Function regexp

packages/ast/parse/combinators.js:1–9

(pattern, callback)

const regexp = (pattern, callback) => (text, position) => {
  const match = text.substring(position).match(pattern);

  if (!match) {
    return null;
  }

  return callback(match, text, position);
};

const explicit = (matcher) => (text, position) => {
  const previous = text[position - 1];