The Collaborative International Dictionary of English v.0.48:

Regretful \Re*gret"ful\ (-f?l), a.
   Full of regret; indulging in regrets; repining. --
   Re*gret"ful*ly, adv.
   [1913 Webster] re-group

WordNet (r) 3.0 (2006):

regretful
    adj 1: feeling or expressing regret or sorrow or a sense of loss
           over something done or undone; "felt regretful over his
           vanished youth"; "regretful over mistakes she had made";
           "he felt bad about breaking the vase" [syn: regretful,
           sorry, bad] [ant: unregretful, unregretting]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "regretful":
   apologetic, ashamed, baffled, balked, betrayed, bilked, blasted,
   blighted, chapfallen, conscience-smitten, conscience-stricken,
   contrite, crestfallen, crossed, crushed, dashed, defeated,
   disappointed, dished, disillusioned, dissatisfied, foiled,
   frustrated, full of remorse, guilty, ill done-by, ill-served,
   let down, mournful, out of countenance, penitent, remorseful,
   repentant, repining, rueful, sad, self-accusing, self-condemning,
   self-convicting, self-debasing, self-flagellating,
   self-humiliating, self-punishing, self-reproaching, shamefaced,
   shamefast, shameful, sorely disappointed, sorrowful, sorry, soured,
   thwarted, unhappy about, wistful