How to validate a text field so it can only contain a four digit number

Question

I\'ve managed to validate my field so it is always four digits, but i need to validate so that it is always a number. I tried adding this block of code but it doesn\'t work

Answer 1

/^[0-9]{4}$/ for four numbers

/^[0-9]*$/ for any amount of numbers

/^[0-9]+$/ for at least one number