Hmmmmmm... Well, just to reiterate, you should only use this in personal books, DEFINITELY NOT FOR SALE.
Calibre or Sigil works. They both edit EPUBs + the steps are extremely similar.
Note: MAKE SURE YOU SAVE A BACKUP COPY of your book. Regular expression are very powerful (and can completely break your book if you do them wrong).
* * *
1. Open Calibre.
2. Right-Click the book's title in the list, then press
Edit Book.
This will open up Calibre's built-in ebook editor.
3. Press
Ctrl+F to pop up the Find/Replace.
4. Below the Find/Replace textboxes, you should see 3 dropdowns and 2 checkboxes:
Mode: [Normal] + [All Text Files] + [Down]
Case Sensitive + Wrap
Change
Mode to Regex.
5. Insert this regular expression in the boxes:
Search: (“[^”<]+”)
Replace: <b>\1</b>
6. DO NOT press "Replace All" yet.
Test your Regular Expression on a small set first:
Press "Find" to see if it highlights the correct text.
Press "Replace".
Make sure it does what you want. Then test again.
If it looks like it's working, then you can "Replace All".
Note: If anything goes seriously wrong, you can always
Edit > Undo or
Edit > Revert. Or as a last resort, go back to your original copy... you did make a backup, didn't you?
No problem.
Sounds like poor writing to me.
But thanks for giving me the use-case.