replace *any* with specific

I have just downloaded v2.43 I have several songs in a folder with various entries under Genre (unknown, alternative, vocal, etc.). I would like to create an action with replace to change any and all genre entries in the selected files to Christmas. I have tried several things but I don't have the syntax correct. I basically want to search for any entry or nothing and replace it with "Christmas."

Replace "Genre": ".*" -> "Christmas"

does not work.

You must use "Replace with regular expression" or that is also very easy:

Action type: Format value
Field: GENRE
Format string: Christmas