Ruby regexp: why does this code not replace what is found but simply insert

Question

I have a ruby regexp searching for urls and I'm trying to replace them with a certain type of urls.

So this regexp detects any url but excludes the type I replace with.

However, when I try this code :

 s.sub(/(?!\S*\/el\/\d*)(?=(?i)\b((?:https?:\/\/|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}\/)(?:[^\s()<‌​>]+|(([^\s()<>]+|(([^\s()<>]+)))*))+(?:(([^\s()<>]+|(([^\s()<>]+)))*))))/, 'http://localhost:8080/el/133710')

if s is s = "blablabla www.google.com bla http://localhost blabla"

It should replace www.google.com with my string http://localhost:8080/el/133710

but I get this instead :

 "blablabla http://localhost:8080/el/133710www.google.com bla http://localhost blabla"

Answer 1

That is because your intended capture www.google.com matches inside a lookahead (?=) , so it is not actually captured. www.google.com only works as a lookahead context. What is captured is only an empty string at the position of match. sub is replacing the captured string (which is an empty string) with http://localhost:8080/el/133710 .