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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 does not match others.


       Regular expressions (“RE”s), as defined by  POSIX,  come  in  two  flavors:  extended  REs
       (“ERE”s)  and  basic REs (“BRE”s).  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 (“ARE”s), 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 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  single  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)  matches a match for re (re is any regular expression) 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    matches the non-alphanumeric character k 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
       Unicode matches any conventional decimal digit. Two ranges may not share an  endpoint,  so
       e.g.   “a-c-e”  is  illegal.  Ranges in Tcl always use the Unicode collating sequence, but
       other  programs  may  use  other  collating  sequences  and  this  can  be  a  source   of
       incompatability between programs.

       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, 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   A "printable" (same as graph, except also including space).

       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 (includes both alnum and punct).

       cntrl   A control character.

       A locale may provide others. A character class may not be used as an endpoint of a range.

              (Note:  the  current  Tcl  implementation  has only one locale, the Unicode locale,
              which supports exactly the above classes.)

       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).

       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 has no multi-character collating elements. This information is only for

       For example, assume  the  collating  sequence  includes  a  ch  multi-character  collating
       element.  Then  the RE “[[.ch.]]*c” (zero or more “chs” 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  ô
       are  the  members  of  an equivalence class, then “[[=o=]]”, “[[=ô=]]”, and “[oô]” are all
       synonymous. An equivalence class may not be an endpoint of a range.

              (Note: Tcl implements only the Unicode locale. It does not define  any  equivalence
              classes. The examples above are just illustrations.)


       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 Unicode, but \135 does not terminate a bracket expression. Beware, however, that some
       applications (e.g., C compilers and the Tcl interpreter if the regular expression  is  not
       quoted  with  braces)  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 is not 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 do not 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