ignorethis.py

import re

for example in ("fooBarIgnoreThis", "fooBarThis"):
    # Zero width negative lookbehind!
    if re.search("(?<!Ignore)This", example):
        print "Matched", example
    else:
        print "Did not match", example

output

(sandbox)jonty@towel:~/sandbox $ python ignorethis.py
Did not match fooBarIgnoreThis
Matched fooBarThis