Hey SIMTEC-Software-Solutions, This regex seems to evaluate correctly on my end. It looks like you need to switch around the order of your ternary. The value that follows the '?' will return when the regex condition is true and the value that follows the ':' will return when the regex condition is false. If the 'Custom rule' input evaluates to a string, it will display that string as the error message. When the 'Custom rule' evaluates to an empty string, then it will not show an error message.
I hope that helps!