Validate sprintf format from input field with regex

Question

I have an input field where both regular text and sprintf tags can be entered.

Example: some text here. %1$s done %2$d times

How do I validate the s

Answer 1

This is what I ended up with, and its working.

// Always use server validation even if you have JS validation
if (!isset($_POST['input']) || empty($_POST['input'])) {
  // Do stuff
} else {
  $matches = explode(' ',$_POST['input']);
  $validInput = true;

  foreach ($matches as $m) {
    // Check if a slice contains %$[number] as it indicates a sprintf format
    if (preg_match('/[%\d\$]+/',$m) > 0) {
      // Match found. Now check if its a valid sprintf format
      if ($validInput === false || preg_match('/^%(?:\d+\$)?[dfsu]$/u',$m)===0) {   // no match found
        $validInput = false;
        break; // Invalid sprintf format found. Abort
      }
    }
  }

  if ($validInput === false) {
    // Do stuff when input is NOT valid
  }
}

Thank you Gumbo for the regex pattern that matches both with and without order marking.

Edit: I realized that searching for % is wrong, since nothing will be checked if its forgotten/omitted. Above is new code.

"$validInput === false ||" can be omitted in the last if-statement, but I included it for completeness.