Every regex that tries to match a substring with .* or .+ is
guaranteed to be wrong. In this case, it was giving incorrect matches
straddling multiple quoted attributes:
<img __ASTRO_IMAGE_="../assets/image.png"> and <a href="link">link</a>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Fixes#7741.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
