Match beginning of two strings

[Ravi]

Hi,

I have about 200GB of data that I need to go through and extract the 
common first part of a line. Something like this.

 >>>a = "abcdefghijklmnopqrstuvwxyz"
 >>>b = "abcdefghijklmnopBHLHT"
 >>>c = extract(a,b)
 >>>print c
"abcdefghijklmnop"

Here I want to extract the common string "abcdefghijklmnop". Basically I 
need a fast way to do that for any two given strings. For my situation, 
the common string will always be at the beginning of both strings. I can 
use regular expressions to do this, but from what I understand there is 
a lot of overhead. New data is being generated at the rate of about 1GB 
per hour, so this needs to be reasonably fast while leaving CPU time for 
other processes.

Thanks
Ravi