keep input to 2 line breaks consecutively in textarea (preview div before submit)

Question

What I am looking to do is allow 2 line breaks for formatting, but preventing 3 or more from displaying (in preview before submit, so cant just strip it out with php)

I started with

var breakTag = (is_xhtml || typeof is_xhtml === 'undefined') ? '<br />' : '<br>';    
return (str + '').replace(/([^>\r\n]?)(\r\n|\n\r|\r|\n){2,}/g, '$1'+ breakTag +'$2');

I tried adding {2,}

var breakTag = (is_xhtml || typeof is_xhtml === 'undefined') ? '<br />' : '<br>';    
return (str + '').replace(/([^>\r\n]?)(\r\n|\n\r|\r|\n){2,}/g, '$1'+ breakTag +'$2');

provided in <textarea>

this is user input
i have inserted 1 line break


i have inserted 3 line breaks



i have inserted 4 line breaks

what I want is

this is user input
i have inserted 1 line break

i have inserted 3 line breaks

i have inserted 4 line breaks

what I am getting is

this is user inputi have inserted 1 line break
i have inserted 3 line breaks
i have inserted 4 line breaks

without the {2,} it displays as input

bonus if we can keep them from entering more than 2 line breaks consecutively in the textarea to begin with, as well as clean it up on preview

* edit *

$('textarea').keyup(function()
{
    var myString = $(this).val();
    myString = nl2br(myString);
    $('#preview').html(myString);
});

function nl2br (str, is_xhtml)
{
    var breakTag = (is_xhtml || typeof is_xhtml === 'undefined') ? '<br />' : '<br>';    
    return (str + '').replace(/([^>\r\n]?)(\r\n|\n\r|\r|\n)/g, '$1'+ breakTag +'$2');
}

Answer 1

Here is code to replace both the value in the textarea and insert <br /> into a div. This isn't in the context of jQuery, but you don't have to use it for something like this. It should be straightforward to convert to jQuery, though.
Edit: avoid error if the textarea is empty.

<script>
function oneBreak(ta) { 
  outDiv = document.getElementById('outDiv'); 
  if (ta.value.length <= 0) {
    outDiv.innerHTML = '';
  } else {
    var val = ta.value + "";
    var ln = "\n";
    //normalize line breaks
    if (val.match(/(\r\n)/g)) ln = "\r\n";
    else if (val.match(/\r/g)) ln = "\r";
    val = val.replace(/(\r\n)/g,"\n");
    val = val.replace(/\r/g,"\n");
    val = val.replace(/(\s*\n{3,})+/g,"\n\n")
    // return to default line breaks
    ta.value = val.replace(/\n/g,ln);
    outDiv.innerHTML = val.replace(/\n/g, "<br />");
  }
  return outDiv.innerHTML; 
}
</script>

<textarea id="breaks" onKeyUp="oneBreak(this);" onChange="oneBreak(this);"></textarea>

<div id="outDiv"></div>

Edit 2: add space-only line removal. JS Fiddle: http://jsfiddle.net/2T2gv/