How to validate an IBAN

IBAN stands for International Bank Account Number
Example : CH10002300A1023502601

Here is a method how to check if an IBAN is valid:

function ChangeAlpha(input: string): string;
// A -> 10, B -> 11, C -> 12 ...
var
a: Char;
begin
Result := input;
for a := 'A' to 'Z' do
begin
Result := StringReplace(Result, a, IntToStr(Ord(a) - 55), [rfReplaceAll]);
end;
end;

function CalculateDigits(iban: string): Integer;
var
v, l: Integer;
alpha: string;
number: Longint;
rest: Integer;
begin
iban := UpperCase(iban);
if Pos('IBAN', iban) > 0 then
Delete(iban, Pos('IBAN', iban), 4);
iban := iban + Copy(iban, 1, 4);
Delete(iban, 1, 4);
iban := ChangeAlpha(iban);
v := 1;
l := 9;
rest := 0;
alpha := '';
try
while v <= Length(iban) do
begin
if l > Length(iban) then
l := Length(iban);
alpha := alpha + Copy(iban, v, l);
number := StrToInt(alpha);
rest := number mod 97;
v := v + l;
alpha := IntToStr(rest);
l := 9 - Length(alpha);
end;
except
rest := 0;
end;
Result := rest;
end;

function CheckIBAN(iban: string): Boolean;
begin
iban := StringReplace(iban, ' ', '', [rfReplaceAll]);
if CalculateDigits(iban) = 1 then
Result := True
else
Result := False;
end;