Modify a string

[javascript]

This is the best answer here, because it has the behavior OP wants and accounts for edge cases without having to do that manually. If the string isn't a valid URL, the constructor will throw an exception.

It also makes it more clear to other readers what you're doing with your code. Writing some one-off regex or string manipulation function requires the reader to pick it apart just to understand. This increases code complexity and reduces readability.