Try javascript

if (value.indexOf('iframe') >= 0) {

JQuery contains is for DOM elements, not strings.

Try doing it like this:

$('#embedModal textarea:contains("iframe")').each(function() {
  //Do something
});

edit

Example

this works:

$.contains( document.documentElement, document.body ); // true

var babyEl = $('div#id');
var daddyEl = $('div#ID');

if($.contains(babyEl, daddyEl)) {
    //do fun stuff
}