Parsing an input string

Hello,
I want to do the following
1. given an input string say "aaababcac."
2. i want to parse it into phrases like a,aa,b,ab,c,ac
3. basically i wan to parse the entire string to check if a character is already present in the dictionary or not.
4. if not present, the character becomes the phrase; if present it checks for a combination of this character and the next to see if that combination exists and so on

i would like to implement it in c but am not able to do it successfully. can you suggest how to go about it. the algorithm is simple but how to implement it

thanks
naveen

hello,
please ignore the original thread. I have successfully implemented it. There was a small bug which i could not figure out before i posted the problem. Its now solved and thanks for your time

Naveen