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       re_syntax - Syntax of Tcl regular expressions.


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


       Regular expressions (``RE''s), as defined by POSIX, come  in  two  flavors:  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 backward 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.


       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

       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 quantifiers, 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 inclusive.

       An atom is one of:

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

               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

               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 character taken as an
               ordinary character, e.g. \\ matches a backslash character

         \c    where c is alphanumeric (possibly followed by other characters), 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  constraints 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 `\'.


       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,  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 literal - 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 special 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  expression
       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-character collating │
       element.  Then the RE [[.ch.]]*c (zero or more ch's followed 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 o^
       are the members of an equivalence class, then `[[=o=]]', `[[=o^=]]',  and  `[oo^]'  are  all
       synonymous.  An equivalence class may not be an endpoint 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 elements!)   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  expressions  [[:<:]]  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 underscore
       (_).  These  special  bracket  expressions  are  deprecated;  users  of  AREs  should  use
       constraint escapes instead (see below).


       Escapes  (AREs  only), which begin with a \ followed by an alphanumeric character, come in
       several varieties:  character  entry,  class  shorthands,  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 character 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  applications  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

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

              (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.

              (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 reference (see below)) the
              character whose octal value is 0xy

         \xyz (where  xyz is exactly three octal digits, and is not a back reference (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 compilers) 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 commonly-used character

         \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 equivalent 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 numbered in the order  of  their  leading  parentheses.   Non-capturing
       parentheses 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 otherwise is taken as octal.


       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-dependent 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

         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 characters

         s  non-newline-sensitive matching (usual default)

         t  tight syntax (usual default; see below)

         w  inverse partial newline-sensitive (``weird'') matching (see MATCHING, 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.


       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  quantifiers  (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 preference 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 substrings, based on their  preferences,  with
       subexpressions  starting earlier in the RE taking priority over ones starting later.  Note
       that outer subexpressions thus take priority over their component subexpressions.

       Note that the quantifiers {1,1} and {1,1}?  can be used  to  force  longest  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 symmetry.


       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 expressions.  All other ARE features use
       syntax which is illegal or has undefined 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  constraints,  and the
       longest/shortest-match (rather than first-match) matching semantics.

       The matching rules for REs containing both normal and non-greedy quantifiers 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 significance:

              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 optimize the search order for  fast  matching  is  obsolete  (AREs  examine  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.)


       BREs  differ  from EREs 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, single-digit back references are available, and \< and \> are synonyms for
       [[:<:]] and [[:>:]] respectively; no other escapes are available.


       RegExp(3tcl), regexp(3tcl), regsub(3tcl), lsearch(3tcl), switch(3tcl), text(3tk)


       match, regular expression, string