正则表达式（JS）：匹配没有协议的URL，忽略带有一个协议的URL

Question

Apologies for yet another regex URL matching question, but I haven't been a able to find a solution in any of the other threads.

I want to run a replace() method on a string, with a pattern that matches all URLs without a protocal (http, https etc) but ignores urls that do have one.

So given this input:

www.google.com www.facebook.com http://www.google.com http://www.facebook.com

It would match www.google.com and www.facebook.com on the first line (without any surrounding whitespace), but ignore the other URLs on the second and third line.

I thought about just looking for www and ignoring matches which have // as preceding characters, which led me to this:

https://www.regex101.com/r/Y3rqxy/1

However, as you can see the second match includes the preceding whitespace. As I want to replace the www with http://www this whitespace buggers things up a little.

Any regex mandarins able to help me out on this one?

Answer 1

Mere seconds after posting this, one of my colleagues came up with a solution. It's a little wacky (thanks javascript) but it works! This example assumes you want to add http:// to any URLs that are missing their protocal.

First you have to reverse the string you're running the .replace() method on:

string.split('').reverse().join('')

Then you can run call the replace method with the following regex (note the backwards http://www !):

string.replace(/www(?!\\/\\/)/gi, 'www//:ptth')

Then you just reverse your string again:

string.split('').reverse().join('')

And any URLS that are missing a protocal in that string will now have them.

It's not going to win any awards for cleanliness, but it works!