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A parser is a piece of software which takes a stream of input with a known syntax and turns it into a sequence of 'tokens' which have a higher-level semantics. The output may be a simple list of tokens, or, more usually, a structured tree. The input is usually a simple stream of characters, but may be some other form of data.

The most common forms of parsers are 'recursive descent', in which a series of functions may call each other, and 'table-driven', in which a formal grammar table is used to do the parsing.