Replace last occurrence of a character in a field with awk - regex

Question

I'm trying to replace the last occurrence of a character in a field with awk. Given is a file like this one:
John,Doe,Abc fgh 123,Abc
John,Doe,Ijk-nop 45D,Def
John,Doe,Qr s Uvw 6,Ghi
I want to replace the last space " " with a comma ",", basically splitting the field into two. The result is supposed to look like this:
John,Doe,Abc fgh,123,Abc
John,Doe,Ijk-nop,45D,Def
John,Doe,Qr s Uvw,6,Ghi
I've tried to create a variable with the number of occurrences of spaces in the field with
{var1=gsub(/ /,"",$3)}
and then integrate it in
{var2=gensub(/ /,",",var1,$4); print var2}
but the how-argument in gensub does not allow any characters besides numbers and G/g.
I've found a similar thread here but wasn't able to adapt the solution to my problem.
I'm fairly new to this so any help would be appreciated!

Answer 1

With GNU awk for gensub():
$ awk 'BEGIN{FS=OFS=","} {$3=gensub(/(.*) /,"\\1,","",$3)}1' file
John,Doe,Abc fgh,123,Abc
John,Doe,Ijk-nop,45D,Def
John,Doe,Qr s Uvw,6,Ghi
Get the book Effective Awk Programming by Arnold Robbins.
Very well-written question btw!

Answer 2

Here is a short awk
awk '{$NF=RS$NF;sub(" "RS,",")}1' file
John,Doe,Abc fgh,123,Abc
John,Doe,Ijk-nop,45D,Def
John,Doe,Qr s Uvw,6,Ghi
Updated due to Eds comment.
Or you can use the rev tools.
rev file | sed 's/ /,/' | rev
John,Doe,Abc fgh,123,Abc
John,Doe,Ijk-nop,45D,Def
John,Doe,Qr s Uvw,6,Ghi
Revers the line, then replace first space with ,, then revers again.