Mutation testing is widely used in research (even if not in practice). Mutation testing tools usually target only one programming language and rely on parsing a program to generate mutants, or operate not at the source level but on compiled bytecode. Unfortunately, developing a robust mutation testing tool for a new language in this paradigm is a difficult and time-consuming undertaking. Moreover, bytecode/intermediate language mutants are difficult for programmers to read and understand. This paper presents a simple tool, called universalmutator, based on regular-expression-defined transformations of source code. The primary drawback of such an approach is that our tool can generate invalid mutants that do not compile, and sometimes fails to generate mutants that a parser-based tool would have produced. Additionally, it is incompatible with some approaches to improving the efficiency of mutation testing. However, the regexp-based approach provides multiple compensating advantages. First, our tool is easy to adapt to new languages; e.g., we present here the first mutation tool for Apple's Swift programming language. Second, the method makes handling multi-language programs and systems simple, because the same tool can support every language. Finally, our approach makes it easy for users to add custom, project-specific mutations.