Can I use regular rain pants instead of cycling-specific overpants?

You can, but you'll notice significant differences. Regular rain pants lack articulated knees (causing fabric bunching behind the knee on every pedal stroke), don't have cycling-specific ankle closures to prevent chainring contact, and often have seams in places that chafe in the riding position. Cycling overpants are cut for the forward-leaning riding position and include features like reflective elements, saddle-compatible crotch construction, and pedal-friendly ankle design. For occasional use, regular rain pants work; for regular riding, cycling-specific overpants are worth the investment.