grammar.js

const grammar = compileGrammar`
​
Expr
    = Alternation                       ${ nth(1) }
​
Alternation
    = Alternation '|' Sequence          ${ leftAssoc(pick('alternation', 1, 3)) }
    | Sequence                          ${ nth(1) }
​
Sequence
    = Sequence Kleene                   ${ leftAssoc(pick('seq', 1, 2)) }
    | Kleene                            ${ nth(1) }
​
Kleene
    = Element '*'                       ${ pick('kleene', 1) }
    | Element '?'                       ${ pick('alternation', 1, null) }
    | Element '+'                       ${ pick('plus', 1) }
    | Element '{' 'int' '}'             ${ pick('repeat', 1, 3) }
    | Element '{' 'int' ',' '}'         ${ pick('repeat', 1, 3, true) }
    | Element                           ${ nth(1) }
​
Element
    = '(' Expr ')'                      ${ pick('group', 2) }
    | Charset                           ${ nth(1) }
    | '.'                               ${ () => 'any' }
    | 'char'                            ${ nth(1) }
​
Charset
    = '[' CharsetInner ']'              ${ pick('charset', -2) }
    | '[' '^' CharsetInner ']'          ${ pick('charset^', -3) }
​
CharsetInner
    = CharsetInner 'char' '-' 'char'    ${ pick(-1, ['range', 2, 4]) }
    | CharsetInner 'char'               ${ pick(-1, 2) }
    | null                              ${ () => [] }
​
`;