What RegExp flag can I use to help me find numbers within a string?

Question

"Write a function called countNumbers which accepts a string of numbers and returns the count of numbers between 0 and 9."

For example:

countNumbers("this is so wonderful") // 0
countNumbers("this is so 1234") // 4

This is what I have so far:

function countNumbers(string) {
    var matches = string.match(/..0.1.2.3.4.5.6.7.8.9/gi)
    var regex = new RegExp(matches, 'gi')
    if (matches) {
        return matches.length
    }
    return 0;
}

I keep getting 0 returned, and it won't count the numbers within the string.