Modifying a regex match value

Question

I want to locate all image tags in my html with src not containing http:// and prepend http:// to the src attribute.

I have got the regex to find all img tags not starting with http://. I'm having some trouble appdening http:// to the src attribute alone. How can I achieve this using regex replace.

<img [^<]*src="(?!http://)(?<source>[^"]*)"[^<]*/>

Source will contain the src value. I just need it to say $2 = "http://" + $2. How can I write this in c# code.

Answer 1

Since you don't want to break existing tags, you will need to assign groups to the parts of the string you are not interested in; in order to be able to include those parts of the match in the replace pattern:

(<img [^<]*src=")(?!http://)(?<source>[^"]*)("[^<]*/>)

Then the replace is trivial:

regex.Replace(input, "$1http://$3$2");

(Also, this might work for your application use case, but I should mention, that in general it is not considered a good idea to parse HTML with regex)