https://en.wikipedia.org/wiki/URL#Syntax
Whatever the user is "throwing at you" if it doesn't have these things in the front, it is not a "URL". It is not even a "protocol-relative URL".
https://en.wikipedia.org/wiki/URL#Syntax
Whatever the user is "throwing at you" if it doesn't have these things in the front, it is not a "URL". It is not even a "protocol-relative URL".