function decode(string, exclude) {
  var cache;

  if (typeof exclude !== 'string') {
    exclude = decode.defaultChars;
  }

  cache = getDecodeCache(exclude);

  return string.replace(/(%[a-f0-9]{2})+/gi, function(seq) {
    var i, l, b1, b2, b3, b4, chr,
        result = '';

    for (i = 0, l = seq.length; i < l; i += 3) {
      b1 = parseInt(seq.slice(i + 1, i + 3), 16);

      if (b1 < 0x80) {
        result += cache[b1];
        continue;
      }

      if ((b1 & 0xE0) === 0xC0 && (i + 3 < l)) {
        // 110xxxxx 10xxxxxx
        b2 = parseInt(seq.slice(i + 4, i + 6), 16);

        if ((b2 & 0xC0) === 0x80) {
          chr = ((b1 << 6) & 0x7C0) | (b2 & 0x3F);

          if (chr < 0x80) {
            result += '\ufffd\ufffd';
          } else {
            result += String.fromCharCode(chr);
          }

          i += 3;
          continue;
        }
      }

      if ((b1 & 0xF0) === 0xE0 && (i + 6 < l)) {
        // 1110xxxx 10xxxxxx 10xxxxxx
        b2 = parseInt(seq.slice(i + 4, i + 6), 16);
        b3 = parseInt(seq.slice(i + 7, i + 9), 16);

        if ((b2 & 0xC0) === 0x80 && (b3 & 0xC0) === 0x80) {
          chr = ((b1 << 12) & 0xF000) | ((b2 << 6) & 0xFC0) | (b3 & 0x3F);

          if (chr < 0x800 || (chr >= 0xD800 && chr <= 0xDFFF)) {
            result += '\ufffd\ufffd\ufffd';
          } else {
            result += String.fromCharCode(chr);
          }

          i += 6;
          continue;
        }
      }

      if ((b1 & 0xF8) === 0xF0 && (i + 9 < l)) {
        // 111110xx 10xxxxxx 10xxxxxx 10xxxxxx
        b2 = parseInt(seq.slice(i + 4, i + 6), 16);
        b3 = parseInt(seq.slice(i + 7, i + 9), 16);
        b4 = parseInt(seq.slice(i + 10, i + 12), 16);

        if ((b2 & 0xC0) === 0x80 && (b3 & 0xC0) === 0x80 && (b4 & 0xC0) === 0x80) {
          chr = ((b1 << 18) & 0x1C0000) | ((b2 << 12) & 0x3F000) | ((b3 << 6) & 0xFC0) | (b4 & 0x3F);

          if (chr < 0x10000 || chr > 0x10FFFF) {
            result += '\ufffd\ufffd\ufffd\ufffd';
          } else {
            chr -= 0x10000;
            result += String.fromCharCode(0xD800 + (chr >> 10), 0xDC00 + (chr & 0x3FF));
          }

          i += 9;
          continue;
        }
      }

      result += '\ufffd';
    }

    return result;
  });
}
LinkifyIt#onCompile()

Override to modify basic RegExp-s.