How to pass a variable into the regex expression in javascript?

I'm making a list search tool to check for a specific name in a long list. I have been successful at hard coding the name into the regex, but I need it to be dynamic.
Here is the code:

    function processString(){
  document.getElementById('textArea').value = "";
  var inputString = document.getElementById('textBox').value;

  var userIn = document.getElementById('userInput').value;
  var regex = //use variable 'userIn' for regex
  $('#textArea').html(result);

  if(result != null){
    for(var i = 0; i < result.length; i++){
      document.getElementById('textArea').value += result[i] + '\r\n';
    }
  }


}

codepen