get video from content

by

Assuming your content is in the variable $content you would begin by removing all html tags using wp_strip_all_tags:

$new_content = wp_strip_all_tags( $content );

Then you will want to trim that to 200 characters using substr:

$new_content = substr( $new_content, 0, 200 );

To retrieve the YouTube video, you’ll need to use preg_match:

preg_match( '/(<iframe.*?src="https://wordpress.stackexchange.com/questions/154352/(.*?youtube.*?)".*?<\/iframe>)/', $html, $matches );
if ( $matches ) {
  $iframe = $matches[1];
  $url = $matches[2];
}

(I wasn’t sure whether you wanted the iframe for embedding, or the src URL, so I’ve included matching for both.)