How to do two-way binding with directive variable and a parent scope?

I want to create a simple directive called form-field. The directive is as follow: a grouping of form label, form input, form error text box. My goal is create a directive that can contain all these in one single line.

  <form-field label="Username:" regex="^[a-z]{5,7}$" constrainsViolationMessage="username should be between 5 and 7 chars" model="username" place-holder="username"> </form-field> 

The problem that I encountered is that I am unable to bind the input variable value to a parent scope variable.

In the fiddle, binding to parent scope does not work at all, on localhost the directive variable is on step after the parent scope variable ! For example if binding variable is 123 parent scope variable is 12.

Here is a fiddle, I hope someone help me with some suggestion .

On local host the problem is that the directive scope is one step after the parent scope the image below depicts the prblem.