Allows lexers to be defined using regular expressions ''a la'' cl-ppcre.  A lexer will convert (or ''tokenize'') a text string into a list of tokens (consisting of a token-class and the token's image).  For example:

{{{
(defun snip (s) (subseq s 1 (1- (length s))))

(defun un-squote (s) (regex-replace-all "''" (snip s) "'"))
(defun un-dquote (s) (regex-replace-all "\"\"" (snip s) "\""))

(deflexer scan-csv
  ("[^\"',]+" value)
  ("'(?:[^']|'')*'" value un-squote)
  ("\"(?:[^\"]|\"\")*\"" value un-dquote)
  ("," comma))
}}}

See the distribution for a more complete example.