Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function ValidateISBN(const ISBN: string): Boolean;
 // 
// References: 
// =========== 
// [1] http://isbn-international.org/userman/chapter4.html 
// 
type
   TISBNPart = (ipGroupID, ipPublisherID, ipTitleID, ipCheckDigit);
   TISBNPartSizes = array [TISBNPart] of Integer;
 const
   ISBNSize = 13;
   ISBNDigits = ['0'..'9'];
   ISBNSpecialDigits = ['x', 'X'];
   ISBNSeparators = [' ', '-'];
   ISBNCharacters = ISBNDigits + ISBNSpecialDigits + ISBNSeparators;
 var
   CurPtr, EndPtr: PAnsiChar;
   Accumulator, Counter: Integer;
   Part: TISBNPart;
   PartSizes: TISBNPartSizes;

   // begin local function 

  function IsPartSizeValid(APart: TISBNPart): Boolean;
   const
     MaxPartSizes: TISBNPartSizes = (5, 7, 6, 1);
   begin
     Result := PartSizes[APart] <= MaxPartSizes[APart];
   end;

   // end local function 

begin
   Result := False;
   // At first, check the overall string length. 
  if Length(ISBN) <> ISBNSize then
     Exit;

   CurPtr := @ISBN[1];
   EndPtr := CurPtr + Pred(ISBNSize);
   Accumulator := 0;
   Counter := 10;
   Part := ipGroupID;
   ZeroMemory(@PartSizes[Low(PartSizes)], SizeOf(PartSizes));

   while Cardinal(CurPtr) <= Cardinal(EndPtr) do
   begin
     if CurPtr^ in ISBNCharacters then
     begin
       if CurPtr^ in ISBNSeparators then
       begin
         // Switch to the next ISBN part, but take care of two conditions: 
        // 1. Do not let Part go beyond its upper bound (ipCheckDigit). 
        // 2. Verify if the current ISBN part does not exceed its size limit. 
        if (Part < High(Part)) and IsPartSizeValid(Part) then
           Inc(Part)
         else
           Exit;
       end
       else // CurPtr^ in [ISBNDigits, ISBNSpecialDigits] 
      begin
         // Is it the last character of the string? 
        if (CurPtr = EndPtr) then
         begin
           // Check the following conditions: 
          // 1. Make sure current ISBN Part equals to ipCheckDigit. 
          // 2. Verify if the check digit does not exceed its size limit. 
          if (Part <> High(Part)) and not IsPartSizeValid(Part) then
             Exit;
         end
         else
           // Special check digit is allowed to occur only at the end of ISBN. 
          if CurPtr^ in ISBNSpecialDigits then
             Exit;

         // Increment the size of the current ISBN part. 
        Inc(PartSizes[Part]);

         // Increment the accumulator by current ISBN digit multiplied by a weight. 
        // To get more detailed information, please refer to the web site [1]. 
        if (CurPtr = EndPtr) and (CurPtr^ in ISBNSpecialDigits) then
           Inc(Accumulator, 10 * Counter)
         else
           Inc(Accumulator, (Ord(CurPtr^) - Ord('0')) * Counter);
         Dec(Counter);
       end;
       Inc(CurPtr);
     end
     else
       Exit;
   end;
   // Accumulator content must be divisible by 11 without a remainder. 
  Result := (Accumulator mod 11) = 0;
 end;