Procedure.s lcs(a$,b$)
  If Len(a$)>Len(b$) : Swap a$,b$ : EndIf
  l=Len(a$)
  For c=1 To l
    For i=1 To 1+l-c
      If FindString(b$,Mid(a$,i,c))
        res$=Mid(a$,i,c)
      EndIf
    Next
  Next
  ProcedureReturn res$
EndProcedure

t1$="testing123testing"
t2$="thisisatest"
Debug lcs(t1$,t2$)