RosettaCodeData/Task/Longest-common-substring/QBasic/longest-common-substring.basic

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CALL LCS("thisisatest", "testing123testing")
END
SUB LCS (a$, b$)
IF LEN(a$) = 0 OR LEN(b$) = 0 THEN PRINT "": EXIT SUB
WHILE LEN(b$)
FOR j = LEN(b$) TO 1 STEP -1
IF INSTR(a$, LEFT$(b$, j)) THEN PRINT LEFT$(b$, j): EXIT SUB
NEXT j
b$ = MID$(b$, 2)
WEND
END SUB