What are some of the chemicals like hcl that regulate digestion?

Not a simple answer: First of all, hcl doesn't regulate digestion, it just facilitates conversion of pepsinogen to pepsin. However, regulation of digestion is incredibly complex as mediated by nerve signal interplay locally within the gut, centrally with the brain, along with neurotransmitters, hormones, a multitude of chemical signaling and feedback loops that all interact with immune system, food, environment, etc.