MutRex: A Mutation-Based Generator of Fault Detecting Strings for Regular Expressions

Regular expressions (regexes) permit to describe set of strings using a pattern-based syntax. Writing a correct regex that exactly captures the desired set of strings is difficult, also because a regex is seldom syntactically incorrect, and so it is rare to detect faults at parse time. We propose a fault-based approach for generating tests for regexes. We identify fault classes representing possible mistakes a user can make when writing a regex, and we introduce the notion of distinguishing string, i.e., a string that is able to witness a fault. Then, we provide a tool, based on the automata representation of regexes, for generating distinguishing strings exposing the faults introduced in mutated versions of a regex under test. The basic generation process is improved by two techniques, namely monitoring and collecting. Experiments show that the approach produces compact test suites having a guaranteed fault detection capability, differently from other test generation approaches.