That's how I had it originally... and when you've got 23 different possibilities, it adds unnecessary complexity. There's already 23 possibilities wherever I create the regular expressions, but now there's 23 possibilities wherever I use it as well.
Worse is that some of the regular expressions are used multiple places. When I modify a regular expression, I'd like to have it be done in one place (wherever the regexp is created) and not have to worry about it in some other place or places (wherever it's used). As it stands now, I can add new ways to express a date in one place, and it'll automatically -- the routine where I create all my regexps, and it'll automatically go into affect in the various places it might be used.
Not a big problem of course... but I'm a huge fan of Larry's principle of laziness.