Regulators are expected to put in place in the coming days new rules aimed at preventing a reoccurrence of last month's stunning stock market "flash crash."

The Securities and Exchange Commission is expected to approve the rules calling for U.S. stock exchanges to briefly halt trading of some stocks that mark big swings. The rules could take effect at different times for various exchanges, starting a few days after the SEC approval, says a person familiar with the matter who wasn't authorized to speak publicly.

There would be a six-month trial period for the new "circuit breakers," which were worked out by the SEC and the major exchanges following the May 6 market plunge when the Dow Jones industrials lost nearly 1,000 points in less than a half-hour.