A key step in technology mapping for non lookup-table (such as multiplexor) based FPGAs is to determine whether a given function can be implemented by the logic module. A new algorithm is presented for solving this problem. The algorithm is based on a character string representation of binary decision diagrams. Such representation leads to a matching algorithm which requires only a few string comparisons for each matching operation. Comparing to the matching algorithm by searching for isomorphism on all different BDDs, the new algorithm is much faster with a modest increase of memory requirement.