<h1/> is also a typo H1 is not a self closing tag.
Code:
<h1>(.*?)\s*<br />\s*(.*?)</h1>
I included
0 or more spaces as part of my search as you do put 1 back in your replace.
FWIW I usually do the reverse. I insert a trailing space after \1.
for use on my small (not a desktop) screen when chapter titles are more the just
Chapter #. The advantage is, the TOC builder works.

You can even resize the second line with a <span class=> and not mess up the TOC inclusion