Federal regulators, who have vacillated for years over whether to curb short selling, are expected to approve new restrictions Wednesday on the practice of betting that a company's shares will decline, according to sources familiar with the plans. The new rules by the Securities and Exchange Commission will seek to strike a middle ground between those who argue that limits on short selling are ineffective and perhaps damaging, and those who argue that they are a necessary measure to prevent speculators from pouncing on a stock when it is quickly declining.