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fickle 2.04 by Jason Tang (tang@jtang.org)

This is a scanner generator program much like flex(1) is to C. If you have no desire to author Tcl programs, particularly those that manipulate text, fickle is not for you. A passing knowledge of flex or some other lex-like program would be useful as that fickle uses nearly identical syntax and commands as flex. Two good references are the flex(1) man page and the O’Reilly book ‘lex & yacc’ by Levine, Mason, and Brown.

Examples of working fickle code may be found in the ‘examples’ directory. See the examples’ README for further details.

fickle is protected by the GNU general public license. See the file COPYING for details.


fickle is to be used as a command-line utility that translates ‘fickle specification files’ into valid Tcl code. Invoke fickle like so:

$ tclsh fickle.tcl some_spec_file.f

and it will generate a file ‘some_spec_file.tcl’ containing the resultant scanner. fickle supports the more popular of flex’s options:

Usage: fickle [options] [FILE]
  FILE     a fickle specification file

  -h          print this help message and quit
  -v          be verbose while generating scanner
  -o FILE     specify name to write scanner
  -d          enable debug mode while running scanner
  -i          generate a case-insensitive scanner
  -l          keep track of line numbers in global variable yylineno
  -s          suppress default rule; unmatched input aborts with errors
  -t          write scanner to standard output
  -I          read input interactively
  -P PREFIX   change default yy prefix to PREFIX
  --version   print fickle version and quit

If no input files are given fickle reads from standard input. Also like flex fickle supports the following ‘%option’ directives (and their “no” counterparts):

caseful or case-sensitive opposite of -i option (default) caseless or case-insensitive -i option debug -d option default opposite of -s option interactive -I option verbose -v option stack enables start states yylineno enables tracking of line numbers yywrap call [yywrap] upon end-of-file

In addition fickle has two additional directives:

%buffersize NUM set size of internal input buffer (default 1024) %option noheaders strips fickle-generated comments from output file


fickle is capable of most of flex’s functionality. In addition to the options listed above the following functions work how one would expect within a Tcl environment:

input, unput, yy_scan_string, yyless, yylex, yyrestart, yywrap, ECHO, YY_FLUSH_BUFFER, and YY_INPUT

as well as these global variables:

::yytext, ::yyleng, ::yyin, and ::yyout

With debug mode enabled (either -d flag or %option debug) fickle adds a global variable ::yy_flex_debug. Set this to non-zero to display to standard error every time the scanner matches a pattern as well as when it reaches the end of a file.

With start states enabled (%option stack) one now can call the functions yy_push_state, yy_pop_state, yy_top_state, and BEGIN. Like flex fickle allows for both inclusionary (%s directive) and exclusionary (%x) states.

With line numbers enabled (%option yylineno) fickle will keep track of newlines within the input file. The line number may be accessed through the global variable ::yylineno.

See a generated file for full documentation of these fickle-supplied functions, assuming that one did not call ‘%option noheaders’.


fickle does its best to emulate flex but there are some important differences to note. The following functions/macros are not supported by fickle:

output, yymore, yy_*_buffer, REJECT, YY_CURRENT_BUFFER, or YY_DECL

nor does it support the declarations %T, %unused, or %used. Unlike flex, unput() is a procedure that takes accepts any string not just a character at a time.

Textual substitutions of definitions is kind of blind, and will ignore backslashes preceding opening braces. For example, if there exists a definition ‘foo’ then it would be substituted into the patterns “{foo}” as well as “{foo}”. Substitutions are performed by order of appearance. Thus if the result of one substitution creates a pattern that looks like a second definition then a second substitution occurs. To prevent this behavior place definitions that might result in creating a valid name higher up in the file. Furthermore fickle will not issue any warnings whenever a pattern has an undefined name.

Interactive mode differs somewhat from flex. fickle reads from $::yyin a block of bytes at a time; by default this block is 1024 bytes though it may be changed with %buffersize. This is akin to flex’s batch processing mode. However this behavior is very undesirable for interactive programs; fickle would block until a user types in 1024 characters. Instead when in interactive mode, set by either the -I command line option or %interactive directive, fickle reads a line at a time from $::yyin through the [gets] procedure. Unlike flex, fickle defaults to batch mode and not interactive mode.

The start state INITIAL is exactly that – the literal “INITIAL” and not the value zero. In addition fickle does not support start condition scopes.

fickle calls Tcl’s [regexp] to handle pattern matching, so any valid Tcl regexp is valid under fickle. This does lead to some incompatiblities with flex-style regexps. <> is unsupported. Circumflexes (^) may behave oddly; fickle tries to handle ^ sanely by modifying its internal buffer whenever it matches newlines. Finally, Tcl regexps do not treat double quotation marks as metacharacters. For example, given the regular expression "/*" the call:

regexp – {"/*"} $string

attempts to match any number of forward slashes rather than a C-style comment token. fickle rewrites patterns containing double quotes to explicitly escape metacharacters within. Therefore fickle instead interprets the above pattern as:

regexp – {/*} $string


fickle, like flex, allows the user to change all ‘yy’ prefaces through the -P flag. The argument to -P will automagically be downcased. However, the pre-defined macro ‘BEGIN’ does not have a prefix. To get around this limitation it takes an optional second parameter which will direct it to the correct parameter. For example suppose one invokes fickle with ‘-P zz’. All internal calls to ‘BEGIN’ will set “zz” as the second parameter. Any of your code which calls ‘BEGIN’ will need to pass “zz” as well, otherwise ‘BEGIN’ will default to using the “yy” internally.

Finally, fickle will exhaust its internal buffer prior to calling yywrap. That means regular expressions cannot match across file boundaries.