We present the AS-Index, a new index structure for exact string search in disk resident databases. AS-indexrelies on a classical inverted file structure, its main innovation being a probabilistic search based on the properties of algebraicsignatures used both for n-grams hashing and pattern search. Specifically, the properties of our signatures allow to carry outa search by inspecting only two of the posting lists. The algorithm thus enjoys the unique feature of requiring a constantnumber of disk accesses, independently from both the pattern size and the database size. We conduct extensive experimentson large datasets to evaluate our index behavior. They confirm that it steadily provides a search performance proportionalto the two disk accesses necessary to obtain the posting lists. This makes our structure a choice of interest for the class ofapplications that require very fast lookups in large textual databases.We describe the index structure, our use of algebraic signatures and the search algorithm. We discuss the operationaltrade-offs based on the parameters that affect the behavior of our structure, and present the theoretical and experimentalperformance analysis. We next compare the AS-Index to the state-of-the-art alternatives and show that (i) the constructiontime matches that of the competitors, due to the similarity of structures, (ii) the search time constantly outperforms thestandard approach, thanks to the economical access to data complemented by signature calculations, which is at the core ofour search method.