@vanitharaj1208 both regex patterns works for me:
1. with @
Result during ingestion (before regex):
2. without @
To view values after mapping process I clicked on "Show results" under 3-dot menu:
I think it is working, to validate the result you need to click 3-dot icon as shown on a screenshot:
When you howerover your mouse above "green" mapped value, it shows only original value, but not end result of mapping.