Would re.sub work for what you're doing? http://www.regular-expressions.info/python.html
Something like this?
#!/usr/bin/python
import re
import fileinput
for line in fileinput.input("test.txt"):
print re.sub("(APP[a-z]{2}[0-9]{3})", "<a href=\"\\1\">\\1</a>", line)
Here's a test run:
-> cat test.txt
APPsd222
APPxx333
-> python test.py
<a href="APPsd222">APPsd222</a>
<a href="APPxx333">APPxx333</a>
I hope this helps! I'm also a python noob :)