Parseq (pronounced parsec) is a parsing library for common lisp. It can be used for parsing lisp's sequences types: strings, vectors (e.g. binary data) and lists. Furthermore, parseq is able to parse nested structures such as trees (e.g. lists of lists, lists of vectors, vectors of strings).

Parseq uses parsing expression grammars (PEG) that can be defined through a simple interface. Extensions to the standard parsing expressions are available. Parsing expressions can be parameterised and made context aware. Additionally, the definition of each parsing expression allows the arbitrary transformation of the parsing tree.

The library is inspired by Esrap and uses a very similar interface. No code is shared between the two projects, however. The features of Esrap are are mostly included in parseq and complemented with additional, orthogonal features. Any resemblance to esrap-liquid is merely coincidental.