Question

I have a small form where users enter a deposit in a dollar amount. This amount can be positive or negative. I am attempting to validate the form each time a key is pressed to prevent letters or characters from being entered by accident.

Consider the function below:

var deposit = document.getElementById("deposit");

deposit.onkeyup = function() {
    var PATTERN = /\d$/;

    if (!deposit.value.match(PATTERN)) {
        deposit.value = deposit.value.replace(deposit.value.slice(-1), "");
    }
}

This works fine and dandy if you only want your user to be able to enter numbers. Any other character will simply be erased. Where I am having trouble is coming up with a pattern that will work on the fly to match a positive or negative currency (without the dollar sign). Users need to be able to enter an optional negative sign, at least one number, followed by a decimal point, followed by 2 more numbers.

Example deposit amounts:

0.09

100.45

4032.34

-0.90

-54.56

-1353.00

Any help coming up with a pattern, or perhaps even a better method, would be greatly appreciated. Please let me know if more detail is needed.

Answer 1

You can use this validation pattern to match those number values.

^-?\d+\.\d{2}$