const RegExpRule = /(\/?)(.+)\1([a-z]*)/;
const FlagRule = /^(?!.*?(.).*?\1)[AJUXgimsux]+$/;

export const parseRegex = (subject: string): RegExp => {
  const match = RegExpRule.exec(subject);

  if (!match) {
    throw new Error('Invalid RegExp.');
  }

  if (match[3] && !FlagRule.test(match[3])) {
    return new RegExp(subject);
  }

  return new RegExp(match[2], match[3]);
};