idea: filter with regular expression

It may be useful to be able to use a regular expression to accept or reject candidates in place of external filter. Such feature could replace List.External:Filter_Alnumspace, Filter_Alnum, Filter_LowerNum, and so on. But it will not be able to replace List.External:Filter_LM_ASCII that does uppercase.

Realistic use-case may be the following: we run incremental mode but pick only candidates with a substring (or of some complicated form). It may be impossible to make good incremental picking by substring before creation of charset file, so we might generate charset from full set of cracks and then pick candidates. Pair option to reject candidates by substring may be used to try remaining candidates on other machine.

Start by reviewing the existing List.External:Filter_Alnumspace, Filter_Alnum, Filter_LowerNum, and Filter_LM_ASCII options, then trace how incremental mode generates candidates and uses charset files. The issue is done when regular-expression acceptance and rejection are defined and usable for the described incremental-candidate workflow, with behavior and option details established.