Snapping three days of gains, the Sensex at the Bombay Stock Exchange lost 120 points, or 0.7 percent, to 17,158, on reports that the Reserve Bank plans to tighten regulations for loan restructuring, and amid weak Asian and European markets, today. The Sensex had gained 176 points in the last three trading sessions. The Nifty at the National Stock Exchange fell 38 points, or 0.7 percent, to 5,205. Stock markets in Japan, China, and Singapore lost between 0.4 percent and 1.4 percent, on weak US economic data.