StartWith[x_, y_] := MemberQ[Flatten[StringPosition[x, y]], 1]
EndWith[x_, y_] :=  MemberQ[Flatten[StringPosition[x, y]], StringLength[x]]
StartWith["XYZaaabXYZaaaaXYZXYZ", "XYZ"]
EndWith["XYZaaabXYZaaaaXYZXYZ", "XYZ"]
StringPosition["XYZaaabXYZaaaaXYZXYZ", "XYZ"]