Parsing expression grammar
A parsing expression grammar, or PEG, is a type of analytic formal grammar that describes a formal language in terms of a set of rules for recognizing strings in the language. A parsing expression grammar essentially represents a recursive descent parser in a pure schematic form that only expresses syntax and is independent of the way an actual parser might be implemented or what it might be used for. Parsing expression grammars look similar to regular expressions or context-free grammars (CFG) in Backus-Naur form (BNF) notation, but have a different interpretation. Unlike CFGs, PEGs are not ambiguous; if a string parses, it has exactly one valid parse tree. This suits PEGs well to parsing computer languages, but not natural languages.