JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<p>[fullwidth blah="1"]
    text line lorem ipsuim blah blah
    [divider value="80"]

    another line with extra space above

    [text] this is test[/text]
         another line
[/fullwidth]</p>

<h2></h2>

JavaScript

var content = $('p').text();
var content_per_line = content.split(/[\r\n]+/g);
var regex = /^((?!\[).)*$/g;
var output = '';
$.each(content_per_line, function(ind ,line) {
	var trimmed_line = $.trim(line);
  console.log(trimmed_line)
	if (trimmed_line !== '\n' && regex.test(trimmed_line) === true) {
  	output += '[text]' + line + '[/text]';
  } else {
  	output += line;
  }
  output += '<br>'
})

console.log(output)

$('h2').html(output)