Is the prayer of one who wore torn trousers between the thighs, such that some of his inner shorts are visible, valid, or must it be repeated?

Covering what is between the navel and the knees is obligatory for men and a condition for the validity of prayer. If a part of the 'awrah (private parts) is uncovered unintentionally for a brief period, the prayer is valid, provided one hastens to cover it. However, if the uncovering was deliberate or prolonged, the prayer becomes invalid. If underwear prevents the 'awrah from being exposed, the prayer is valid.