Configuration

The knobs that change parsing behavior.

Rule.first_match_alternation

A class attribute on a Rule subclass. Selects how alternation resolves:

• False (default) — longest match wins; ties are broken by declaration order.

• True — the first matching alternative is returned immediately.

class MyGrammar(Rule):
    first_match_alternation = True

See Alternation: longest match vs. first match for why this is configurable.

Rule.max_cache_size

An int | None class attribute bounding the LRU parse cache that amortizes backtracking. None (the default) leaves the cache unbounded; set a non-negative integer to cap it. See Backtracking and caching.

ABNF_NO_RUST (environment variable)

When set (to any value), forces the pure-Python backend even if the optional abnf_rust extension is installed. Useful for debugging, benchmarking, or A/B comparison. The dispatch is decided once, at import time.

ABNF_NO_RUST=1 python your_script.py

See The two backends and Use the Rust backend.

Line endings

ABNF uses CRLF (\r\n) as the delimiter between rules in a rulelist. When loading a grammar from text (Rule.create, Rule.load_grammar, Rule.from_file), input is normalized to CRLF by default. Beware that some text editors silently rewrite line endings.