Skip to content
Generates random text based on context-free grammars defined in BNF
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


BNFGen generates random text based on context-free grammars. You give it a file with your grammar defined in BNF-like syntax, it give you a string that follows that grammar.

Can be used for parser fuzzing or amusement.

Grammar syntax

The grammar syntax is an ad hoc extension of BNF (as if there's any other kind of BNF extensions).

Non-terminals are written in angle brackets (<foo>), terminals must be quoted ('foo' or "foo"). It's ok to use the other kind of quotes inside quoted strings without escaping, that is, "foo 'bar' baz" or 'foo "bar" baz' are fine. Same kind of quote must be escaped with a backslash.

To enable writing multi-line rules easily, rules must be separated with a semicolon.

Empty alternatives are not allowed in rules with alternation, i.e. <foo> ::= <bar> | <baz>; is fine, while <foo> ::= | <bar> | <baz>; is not.

Here is a simple example:

$ cat examples/02_english.bnf 
# A trivial subset of English.
<start> ::= <NP> <VP> ;
<NP> ::= <article> <subject> ;
<VP> ::= <verb> <article> <object> ;
<article> ::= "a" | "the" ;
<subject> ::= "customer"
            | "system administrator"
            | "programmer" ;
<object> ::= "system" | "compiler" | "program" ;
<verb> ::= "used" | "installed" | "developed"

$ bnfgen examples/02_english.bnf
a system administrator installed a compiler

The default start symbol is <start>, but you can override it with --start option. For example, bnfgen --start NP examples/02_english.bnf will only produce noun phrases.

By default BNFGen uses space as a token separator, but you can specify any separator with --separator option, including empty string (--separator "").


There is no direct support for repetition, but it can be easily done with recursive rules.

Here are recipes for common repetition patterns:

# <foo>?
<one_or_more_foo> ::= <foo> | "" ;

# <foo>*
<zero_or_more_foo> ::= <foo> <zero_or_more_foo> | "" ;

# <foo>+
<one_or_more_foo> ::= <foo> <one_or_more_foo> | <foo> ;

BNFGen makes no attempt to check if a production will ever terminate, and even terminating rules may product a lot of data. You can limit recursion depth with --max-depth option. Note that --max-depth 0 means "no recursion" rather than infinite. To allow infinite recursion, simply omit that option.

Weighted random

A problem with using simple BNF for producing test cases for parser fuzzing is that if each alternative is taken equally often, there's a 50% chance that production will terminate immediately with the non-recursive case. Likewise, you simply may want some symbols more frequent than others to give your text a more realistic appearance.

The "killer feature" of BNFGen is that it gives you control over recursion depth. You can specify a "weight" for every alternative to choose how often they should be taken. The default weight is 1.

For example, if you want to produce long strings of "a", you can use this rule to make BNFGen choose the recursive alternative hundred times more often.

<start> ::= 100 <start> "a" | "a" ;

You can give different weight to every alternative. For example, the following grammar will product strings with lots of a's, some b'c, and few c's.

<foo> ::= 100 "a" | 50 "b" | "c" ;

<start> ::= 100 <foo> <start> | <foo>


Local installation:

opam pin add bnfgen . -n
opam install bnfgen

You can also find some binaries in Github releases.


Usage: bnfgen [OPTIONS] <BNF file>
  --dump-rules Dump production rules and exit
  --separator <string>  Token separator for generated output, default is space
  --start <string>  Start symbol, default is "start"
  --max-depth <int>  Maximum recursion depth, default is infinite
  --version   Print version and exit
  -help  Display this list of options
  --help  Display this list of options

You can’t perform that action at this time.