Allowing decimal in numbers only field with JavaScript

Question

Hi all, I have a function which I use to limit form inputs to numbers only, or number and decimals depending on the field. Allowing decimals and numbers is easy enough, but I am trying to take it a step further and allow only one decimal, while also making sure that decimal is not the first character in the field. I have successfully allowed only one decimal, and I have also made it so a decimal will only be allowed if "0" is the first digit, but I cannot get it to allow a decimal when ANY number is the first digit for some reason. I could make it work if I made a massive if statement, but I am trying to avoid that. Any suggestions?

            // this allows only one decimal, and only if the first character of the field is a zero
            else if ((('.').indexOf(keychar) > -1) && field == document.form.start_pay && document.form.start_pay.value.indexOf('.') <= -1 && document.form.start_pay.value.charAt(0) == ('0')){
                return true;
            }

Answer 1

Change it to:

        else if ((('.').indexOf(keychar) > -1) && field == document.form.start_pay && document.form.start_pay.value.indexOf('.') <= -1 && /[0-9]+/.test(document.form.start_pay.value.charAt(0))){
            return true;
        }

Answer 2

/^((?:\d\.\d)|(?:\d+))$/.test(document.form.start_pay)

will cover all of your cases. (Should pass 1, 100, 0.7, 8.3 and any other permutations you can think of but disallow .3, 1.21 ... etc.)

Line by line:

/ #Begin regular expression
    ^ #Starting at the beginning of the string
    ( #For group #1
        (?: #Match
            \d\.\d #A number followed by a literal . (\.) followed by a number
        )
    | #Or
        (?: #Match
            \d+ #A number one or more times
        )
    ) #End group 1
    $ #Followed by the end of the string
/ #End regular expression

Answer 3

Change :

document.form.start_pay.value.charAt(0) == ('0')

To :

!isNaN(document.form.start_pay.value.charAt(0))