Erasing all its early gains, the Sensex at the Bombay Stock Exchange declined 90 points, or 0.5 percent, to close at a fresh one-month low of 19,429, amid falling regional markets, on Friday. The Nifty at the National Stock Exchange fell 40 points, or 0.7 percent, to 5,881.Stock markets in Japan, China, Hong Kong, South Korea and Singapore declined between 0.2 percent and 1.8 percent.