Newbie question: string replace

[usgog at yahoo.com]

Now it works:
rex = re.compile(r'(^.*username *=[^"]*")([^"]*)(".*$)')
for line in fileinput.input(FILE, inplace=1):
     m = rex.match(line)
     if m is not None:
         line = "%s%s%s\n" % (m.group(1), new_name, m.group(3))
     print line

But there is an extra line break after each line in FILE. Why is that?
I tried to line = "%s%s%s" % (m.group(1), new_name, m.group(3)) but it
turns out there are two extra line breaks after each line...