A couple of very simple yet handy routines for validation of form fields:

Function to validate an email address (see original post):

function ValidateEmail(email){
    var emailReg = "^[\\w-_\.]*[\\w-_\.]\@[\\w]\.+[\\w]+[\\w]$";
    var regex = new RegExp(emailReg);
    return regex.test(email);
}

Function to validate a number:

function ValidateNumber(num)
{
    return num.match(/^\d+$/);
}

Input mask for a field to only accept numbers:

In your html add the following:

<input name="numericfld"
       onkeypress="return(ForceNumbersOnly(this, event));"
       onchange="RemoveNonNumeric(this)" />

Then implement the following functions:

function ForceNumbersOnly(myfield, e, dec)
{
    var key;
    var keychar;

    if (window.event)
    {
        key = window.event.keyCode;
    }
    else if (e)
    {
        key = e.which;
    }
    else
    {
        return true;
    }
    if(key != 46 && key != 45 && key > 31 && (key < 48 || key > 57))
    {
        return false;
    }
    else
    {
        return true;
    }
}

function RemoveNonNumeric(myfield)
{
    var re = /[^0-9\.\-]/g;
    if(re.test(myfield.value))
    {
        myfield.value = myfield.value.replace(re, '');
        myfield.value = myfield.value.replace(/\./, '');
    }
}