Copy and paste from the import file will not work as it'll copy too many escape characters; try:

Regex:
Code
((?:https?:)?\/\/)?((?:www|m)\.)?((?:youtube\.com|youtu.be))(\/(?:[\w\-]+\?v=|embed\/|v\/)?)([\w\-]+)([a-zA-Z0-9_-]{8,12})(\S+)

Markup:
Code
<div class="video-container"><iframe frameborder="0" width="500" height="300" src="https://www.youtube.com/embed/\\5\\6\\7?wmode=opaque" allowfullscreen></iframe></div>

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