error validating number 4 or 5 digits

I am hoping a javascript guru can help me debug a problem with a validation function.  I have a simple javascript function that validates an input that must be numeric and 4 or 5 digits.  However, it appears to be causing errors.

See below for the validation function.

Thanks for any help.

This code causes errors:

function checkemployeenumber(input) {
        var str=input.value;
        var filter = /^[0-9]{4,5}$/;

        if (filter.test(str)) {
            return true;
        }else{
            return false;
        }
    }

If I change to just validate 5 digits like this it works:

function checkemployeenumber(input) {
        var str=input.value;
        var filter = /^[0-9]{5}$/;

        if (filter.test(str)) {
            return true;
        }else{
            return false;
        }
    }

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