Dic:rein in// rein vs. reign.

─. Reins are the thin leather straps attached round a horse's neck which are used to control the horse. (고삐)

─. To rein in something means to control it.

• His administration's economic policy would focus on reining in inflation.
• Mary spoiled both her children, then tried too late to rein them in. 

─. Reign is sometimes wrongly written for rein in certain phrases:

• He gave full rein (not reign) to his feelings.
• It will be necessary to rein in (not reign in) public spending.

.... Cobuild, Collins

• It's the job of government regulators to reign in [rein in] the power of the all-powerful market maker in many two-sided markets. (Ray Fisman and Tim Sullivan, The Inner Lives of Markets, p.124)