check whether string contains a line break

Line breaks in HTML aren’t represented by \n or \r. They can be represented in lots of ways, including the <br> element, or any block element following another (<p></p><p></p>, for instance).

If you’re using a textarea, you may find \n or \r (or \r\n) for line breaks, so:

var text = $("#theTextArea").val();
var match = /\r|\n/.exec(text);
if (match) {
    // Found one, look at `match` for details, in particular `match.index`
}

Live Example | Source

…but that’s just textareas, not HTML elements in general.