Fuzzy match under PostgreSQL 8.0.15

Good afternoon,

Under PostgreSQL 8.0.15, what is the good function to do the Fuzzy match
between two strings?

For example,
(1) 'abcddd' vs. 'abc'
probably returns 3

(2) 'abcddd' vs. 'ab'
probably returns 4

I am looking for functions that could compare how two strings are
similar to each other.

thanks a lot!

--
Lu Ying