rueful
adjective /ˈruːfl/
  /ˈruːfl/
 - feeling or showing that you are sad or sorry
- a rueful smile
 
Oxford Collocations DictionaryRueful is used with these nouns:- grin
 - laugh
 - smile
 - …
 
Word OriginMiddle English (also in the sense ‘pitiable’): from the noun rue ‘regret’ + -ful.