Modgrammar is a general-purpose library for building language parsers and interpreters using context-free grammar definitions in Python. Language parsing rules (grammars) are defined as standard Python classes, which can then be used to parse and validate input strings or files into meaningful data structures. Possible applications range from simple input validation, to complex expression evaluation, to full-fledged programming language parsing for compilers or interpreters. Some features include: * Pure-Python cross-platform design. * Full Unicode support * Grammars are defined using standard Python syntax. * Supports arbitrarily complex grammars, including recursion. * Defining a grammar automatically creates a working parser in the process (no compilation steps or lengthy startup times). * Parse results contain full parse-tree information, including heirarchical tokenization of the input. * Parse result objects can have custom methods, producing rich data objects. * Modular grammar design supports distributing grammars as python library modules, combining grammars from multiple sources into larger grammars, and even parameterized grammar definitions.