Runtime mode

Runtime mode loads the Ruby specification and interprets its compiled automaton. It is the shortest feedback loop for tests and application development.

require 'flexr'

class Lexer < Flexr::Lexer
  token_kind :struct
  rule(/[ \t\n]+/, skip: true)
  rule(/[0-9]+/) { emit :INTEGER, text.to_i }
  rule(/\+/) { emit :PLUS }
end

lexer = Lexer.new('12 + 3')
lexer.each_token { |token| p token }

Use runtime mode when the specification is the source of truth and startup compilation is acceptable. It also makes it easy to exercise actions and location tracking in unit tests.

Input and iteration

next_token returns one token at a time. each_token iterates until EOF. The exact return shape depends on token_kind; see tokens and locations.

Validate early

Run flexr check SPEC in CI. It can report unsupported regexp constructs, unreachable rules, empty matches, undeclared emitted tokens, and automaton-size warnings before runtime.

For the complete API contract, see the repository’s runtime reference.