX syntax for both BREs and EREs.

       Many  of  the ARE extensions are borrowed from Perl, but some have been
       changed to clean them up, and a few Perl extensions  are  not  present.
       Incompatibilities  of  note  include  `\b',  `\B',  the lack of special
       treatment for a trailing newline, the addition of complemented  bracket
       expressions  to  the things affected by newline-sensitive matching, the
       restrictions on parentheses  and  back  references  in  lookahead  con-
       straints,  and  the  longest/shortest-match  (rather  than first-match)
       matching semantics.

       The matching rules for REs containing both normal and non-greedy  quan-
       tifiers  have  changed  since early beta-test versions of this package.
       (The new rules are much simpler and cleaner, but don't work as hard  at
       guessing the user's real intentions.)

       Henry  Spencer's  original 1986 regexp package, still in widespread use
       (e.g., in pre-8.1 releases of Tcl), implemented  an  early  version  of
       today's  EREs.  There are four incompatibilities between regexp's near-
       EREs (`RREs' for short) and AREs.  In roughly increasing order of  sig-
       nificance:

              In  AREs,  \  followed by an alphanumeric character is either an
              escape or an error, while in RREs, it was just  another  way  of
              writing  the alphanumeric.  This should not be a problem because
              there was no reason to write such a sequence in RREs.

              { followed by a digit in an ARE is the  beginning  of  a  bound,
              while  in  RREs,  {  was  always  an  ordinary  character.  Such
              sequences should be rare, and will  often  result  in  an  error
              because following characters will not look like a valid bound.

              In AREs, \ remains a special character within `[]', so a literal
              \ within [] must be written `\\'.  \\ also  gives  a  literal  \
              within [] in RREs, but only truly paranoid programmers routinely
              doubled the backslash.

              AREs report the longest/shortest match for the RE,  rather  than
              the  first  found  in a specified search order.  This may affect
              some RREs which were written in the expectation that  the  first
              match would be reported.  (The careful crafting of RREs to opti-
              mize the search order for fast matching is obsolete (AREs  exam-
              ine  all  possible matches in parallel, and their performance is
              largely insensitive to their complexity)  but  cases  where  the
              search  order  was  exploited to deliberately find a match which
              was not the longest/shortest will need rewriting.)


BASIC REGULAR EXPRESSIONS
       BREs differ from EREs in several respects.  `|', `+', and ?  are  ordi-
       nary  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, single-digit back references are available, and \<  and
       \>  are synonyms for [[:<:]] and [[:>:]] respectively; no other escapes
       are available.


SEE ALSO
       RegExp(3), regexp(n), regsub(n), lsearch(n), switch(n), text(n)


KEYWORDS
