Use
Code:
<p class="calibre2">\sCHAPTER\s*(\d*)\s</p>
for search and
Code:
<h2 class="calibre3">CHAPTER \1</h2>
for replace.
If you need more than one part of the searched text, surround each needed part with parentheses. In the replacement, use backslash and number where the number is the result of numbering the parentheses from left to right, starting with one.