code: function escapeRegexString( token ) {
  	// taken from: https://github.com/sindresorhus/escape-string-regexp/blob/master/index.js
  	const matchOperatorsRe = /[|\\{}()[\]^$+*?.]/g;
  	return token.update( str => str.replace( matchOperatorsRe, '\\$&' ) );
  }
docs: Escapes a string
  @param {lunr.Token} token The string to escape
  @returns {lunr.Token} An escaped string