$CPU 8086          ' make compatible with XT systems
$LIB ALL OFF       ' turn off all PowerBASIC libraries
$ERROR ALL OFF     ' turn off all PowerBASIC error checking
$OPTIMIZE SIZE     ' optimize for smaller code
 $COMPILE UNIT      ' compile to a UNIT (.PBU)
'$COMPILE EXE       ' compile to a UNIT (.PBU)

DEFINT A-Z         ' Required for all numeric functions, forces PB to not
                   ' include floating point in UNIT (makes it smaller)
'/*------------------------------------------------------------------*/
$CODE SEG "MLIB5"
'/*------------------------------------------------------------------*/
; ISISBN(STRING$)
;     Returns 1 if STRING$ contains a valid ISBN number.
;     Returns 0 if not.
;     EXAMPLE:  T = ISISBN("0137790678")      1
'/*------------------------------------------------------------------*/
  FUNCTION ISISBN(BYVAL A$) PUBLIC AS INTEGER
    FOR X = 1 TO LEN(A$)
      IF MID$(A$,X,1) >="0" AND MID$(A$,X,1) <="9" THEN B$=B$+MID$(A$,X,1)
    NEXT X
    IF RIGHT$(A$,1) = "X" OR RIGHT$(A$,1) = "x" THEN B$=B$ + "X"
    FOR X= 1 TO LEN(B$)-1
      CS = CS + VAL(MID$(B$,X,1)) * X
    NEXT X
    IF RIGHT$(B$,1) = "X" THEN M = 10 ELSE M = VAL(RIGHT$(B$,1))
    IF CS MOD 11 = M THEN FUNCTION = 1 ELSE FUNCTION = 0
  END FUNCTION
'/*------------------------------------------------------------------*/
'  $INCLUDE "C:\CODE\MLIB\MLIB.INC"
'  INPUT A$
'  SAY 10,1,STR$(ISISBN(A$)),31
'  Y$ = GETKEY