function pathtoRegexp(path, keys, options) {
  options = options || {};
  var strict = options.strict;
  var end = options.end !== false;
  var flags = options.sensitive ? '' : 'i';
  keys = keys || [];

  if (path instanceof RegExp) {
    return path;
  }

  if (Array.isArray(path)) {
    // Map array parts into regexps and return their source. We also pass
    // the same keys and options instance into every generation to get
    // consistent matching groups before we join the sources together.
    path = path.map(function (value) {
      return pathtoRegexp(value, keys, options).source;
    });

    return new RegExp('(?:' + path.join('|') + ')', flags);
  }

  path = ('^' + path + (strict ? '' : path[path.length - 1] === '/' ? '?' : '/?'))
    .replace(/\/\(/g, '/(?:')
    .replace(/([\/\.])/g, '\\$1')
    .replace(/(\\\/)?(\\\.)?:(\w+)(\(.*?\))?(\*)?(\?)?/g, function (match, slash, format, key, capture, star, optional) {
      slash = slash || '';
      format = format || '';
      capture = capture || '([^\\/' + format + ']+?)';
      optional = optional || '';

      keys.push({ name: key, optional: !!optional });

      return ''
        + (optional ? '' : slash)
        + '(?:'
        + format + (optional ? slash : '') + capture
        + (star ? '((?:[\\/' + format + '].+?)?)' : '')
        + ')'
        + optional;
    })
    .replace(/\*/g, '(.*)');

  // If the path is non-ending, match until the end or a slash.
  path += (end ? '$' : (path[path.length - 1] === '/' ? '' : '(?=\\/|$)'));

  return new RegExp(path, flags);
}
Normalize the given path string,
returning a regular expression.

An empty array should be passed,
which will contain the placeholder
key names. For example "/user/:id" will
then contain ["id"].

@param  {String|RegExp|Array} path
@param  {Array} keys
@param  {Object} options
@return {RegExp}
@api private