You can also try below code.
string htmlString = "<a href="\"http://www.mysite.com\"" title="Asha Computers in Alkapuri, VADODARA">Asha Computers</a>";
string serachFor = "<a href="\"http";<br" mode="hold" /> int startIndx = htmlString.IndexOf(serachFor);
int endIndx = htmlString.IndexOf('"',startIndx + serachFor.Length);
string href ="http:" + htmlString.Substring(startIndx + serachFor.Length, endIndx - (startIndx + serachFor.Length));