Lexers and parsers are often used as front ends to connect input from the outside world with the internals of a larger software system. These front ends are natural targets for attackers who wish to compromise the larger system. A formally verified tool that performs mechanized lexical analysis would render attacks on these front ends less effective. In this paper we present Verbatim, an executable lexer that is implemented and verified with the Coq Proof Assistant. We prove that Verbatim is correct with respect to a standard lexer specification. We also analyze its theoretical complexity and give results of an empirical performance evaluation. All correctness proofs have been mechanized in Coq.