Regular expression

[Sang DeSibtis]

import re,sys,string
for line in open('junk', 'r').readlines ()
a = re.compile('AAAA')
b = re.compile('BBBB')

re.sub(a, 'aaaa', line)
re.sub(b, 'bbbb', line)

print line   # nothing got changed 


print re.sub(a, 'aaaa', line)   # works
print re.sub(b, 'bbbb', line)    # works, however the line above
doesn't work



 Questions: how do I make a global replacements in the 'line' that
read from the file object (buffer in memory? ).Obviously, this is my
first attemp at Python and I am not sure how things work. I want to
replace all strings (not necessary 'AAAA' or 'BBBB' as illustrated
here) in one pass. Will some one please explain in detail
!!!!!!!!!!!!!!!!

TIA