A few weeks back we made the URL field editable, so you can specify the exact URL in question.
As for regexp support - ideally we want to make LastPass usable by mere mortals.
We'd hate to think that people - even advanced users - have to write regexps to use LastPass.
But, we currently do understand your need for this.
We're going to make some serious efforts shortly to improve our matching algorithms.
We're hoping that after we're done, you and other users wont have the need for regexps.
We're not sure if we can completely rid the need for them, but we want to try.
So, check back in a few weeks and if you still feel that regexps are needed, please let us know.
I wish LastPass could match URL to login data not only at domain level, but also using more precise URL. I often have different sub-sites under the same domain, eg. http://www.domain.com/site1 and http://www.domain.com/site2, so logins to all of them gets mixed up in one AutoFill list. Will you consider such situation when improving LastPass matching algorithms?