RunLengthEncode[s_String] := StringJoin[
  {ToString[Length[#]] <> First[#]} & /@ Split[StringSplit[s, ""]]
  ]

RunLengthDecode[s_String] := StringJoin[
  Table[#[[2]], {ToExpression[#[[1]]]}] & /@
   Partition[StringSplit[s, x : _?LetterQ :> x], 2]
  ]