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Parseparse is a simple tiny backtracking recursive descent parser written in Python.

It is mainly for educational purposes, although I may use it in small personal projects.

It includes a bootstrapped metagrammar so that it can parse a BNF grammar definition.

Parse trees can be transformed with Python expressions on the fly (and is thus suitable for constructing abstract syntax trees, or even interpreting expressions inline.)

S-expression parsing:

# Build a grammar for parsing S-expressions
gram = grammar("""S: '(' S '.' S ')' -> { (s[1], s[3]) }
 | atom -> { s[0] };
atom: /[A-Z]+/ -> { s[0] };
""")

# Parse a test sting
input_str = "(A.(B.(C.NIL)))"
print("PARSE:", parseall(gram, gram["S"], input_str, 0, True))

# output:
# PARSE: ('A', ('B', ('C', 'NIL')))

Please see the source code for details on what parseall does. In short, you give it a grammar (in this case built from a definition), a starting production (in this case the 'S' production), an input string, an offset (0 being the start), and if the parser should be verbose (for debugging).

The last two parameters may be omitted.

It should be trivial to memoize parse, which may lend itself to being a Packrat parser, for a good optimization.