Date matching question

Hi guys,
I'm trying to rename files with FileName_July_2018.extension to FileName_201807.extension.
So I have a name match check for customdate (using month_year) then do a rename with pattern name_customdate.extension.
The files match fine, except they come out to be FileName_July_2018_201807.extension.
How do I tell it to rename with pattern of name minus the date that matched, then add the new formatted date behind it?
I'm trying to rename files with FileName_July_2018.extension to FileName_201807.extension.
So I have a name match check for customdate (using month_year) then do a rename with pattern name_customdate.extension.
The files match fine, except they come out to be FileName_July_2018_201807.extension.
How do I tell it to rename with pattern of name minus the date that matched, then add the new formatted date behind it?