10 INPUT X$,Z$,Y$:IF LEN(Z$)><LEN(Y$) THEN 10
20 FOR J=1 TO LEN(X$)-LEN(Z$)+1
30 IF Z$<>MID$(X$,J,LEN(Z$)) THEN 50
40 MID$(X$,J,LEN(Y$))=Y$
50 NEXT J:PRINT X$:END