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