Grammar

LBNF

Lux uses LBNF (BNF variant) to describe the grammar.

Defining rules

To define rule simply do this:

rule-name = expression;

Rule name must only contain letters, digits, hyphens and underscores:

first-valid-rule-name
invalid $rule name&
2nd-valid-rule-name

Root rule

To define root rule (rule that will be used to match the whole input) use !:

!root-rule-name = expression;

Rule name can be only defined once!

Patterns

Basic rule for LBNF is pattern. Pattern must be surrounded by quotes:

"LBNF pattern"

Don’t use whitespaces and line breaks in patterns: use inline and multiline indent instead!

Includes

You can include rules by name and use them in other rules:

first-rule = "pattern";
second-rule = first-rule;

Groups

You can group rules by using parentheses:

rule = (a | b) c;

Chains

Chains are rule sets that can be used to define a rule sequence or a rule variants.

rule = "a" | "b"; -- "or" chain
rule = "a" "b"; -- "and" chain

Modifiers

Rule modifiers describe how many times the rule can be repeated:

rule = [abc]; -- optional (0 or 1)
rule = {abc}; -- repeation (0 or more)

You can specify repeation count:

rule = {abc}<min..max>;

{abc} is same as {abc}<0..inf>
[abc] is same as {abc}<0..1>

Advanced usage

You can create rules manually using Grammar._or, Grammar._and, Grammar.repeation, Grammar.optional, Grammar.pattern, Grammar.include and Grammar.custom.

Custom rules

You can define custom rules using Grammar.custom:

Grammar.custom(function(parser, elements)
    return true, elements, #elements -- success, result, count
end)