$ perl -e '$_="foobar_";s/[^a-zA-Z]+$//;print'
foobar

$ perl -e '$_="foobar_";s/\W+$//;print'
foobar_

$ perl -e '$_="foobar9";s/\W+$//;print'
foobar9

##</code><code>##

$ perl -e '$_="foobar_";s/[[:^alpha:]]+$//;print'
foobar