Naive approach:
str = """
<div class=wp-block-supertrends-figure><div style=overflow: hidden; padding-top:
56.25%; position: relative;><iframe src=https://player.vimeo.com/video/687889699
frameborder=0 allowfullscreen=></iframe></div><div class=title>Video</div><div
class=text>Video description</div></div>
"""
Regex.run(~R|https://player.vimeo.com/video/[0-9]+|, str)
# ["https://player.vimeo.com/video/687889699"]
EDIT: Sorry, I just realized, there can be more URL formats. So consider this irrelevant.
