Competitive markets will always tend toward monopoly because that is the state at which the highest profits can be extracted. Regulation can, in some cases, help push it a bit faster in that direction by making barriers to enter the market even more onerous and expensive thus limiting or even eliminating new competitors. This allows the existing large competitors to either continue to knock out or buy up smaller ones, and the few remaining large ones to either merge to a single entity or settle into a state of non-competitive action (effectively collusion) where the market then either is or behaves as a monopoly.
It depends what you mean by regulation, right?
Many industries have extremely onerous and costly regulation which creates barriers for new entrants to those markets.
I'm not aware of how regulation of competitive markets would lead to monopoly. Are you referring to "regulation" in the form of state-endorsed/controlled monopolies? Those usually exist in situations where barriers to entry and economies of scale are so high that the state is simply "getting ahead" of the inevitable monopoly (e.g. national infrastructure).
Of course, corruption could also be what you mean by "regulation" that leads to monopolies, but all of this is predicate on the absence of corruption or cartels. If that's not the case you can throw all your assumption out of the window anyway.