My regex fu must be on the blink.
This regex search should work, but it's not working in Sigil, does work in EditPad though, so perhaps it's a problem with the regex engine?
Code:
(?<=<span\sclass="(bold|italics)">)(.+?)(?=</span>)
The group that stores the text will be \2 (not the usual \1 - which here will be the bold|italics class)
Is that any use?
EDIT: This works in Sigil (with your example).
Search for