Regex not working in HTML5 pattern

So I have this regex intended to let pass all text but those that contain as initial chars the "34" sequence:

^(?!34)(?=([\w]+))

The regex is working fine for in http://ift.tt/1J0wujt

Any Idea why it isn't working in my form?

<form>
    <input pattern="^(?!34)(?=([\w]+))" type="text">
    
        <button type="submit">Submit!</button>
</form>