yep:
find:
<a href="#id
followed by a capturing group `()` that contains one or more integers `\d+`
(\d+)
followed by
"
replace:
<a epub:type="noteref" href="#id
followed by a back-reference to the captured group above
\1
followed by
" xmlns:epub="http://www.idpf.org/2007/ops"
this site is an invaluable reference for anything regex, basic to advanced:
http://www.regular-expressions.info