The Sensex today opened over 124 points up while the wide-based Nifty regained the 5,100-points mark on heavy buying, particularly in blue-chip stocks. The Sensex, which had ended lower by 28 points yesterday, gained 124 points or 0.73 per cent to 17,096 points. The National Stock Exchange index Nifty regained the 5,100-points level by rising 34 points to trade at 5,114 points.