var text = 'This is a pen.';
var reg1 = /This is a (.*)\./;
var result1 = text.match(reg1);
$('#reg1').html(result1[1]);
var reg2 = new RegExp('This is a (.*)\\.');
var result2 = text.match(reg2);
$('#reg2').html(result2[1]);
<div id="reg1"></div> <div id="reg2"></div>