[Vuejs]-Using a reg expression to ensure the user entered a decimal?

Your function works fine. I suggest reversing the returning value like this:

decimal: function (val/*,rule*/){
    if(!val){
        return true;
    }
    return !/^[0-9]{1,6}(?:[,.][0-9]{0,5})?$/.test(val);
}

This regex works to ensure the user has entered a decimal value (upto 2 points (eg.785747.12))..

\d+(\.\d{1,2})?

I got this from
Simple regular expression for a decimal with a precision of 2

I hope this helps.