s = 'The only true (wisdom) is in knowing you know (nothing)'
s.match(/(([^)]+))$/).captures

s = 'The only true (wisdom) is in knowing you know (nothing)'
s.match(/(?<=()([^)]+)(?=)$)/).captures

str = "The only true (wisdom) is in knowing you know (nothing)"
str.scan(/((.+?))/).last
#=> "nothing"

s = 'The only true (wisdom) is in knowing you know (nothing)'
s.match(/(([^)]+))$/).captures # => ["nothing"]

"...knowing you know ((almost) nothing)"[/(((?:[^()]*|(g<1>))*))$/, 1]
#=> "(almost) nothing"

s = 'The only true (wisdom) is in knowing you know (nothing)'

r = s.reverse
r[(r.index(')') + 1)...(r.index('('))].reverse
  #=> "nothing"