Retrieving music pieces based on their content is an emerging topic in the database management and information retrieval fields. This paper tackles the challenge of retrieving musical scores containing a given melodic pattern while allowing flexibility in the matching. It provides two contributions: (i) a fuzzy-logic-based theoretical framework that makes it possible to support customizable and explainable retrieval of approximate answers, including transposition-aware searches; (ii) an implementation of this framework within a real-world digital score library, along with some experimental performance results.