Three possibilities on creating a regex-generality-rating come into my mind:

1. generate all strings of a given length L and count how many are matched by the regex
2. use the regex to calculate the number of all possible strings (up to a given length?!)
3. do some inexact estimation

1. would be easy to implement (generate all strings, match) - but is probably much too slow (just counting the letters and numbers would give you 26+26+10 different characters at each position of your string ...
... so this approach is not feasible :-(

2. is more tricky - you need to analyze the regex, but this is possible (see the previous answers, or here for some tools for this.)

I would then calculate two ratings for this and combine them in a suitable way (however suitable looks like ;-)

• a freedom rating:
  • each time you encounter some freedom (e.g. a-z you multiply the rating by the # of possibilities (26 in this example)
  • this covers stutations like OR, ranges, numbers of occurences {2-5}, ? ...
• a fixed rating:
  • this counts the number of fixed characters in the regex
  • I would use this to distinguish between "Hi" and "Hello" - having both the same "freedom", but "Hello" is more specific

However I see some problems with this approach:

• how to "tweak" the formula to identify that all the following are the same (and have the same rating)

  Hi  =  ^.*Hi   =  ^.*.?Hi   =  .*Hi.*    =  ^.*.?.*Hi.*$
  [download]

  you might overcome this by adding "^.*" and ".*$" to your regexes before analysis ...
• how will this work with complex regexes (e.g. lookaheads/lookbehinds or even (?{ code }) )
• ...

3. If I had the task you describe, I'd first check my possible input-regexes (maybe most of them are just simple?) and check if there is an issue if some are not ordered correctly. Probably it is possible to create a simplified formula that works well in most cases ...

HTH, Rata (very interested on how you sort this problem out!)

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.