Detect if string contains a float?

Question

How can I detect if a string contains a float. For example: '0.004'

But without using StrToFloat because that function are slow but rather by iterating through chars.

function IsInteger(const S: String): Boolean;
var
  P: PChar;
begin
  P := PChar(S);
  Result := True;
  while not (P^ = #0) do
  begin
    case P^ of
      '0'..'9': Inc(P);
    else
      Result := False;
      Break;
    end;
  end;
end;

This will check if string is a positive integer but not a float..

Answer 1

I would use TryStrToFloat():

if TryStrToFloat(str, value, FormatSettings) then
  ....

If you are prepared to use the default system wide format settings then you can omit the final parameter:

if TryStrToFloat(str, value) then
  ....

Answer 2

Can you use a RegEx here? Something like:

([+-]?[0-9]+(?:\.[0-9]*)?) 

Answer 3

The problem with this question is that saying "is too slow" doesn't tell much. What does the profiler tells to you? Do you have an informed idea about the input data? What about different notations, for example, 6.02e23?

If your input data is mostly noise, then using regular expressions (as answered here) may improve things but only as a first filter. You could then add a second step to actually obtain your number, as explained by David's answer.