WordNet (r) 3.0 (2006):

regrettable
    adj 1: deserving regret; "regrettable remarks"; "it's
           regrettable that she didn't go to college"; "it's too bad
           he had no feeling himself for church" [syn:
           regrettable, too bad]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "regrettable":
   abominable, affecting, afflictive, arrant, atrocious, awful, base,
   beastly, beneath contempt, bitter, blameworthy, bleak, brutal,
   calamitous, cheerless, comfortless, contemptible, deplorable,
   depressing, depressive, despicable, detestable, dire,
   discomforting, disgusting, dismal, dismaying, distressful,
   distressing, dolorific, dolorogenic, dolorous, dreadful, dreary,
   egregious, enormous, execrable, fetid, filthy, flagrant, foul,
   fulsome, grievous, gross, hateful, heartbreaking, heinous,
   horrible, horrid, infamous, joyless, lamentable, loathsome, lousy,
   monstrous, mournful, moving, nasty, nefarious, noisome, notorious,
   obnoxious, odious, offensive, outrageous, painful, pathetic,
   piteous, pitiable, pitiful, poignant, rank, reprehensible,
   repulsive, rotten, rough, rueful, sad, saddening, scandalous,
   schlock, scurvy, shabby, shameful, shaming, sharp, shocking,
   shoddy, sordid, sore, sorrowful, squalid, terrible, too bad,
   touching, tough, unclean, uncomfortable, unfortunate, unhappy,
   unlucky, upsetting, vile, villainous, woebegone, woeful, worst,
   worthless, wretched, wrong