isin is a poor link matching algorithm, if it works at all. The least you could do is run a regex pattern to better ensure the string posted is a link.

$regex($1-,http:\/\/\.(.*)\.(com|xxx|net|org|me|gov|tv|tk))

I'm terrible at regex so this is a first pass at something to better capture links. I'm sure someone will comment soon enough with a more refined example but may this serve as a base.