Если нужна просто проверка
re=/<iframe [\s\S]+?https:\/\/www.youtube\.com\/[\s\S]+?\/iframe>/i
alert([
re.test(
'<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>'
+'FOO'
+'<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>'
+'FOO'
+'<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>'
+'FOO'
+'<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>'
+'FOO'
),
re.test("foo")
])
//[true, false]