Traversing strings for multiple instances of substrings - freepascal or delphi - freepascal

Question

Platform : Lazarus 1.1, FreePascal 2.7.1, Win 7 32-bit.
I have a string value as follows:
FileName[12345][45678][6789].jpg
By default (assume this is default behaviour 0), my program currently pulls out the last set of numbers from the last pair of square brackets to the farthest right of the filename, i.e. 6789. It does so using this code:
if chkbxOverrideUniqueID.Checked then
IDOverrideValue := StrToInt(edtToggleValue.Text);
// User is happy to find the right most unique ID
if not chkbxOverrideUniqueID.Checked then
LastSquareBracket := RPos(']', strFileName);
PreceedingSquareBracket := RPosEx('[', strFileName, LastSquareBracket) + 1;
strFileID := AnsiMidStr(strFileName, PreceedingSquareBracket, LastSquareBracket - PreceedingSquareBracket)
else // User doesn't want to find the rightmost ID.
// and now I am stuck!
However, I have now added an option for the user to specify a non-default behaviour. e.g if they enter '1', that means "look for the first ID in from the farthest right ID". e.g. [45678], because [6789] is default behaviour 0, remember. If they enter 2, I want it to find [12345].
My question : How do I adapt the above code to achieve this, please?

Answer 1

The following code will return just the numeric value between brackets:
uses
StrUtils;
function GetNumber(const Text: string; Index: Integer): string;
var
I: Integer;
OpenPos: Integer;
ClosePos: Integer;
begin
Result := '';
ClosePos := Length(Text) + 1;
for I := 0 to Index do
begin
ClosePos := RPosEx(']', Text, ClosePos - 1);
if ClosePos = 0 then
Exit;
end;
OpenPos := RPosEx('[', Text, ClosePos - 1);
if OpenPos <> 0 then
Result := Copy(Text, OpenPos + 1, ClosePos - OpenPos - 1);
end;
If you'd like that value including those brackets, replace the last line with this:
Result := Copy(Text, OpenPos, ClosePos - OpenPos + 1);