Is it obligatory to repeat the prayer upon feeling something at the anus without being certain of the emission of gas, knowing that one was exposed to gas pressure before that? And is the principle "certainty is not removed by doubt" used for obsessive thoughts in all matters of Islamic law, or is it specific to purity and prayer?

Prayer is not valid when one is suppressing the urge to relieve oneself of the two impurities (urine and feces), due to the Prophet's (peace and blessings be upon him) saying: "There is no prayer in the presence of food, nor when one is suppressing the urge to relieve oneself of the two impurities." If a worshipper begins prayer while suppressing the urge to relieve himself of the two impurities, his prayer is valid but incomplete. However, if a worshipper feels the urge to relieve himself of the two impurities during prayer, his prayer is valid and there is no dislikeability in it, as long as this urge does not prevent him from completing the prayer. As for one who doubts whether wind has been released from him, his ablution is not invalidated by this mere doubt, and he should continue his prayer unless he is certain that an impurity has been released. The jurisprudential rule is that certainty is not removed by doubt.