CL-LEXER package The CL-LEXER package implements a lexical-analyzer-generator called DEFLEXER, which is built on top of both REGEX and CLAWK. Many of the optimizations in the recent rewrite of the regex engine went into optimizing the sorts of patterns generated by DEFLEX. The default lexer doesn't implement full greediness. If you have a rule for ints followed by a rule for floats, the int rule will match on the part before the decimal before the float rule gets a chance to look at it. You can fix this by specifying :flex-compatible as the first rule. This gives all patterns a chance to examine the text and takes the one that matches the longest string (first pattern wins in case of a tie). The down side of this option is that it slows down the analyser. If you can solve the issue by reordering your rules that's the way to do it. I'm currently writing an AWK->CLAWK translator using this as the lexer, and it's working fine. As far as I can tell, the DEFLEXER-generated lexing functions should be fast enough for production use. Currently, the LEX/FLEX/BISON feature of switching productions on and off using state variables is not supported, but it's a pretty simple feature to add. If you're using CL-LEXER and discover you need this feature, let me know. It also doesn't yet support prefix and postfix context patterns. This isn't quite so trivial to add, but it's planned for a future release of regex, so CL-LEXER will be getting it someday. Anyway, Here's a simple DEFLEXER example: (deflexer test-lexer ("[0-9]+([.][0-9]+([Ee][0-9]+)?)" (return (values 'flt (num %0)))) ("[0-9]+" (return (values 'int (int %0)))) ("[:alpha:][:alnum:]*" (return (values 'name %0))) ("[:space:]+") ) > (setq *lex* (test-lexer "1.0 12 fred 10.23e45")) <closure> > (funcall *lex*) FLT 1.0 > (funcall *lex*) INT 12 > (funcall *lex*) NAME "fred" > (funcall *lex*) FLT 1.0229999999999997E46 > (funcall *lex*) NIL NIL You can also write this lexer using the :flex-compatible option, in which case you can write the int and flt rules in any order. (deflexer test-lexer :flex-compatible ("[0-9]+" (return (values 'int (int %0)))) ("[0-9]+([.][0-9]+([Ee][0-9]+)?)" (return (values 'flt (num %0)))) ("[:space:]+") )