Both anti-trust policy and economic regulation deal with monopoly. What distinguishes the two approaches? How does the government decide to use one

Both anti-trust policy and economic regulation deal with monopoly. What distinguishes the two approaches? How does the government decide to use one form of remedy rather than the other?I understand:An anti-trust policy reflects the government's attempt to reduce anticompetitive behavior and promote a market structure that leads to greater competition and also attempts to promote socially desirable market performance.Anti-trust policies tries to promote competition in those markets where competition seems desirable.I guess I'm not correlating what the "distinguishing approach" or how the government decides to use one or the other?