runlengthEncode: function [s][
    join map chunk split s => [&] 'x ->
        (to :string size x) ++ first x
]

runlengthDecode: function [s][
    result: new ""
    loop (chunk split s 'x -> match? x {/\d+/}) [a,b] ->
        'result ++ repeat first b to :integer join to [:string] a
    return result
]

str: "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW"

encoded: runlengthEncode str
print ["encoded:" encoded]

decoded: runlengthDecode encoded
print ["decoded:" decoded]

if decoded=str -> print "\nSuccess!"