RunLengthEncode[input_String]:=StringJoin@@Sequence@@@({ToString @Length[#],First[#]}&/@Split[Characters[input]])
RunLengthDecode[input_String]:=StringJoin@@ConstantArray@@@Reverse/@Partition[(Characters[input]/.(ToString[#]->#&/@Range[0,9]))//.{x___,i_Integer,j_Integer,y___}:>{x,10i+j,y},2]