Is the Hadd penalty obligatory for one who committed fornication from behind clothing, then regretted it and repented?

The hadd (prescribed punishment) is not obligatory except upon penetration. And it is not a condition for the validity of repentance that the hadd be carried out. If you have committed something that necessitates the hadd, then repent between yourself and Allah, and conceal yourself with Allah's concealment, and regret your action, for regret is repentance. Be sincere in seeking refuge with Allah, and increase your istighfar (seeking forgiveness) and the performance of expiating good deeds, for "indeed, good deeds do away with evil deeds."