markRegExp(regexp, opt) {
    this.opt = opt;
    this.log(`Searching with expression "${regexp}"`);
    let totalMatches = 0,
      fn = 'wrapMatches';
    const eachCb = element => {
      totalMatches++;
      this.opt.each(element);
    };
    if (this.opt.acrossElements) {
      fn = 'wrapMatchesAcrossElements';
    }
    this[fn](regexp, this.opt.ignoreGroups, (match, node) => {
      return this.opt.filter(node, match, totalMatches);
    }, eachCb, () => {
      if (totalMatches === 0) {
        this.opt.noMatch(regexp);
      }
      this.opt.done(totalMatches);
    });
  }
Marks a custom regular expression
@param  {RegExp} regexp - The regular expression
@param  {Mark~markRegExpOptions} [opt] - Optional options object
@access public