[Gunnerp245]

It seems you are opening "Search and Replace" from the preference menu, as there is no 'open' button in the convert menu.

Procedure:
1. Open the Search & Replace menu/setup.
2. Put my regex into the "Search Regular Expression" (SRE) field
3. Select "Add", just in case, LoL.
4. Bring the added regex back to the SRE field
5. Select the "Wizard" button to the right of the SRE field
6. The "Regex Builder" window opens, with my regex in the "Regex:" field at the top
7. But, the "Preview" window is blank, and pressing the "Test" button does nothing
8. So, at the bottom there is a "Open" button, which presumably lets you open a document(?) that contains the data one is trying to search and replace
9. So, select the EPUB document and try to open it. Nothing, no document. Try a .html document. Nothing shows in the "Preview" page! Neither will it let me PASTE any text in it.

The Preferences action allows you to change the way various aspects of calibre work on every file every time. To see what you are trying to do you need to be in the convert menu.

Spoiler:

Main calibre Menu, Book to convert selected.

Spoiler:

After selecting 'convert book'.

Spoiler:

Selected 'Search and Replace'.

Spoiler:

Selected 'Wizard' icon.

calibre online User Manual.