validates_url | URL Validation for Rails | Validation library

I have a situation like that, I wanna check a field before save if it's a correct URL then save it, if not then change it to a correct way, my question is the validates function(regexp) is similar to Rails origin validates function, I don't wanna repeat it cause DRY principle, so is there any way can call the Rails validates alone?

I know I can do that by the way in below, but the syntax means check model with entire validates, so if other filed validate fail, it will change URL format, that's what I want, I wanna change URL format to correct format only when the URL format is not correct before save.

What's the best way to do it?