Pattern-matching based document compression systems rely on finding a small set of patterns that can be used to represent the whole document. When analyzing and comparing this kind of systems two factors have to be considered: the compression rate attained and the speed and associated complexity of the code-book building. In order to reduce the computational burden of the pattern matching operation while keeping a good compression ratio, we propose a new fast algorithm to carry out a WAN (weighted AND-NOT) matching process. Thus, code-book building is performed in two stages: the first step is based on FWAN (fast WAN) with a loose threshold; in the second one a more accurate but slower method (CTM, EPM) is applied over the initial approximate code-book. This screening greatly reduces the search space for the clustering procedure implicit in obtaining the library without altering the compression ratio. Experimental results show a very good speed performance for this new algorithm: at least three times faster than the usual WAN.