Techniques are described to improve the performance of regular expression (regex) evaluation in a database management system (DBMS) by a speculative execution of a regex engine. In an embodiment, the DBMS determines properties of strings and generates descriptors for the string input data. Based on the descriptors, the regex engine validates an existing assertion for the execution logic to, at least in part, evaluate the regex pattern on the strings. Based on validating the existing assertion for selecting the execution logic, evaluating the regex pattern on the strings.