Python Regex Partial Replace

Replacing only part of the matched string.

Example:
For the string http://subdomain.mydomain.com/hello.html I want to identify mydomain.com url and replace the subdomain only.

import reurl = "http://subdomain.mydomain.com/hello.html"re.sub(r"(://)([^.]+)(\.mydomain\.com)", r"\1www\3", url)# Outputhttp://www.mydomain.com/hello.html