code: function hostregexp (val) {
    var source = !isregexp(val)
      ? String(val).replace(ESCAPE_REGEXP, ESCAPE_REPLACE).replace(ASTERISK_REGEXP, ASTERISK_REPLACE)
      : val.source
  
    // force leading anchor matching
    if (source[0] !== '^') {
      source = '^' + source
    }
  
    // force trailing anchor matching
    if (!END_ANCHORED_REGEXP.test(source)) {
      source += '$'
    }
  
    return new RegExp(source, 'i')
  }
docs: Generate RegExp for given hostname value.
  
  @param (string|RegExp} val
  @private