re.sub and variables

[fuglyducky]

I have a function that I am attempting to call from another file. I am
attempting to replace a string using re.sub with another string. The
problem is that the second string is a variable. When I get the
output, it shows the variable name rather than the value. Is there any
way to pass a variable into a regex?

If not, is there any other way to do this? I need to be able to dump
the variable value into the replacement string.

For what it's worth this is an XML file so I'm not afraid to use some
sort of XML library but they look fairly complicated for a newbie like
me.

Also, this is py3.1.2 is that makes any difference.

Thanks!!!

#####################################################

import random
import re
import datetime

def pop_time(some_string, start_time):
    global that_string

    rand_time = random.randint(0, 30)
    delta_time = datetime.timedelta(seconds=rand_time)

    for line in some_string:
        end_time = delta_time + start_time
        new_string = re.sub("thisstring", "thisstring\\end_time",
some_string)
        start_time = end_time

    return new_string