Any time we use common string functions like \texttt{split}, \texttt{trim}, or \texttt{slice}, we effectively perform parsing. Yet no one ever bothers to write down grammars for such \emph{ad hoc} parsers. We propose a grammar inference system that allows programmers to get input grammars from unannotated source code ``for free,'' enabling a range of new possibilities, from interactive documentation to grammar-aware semantic change tracking. To this end, we introduce \textsc{Panini}, an intermediate representation with a novel refinement type system that incorporates domain knowledge of ad~hoc parsing.