This paper introduces a technique for synthesizing hash functions specialized to particular byte formats. This code generation method leverages three prevalent patterns: (i) fixed-length keys, (ii) keys with common subsequences, and (iii) keys ranging on predetermined sequences of bytes. Code generation involves two algorithms: one identifies relevant regular expressions within key examples, and the other generates specialized hash functions based on these expressions. Comparative analysis demonstrates that the synthetic functions outperform the general-purpose hashes in the C++ Standard Template Library and the Google Abseil Library when keys are given in ascending, normal or uniform distribution. In applications where low-mixing hashes are acceptable, the synthetic functions achieve speedups ranging from 2% to 11% on full benchmarks, and speedups of almost 50x once only hashing speed is considered.