#language-model #bnf #forms #token #recursion #schema #sampler


A crate that uses recursive descent algorithm to ensure tokens produced by a large language model follow a Backus Naur Form schema

13 releases

0.3.6 Mar 27, 2024
0.3.5 Oct 24, 2023
0.3.1 Sep 15, 2023
0.2.5 Sep 15, 2023
0.1.2 Aug 31, 2023

#206 in Database interfaces

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This is a language model sampler that uses recursive descent algorithm to ensure tokens produced by a large language model follow a schema based on Backus Naur Form(BNF).

Tokens must be encoded in UTF-8.


  1. Very fast.
  2. Compatible with any type of vocabulary of language model.
  3. Easy to use.

How to try it?

  1. Install Rust.
  2. Run cargo run --release to run the console_playground program. Your console input is considered as tokens. Try cargo run --release -- --help to check all possible command line configurations. Modify `assets/grammar.bnf`` to change schema. (see Grammar schema section and Listing possible tokens section)

Or you can download the pre-compiled binaries from the release page and run.

Use in Your Project

To use in your own rust project, simply add bnf_sampler = "0.3.1" as a dependency in your Cargo.toml.


Copy paste one of these examples into assets/grammar.bnf to try by yourself.

DNA Sequence






Grammar schema

In this project, a slightly modified version of BNF is used. The key differences are:

  • Left recursion is not supported. (plan to support in the future.)

  • Consecutive terminals are merged into one terminal. e.g. 'b''o''y' becomes 'boy'.

  • <any!> is added as a special nonterminal which matches any token in the given vocabulary.

  • <except!(excepted_literals)> is added as a special nonterminal which:

    • matches any token in the given vocabulary that does not contain any of the excepted_literals.

    • matches the slice token[:the beginning of the first appearing excepted literal] if the token contains any of the excepted_literals and at least one possible prefix of the slice equals any token in the given vocabulary.

    • <except!(excepted_literals)> has two forms:

      • <except!('excepted_literal')> or <except!("excepted_literal")> which specifies one and only one excepted_literal.
        • e.g. <except!('ar')> specifies ar as the excepted_literal. It will match c in card(given c is one valid token), and pass ard to next term in grammar.
      • <except!([nonterminal])> which specifies any token accepted by the nonterminal belongs to excepted_literals.
        • WARNING: the nonterminal itself and all the nonterminals expanded from the nonterminal should not be <except!([nonterminal])>, or the program may panic.
        • e.g. given <abc> ::= 'a'|'b'|'c', <sequence>::= <abc>|<abc><sequence> <except!([sequence])> specifies all tokens which only contains a,b and c as excepted_literals.
  • In terminals and excepted_literals, escape sequences like \t, \r, \n, \u1234 are recognized and converted to corresponding UTF-8 bytes. \x<hex><hex>, like \x00, are converted to raw bytes however.

Listing possible tokens

The possible tokens listed are the tokens that can be accepted by the sampler in its current state. The following rule defines whether a token is listed in the return value of Sampler::all_possible_tokens with a given BNF:

  • The sampler has not terminated or gets into an invalid state. In other words, the current input token can be accepted by the sampler, and no path exists such that all the terminals and nonterminals are consumed in the path.

    • e.g. With <start>::=<A><B><C>, <A>::='boy', <B>::='next', <C>::='door',<start> will create a sampler that terminates after boy,next,door are inputed in this exact sequence, and goes into an invalid state otherwise.
    • e.g. <sequence>::=<any!>|<any!><sequence> will create a sampler that terminates after any input token because of the path where <sequence> become <any!>. In other words, <any!> is the only nonterminal in the path and is consumed.
  • For a given terminal, only the longest possible token is listed.

    • e.g. terminal 'apple' will only list token apple given that token exists. Tokens like a,ap,app will not be listed.
  • A terminal can be partially matched and consumed.

    • e.g. terminal apple66666 will only list token apple(see rule 1), given apple66666 is not a valid token. After apple is inputed, the terminal becomes 66666 because the prefix apple is matched.
  • A token can be matched by multiple terminals on byte level.

    • e.g. Given <byte> ::= '\xf0'|'\xa0'|'xb0', <sequence>::= <byte>|<byte><sequence>,<sequence> will list any token whose UTF-8 encoding only contains byte value 240,160 and 176.


  1. Add more examples and ready-to-use BNF schema.
  2. Add more tests.
  3. Test more advanced parser algorithms(like Earley) and see whether the speed can be improved.
  4. Python binding.
  5. Huggingface transformers integration.


~141K SLoC