The Collaborative International Dictionary of English v.0.48:

Rueful \Rue"ful\ (r[udd]"f[.u]l), a.
   1. Causing one to rue or lament; woeful; mournful; sorrowful.
      [1913 Webster]

   2. Expressing sorrow. "Rueful faces." --Dryden.
      [1913 Webster]

            Two rueful figures, with long black cloaks. --Sir W.
                                                  Scott.
      [1913 Webster] -- Rue"ful*ly, adv. -- Rue"ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rueful
    adj 1: feeling or expressing pain or sorrow for sins or offenses
           [syn: contrite, remorseful, rueful, ruthful]