Jquery function to trim float returns null

Question

I was looking for a way to trim floats to a certain number of decimal places without rounding. After some googling, I found this:

15.7784514000.toString().match(/^\d+(?:\.\d{0,2})?/)

This works great, returning 15.77. I'm building an application which will include different floats which need to be trimmed to a given number of places, so I'm trying to convert this into a function that I can give my float, and the number of decimal places I want, and get the result.

I've come up with this:

function floatTrim(num) {
    var dec = '2',
        exFrame = '^\d+(?:\.\d{0,' + dec + '})?',
        ex = new RegExp(exFrame),
        j = num.toString(),
        out = j.match(ex);
    console.log(out);
}

... where the 'dec' variable equals the number of decimal places to trim to. However, when I run this function with the float 15.7784514000 from above, it returns null, and I can't figure out why. Can anyone offer some insight?