If I try to replace some filed completely and to use regex ".*" to be replaced with something new. It will display the replacement twice. For example,
IN ARTIST field,
I use Actions -> Quick Actions -> Replace with Regular Expression.
Regex: .*
Replace: ReplacementText
It will be changed to ReplacementTextReplacementText instead of just ReplacementText
Also, if the field is is blank. This regex does not make any changes.