Возможно вот этого достаточно будет
alert(
(
'<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'
)
.match(/<iframe [\s\S]+?https:\/\/www\.youtube.com\/[\s\S]+?\/iframe>/ig)
)
//[ '<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>',
// '<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>',
// '<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>',
// '<iframe width="560" height="315" src="https://www.youtube.com/embed/11111" frameborder="0" allowfullscreen></iframe>' ]