Regular Expression to get the SRC of images in C#

I'm looking for a regular expression to isolate the src value of an img. (I know that this is not the best way to do this but this is what I have to do in this case)

I have a string which contains simple html code, some text and an image. I need to get the value of the src attribute from that string. I have managed only to isolate the whole tag till now.

string matchString = Regex.Match(original_text, @"(<img([^>]+)>)").Value;