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I recently had to implement version string comparison for InnoSetup, because the solutions that I found on the internet did not suit my needs. Hopefully this will be useful for somebody else.

// Procedure to split a string into an array of integers 
procedure Explode(var Dest: TArrayOfInteger; Text: String; Separator: String);
var
  i, p: Integer;
begin
  i := 0;
  repeat
    SetArrayLength(Dest, i+1);
    p := Pos(Separator,Text);
    if p > 0 then begin
      Dest[i] := StrToInt(Copy(Text, 1, p-1));
      Text := Copy(Text, p + Length(Separator), Length(Text));
      i := i + 1;
    end else begin
      Dest[i] := StrToInt(Text);
      Text := '';
    end;
  until Length(Text)=0;
end;

// Function compares version strings numerically:
//     * when v1 = v2, result = 0  
//     * when v1 < v2, result = -1  
//     * when v1 > v2, result = 1
//
// Supports version numbers with trailing zeroes, for example 1.02.05.
// Supports comparison of two version number of different lengths, for example
//     CompareVersions('1.2', '2.0.3')
// When any of the parameters is '' (empty string) it considers version number as 0
function CompareVersions(v1: String; v2: String): Integer;
var
  v1parts: TArrayOfInteger;
  v2parts: TArrayOfInteger;
  i: Integer;
begin
  if v1 = '' then
  begin
    v1 := '0';
  end;

  if v2 = '' then
  begin
    v2 := '0';
  end;

  Explode(v1parts, v1, '.');
  Explode(v2parts, v2, '.');
  
  if (GetArrayLength(v1parts) > GetArrayLength(v2parts)) then
  begin
    SetArrayLength(v2parts, GetArrayLength(v1parts)) 
  end else if (GetArrayLength(v2parts) > GetArrayLength(v1parts)) then
  begin
    SetArrayLength(v1parts, GetArrayLength(v2parts)) 
  end; 
  
  for i := 0 to GetArrayLength(v1parts) - 1 do 
  begin
    if v1parts[i] > v2parts[i] then
    begin
      { v1 is greater }
      Result := 1;
      exit;
    end else if v1parts[i] < v2parts[i] then
    begin
      { v2 is greater }
      Result := -1;
      exit;
    end;
  end;
  
  { Are Equal }
  Result := 0;
end;

To test the code in InnoSetup, you can use the following function:

procedure TestVersions(v1: String; v2: String);
begin
  Log(v1 + ' : ' + v2 + ' = ' + IntToStr(CompareVersions(v1, v2)));
end;

And call it with some parameters:

  TestVersions('1', '2');
  TestVersions('2', '1');
  TestVersions('3', '3');
  
  TestVersions('1.1', '1');
  TestVersions('2.1', '1');
  TestVersions('1.1', '2');

  TestVersions('2.12.11', '2.12.25');
  TestVersions('', '2.12.25');
  TestVersions('2.12.25', '');
  TestVersions('', '');
  TestVersions('2.12.11', '2.012.11');

A practical example can be used as follows:

if CompareVersion(currentVersion, newVersion) = -1 then
begin
  //do the upgrade
end