Validate url in forms using regex

Hello,

I'm trying to validate that a user enters a URL in a text field.  I am trying to use regex validation with the following string.

^(http://www.|https://www.|http://|https://)?[a-z0-9]+([-.]{1}[a-z0-9]+)*.[a-z]{2,5}(:[0-9]{1,5})?(/.*)?$

It works on a regex tester but not on the form.  Is there something I need to do to use it on the form, or is the pattern bumpkis?

Thanks for your help!