I just don't understand why so many people like to reinvent the wheel all the time. In PHP, there's filter_var() family of functions with plenty of filter types

I just don't understand why so many people like to reinvent the wheel all the time. In PHP, there's filter_var() family of functions with plenty of filter types, like FILTER_VALIDATE_URL, or you could try to use parse_url() if you need to add further constrains to validated URLs - like forbidding localhost, etc. IMHO complex regular expressions should be avoided as they make debugging a PITA and are usually a performance bottleneck, too.