replacing a part of a string using a regexp

[Rajarshi Guha]

Hi,
  I'm trying to parse some HTML tags and I have the following situation:

a line contains:

<a href="www.myurl.com">

And I want to replace the url and make the line:

<a href="http://myserver.com/prog.py?www.myurl.com">

Using re.sub I can substitute the whole string bewteen quotes, but the
substituion needs to use the string that is *found* in the quotes.

How can I access the matched portion in a re.sub() expression?

TIA