Convert multibyte hex back to UTF-8 string - string

Question

In Delphi 10.4, I have a hexadecimal representation of a string:
function TForm1.Button2Click(Sender: TObject);
var
i, nr : integer;
Input, HexStr, h: String;
begin
HexStr := '';
Input := Edit2.Text;
for i:=1 to Length(Input) do begin
nr := Ord(Input[i]);
h := IntToHex(nr, 0);
if Length(h) = 1 then
h := '0' + h;
HexStr := HexStr + h;
end;
Edit3.Text := HexStr;
end;
For "abc€", I get "61626320AC" here. Note the € sign converts to "20AC".
Now for converting it back to a normal string, I split that hex into 2-char pairs, using StrToInt() with a '$' prefix. I have no indicator for it being 4 chars long for an € sign, instead of 2 chars, and this breaks the euro sign:
How do I convert back such a hex string without breaking multibyte characters?

Answer 1

I have no indicator for it being 4 chars long for an € sign, instead of 2 chars
And because of that reason alone, you simply won't be able to convert "61626320AC" back to "abc€", because you don't know which hex codes are 2 digits and which are 4 digits, you didn't deliminate them. For instance, the hex codes 6162 and 6263 also represent valid Unicode characters, too.
As #TomBrunberg mentioned in comments, you can use 4-digit hex codes for every character, eg:
function TForm1.Button2Click(Sender: TObject);
var
i, nr : integer;
Input, HexStr, h: String;
begin
HexStr := '';
Input := Edit2.Text;
for i := Low(Input) to High(Input) do begin
nr := Ord(Input[i]);
h := IntToHex(nr, 4);
HexStr := HexStr + h;
end;
Edit3.Text := HexStr;
end;
Then you can convert back by splitting up the hex string into 4-character groups when calling StrToInt(), eg:
function TForm1.Button3Click(Sender: TObject);
var
i, nr : integer;
Output, HexStr, h: String;
begin
HexStr := Edit3.Text;
Output := '';
i := Low(HexStr);
while i <= High(HexStr) do begin
h := Copy(HexStr, i, 4);
nr := StrToInt('$' + h);
Output := Output + Char(nr);
Inc(i, 4);
end;
Edit2.Text := Output;
end;
Or, as #AmigoJack hinted at, you can use UTF-8 instead, eg:
function TForm1.Button2Click(Sender: TObject);
var
i, nr : integer;
HexStr, h: String;
Input: UTF8String;
begin
HexStr := '';
Input := UTF8String(Edit2.Text);
for i := Low(Input) to High(Input) do begin
nr := Ord(Input[i]);
h := IntToHex(nr, 2);
HexStr := HexStr + h;
end;
Edit3.Text := HexStr;
end;
function TForm1.Button3Click(Sender: TObject);
var
i, nr : integer;
HexStr, h: String;
Output: UTF8String;
begin
HexStr := Edit3.Text;
Output := '';
i := Low(HexStr);
while i <= High(HexStr) do begin
h := Copy(HexStr, i, 2);
nr := StrToInt('$' + h);
Output := Output + AnsiChar(nr);
Inc(i, 2);
end;
Edit2.Text := string(Output);
end;