Edit in JSFiddle

//Function to convert hex format to a rgb color
function rgb2hex(rgb){
 rgb = rgb.match(/^rgba?[\s+]?\([\s+]?(\d+)[\s+]?,[\s+]?(\d+)[\s+]?,[\s+]?(\d+)[\s+]?/i);
 return (rgb && rgb.length === 4) ? "#" +
  ("0" + parseInt(rgb[1],10).toString(16)).slice(-2) +
  ("0" + parseInt(rgb[2],10).toString(16)).slice(-2) +
  ("0" + parseInt(rgb[3],10).toString(16)).slice(-2) : '';
}

$('button').click(function(){
    var hex = rgb2hex( $('input').val() );
    $('.result').html( hex );
});

Convert RGB/RGBA to hex:<br>
(opacity of rgba is ignored)<br>
<input type="text" value="rgba(34, 34, 34, 1)"> <button>Convert</button><br>
<br>
Result <span class="result"></span>