NAME

       string::token - Regex based iterative lexing

SYNOPSIS

       package require Tcl  8.5

       package require string::token  ?1?

       package require fileutil

       ::string token text lex string

       ::string token file lex path

       ::string token chomp lex startvar string resultvar

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DESCRIPTION

       This package provides commands for regular expression based lexing (tokenization) of strings.

       The complete set of procedures is described below.

       ::string token text lex string
              This  command takes an ordered dictionary lex mapping regular expressions to labels, and tokenizes
              the string according to this dictionary.

              The result of the command is a list of tokens, where each token is  a  3-element  list  of  label,
              start- and end-index in the string.

              The command will throw an error if it is not able to tokenize the whole string.

       ::string token file lex path
              This  command  is  a  convenience  wrapper  around  ::string  token text above, and fileutil::cat,
              enabling the easy tokenization of whole files.  Note that this command loads the file wholly  into
              memory before starting to process it.

              If  the  file  is  too large for this mode of operation a command directly based on ::string token
              chomp below will be necessary.

       ::string token chomp lex startvar string resultvar
              This command is the work horse underlying ::string token text above. It is exposed to enable users
              to write their own lexers, which, for example may apply different lexing dictionaries according to
              some internal state, etc.

              The command takes an ordered dictionary lex mapping regular  expressions  to  labels,  a  variable
              startvar  which  indicates  where  to  start  lexing  in  the  input string, and a result variable
              resultvar to extend.

              The result of the command is a tri-state numeric code indicating one of

              0      No token found.

              1      Token found.

              2      End of string reached.

              Note that recognition of a token from lex is started at the character index in startvar.

              If a token was recognized (status 1) the command will update the index in startvar to point to the
              first  character of the string past the recognized token, and it will further extend the resultvar
              with a 3-element list containing the label associated with the regular expression  of  the  token,
              and the start- and end-character-indices of the token in string.

              Neither startvar nor resultvar will be updated if no token is recognized at all.

              Note that the regular expressions are applied (tested) in the order they are specified in lex, and
              the first matching pattern stops the process. Because of this it is  recommended  to  specify  the
              patterns to lex with from the most specific to the most general.

              Further  note  that  all  regex  patterns  are implicitly prefixed with the constraint escape A to
              ensure that a match starts exactly at the character index found in startvar.

BUGS, IDEAS, FEEDBACK

       This document, and the package it describes, will undoubtedly contain bugs and  other  problems.   Please
       report  such  in  the  category  textutil  of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist].
       Please also report any ideas for enhancements you may have for either package and/or documentation.

       When proposing code changes, please provide unified diffs, i.e the output of diff -u.

       Note further that attachments are strongly preferred over inlined patches. Attachments  can  be  made  by
       going  to the Edit form of the ticket immediately after its creation, and then using the left-most button
       in the secondary navigation bar.

KEYWORDS

       lexing, regex, string, tokenization

CATEGORY

       Text processing