FUNCTION Common_Sub$(haystack$, needle$)

    WHILE LEN(needle$)
        FOR x = LEN(needle$) DOWNTO 1
            IF INSTR(haystack$, LEFT$(needle$, x)) THEN RETURN LEFT$(needle$, x)
        NEXT
        needle$ = MID$(needle$, 2)
    WEND
    EXIT

ENDFUNC

PRINT Common_Sub$("thisisatest", "testing123testing")