parseRegExp.js

// new RegExp('/test/i')
// is interpreted as /\/test\/i/
// this function interprets it as
// /test/i
function parseRegExp(regExpStr) {
  if (typeof regExpStr !== 'string') {
    throw new TypeError('Invalid argument: not a string.');
  }

  const start = regExpStr.indexOf('/');
  const end = regExpStr.lastIndexOf('/');
  if (start === -1 || start === end) {
    throw new TypeError('Invalid RegExp syntax, matcher should be surrounded with slashes.');
  }

  const matcher = regExpStr.substring(start + 1, end);
  if (matcher === '') {
    throw new TypeError('Invalid RegExp syntax, empty matcher.');
  }
  
  const options = regExpStr.substr(end + 1);

  return new RegExp(matcher, options);
}