Is a circle a regular polygon?

No, a circle is not a regular polygon because it is not even a polygon.
A polygon must have at least 3 sides and 3 angles (a circle has 1 side and has 0 angles), it has to be a closed shape, and it cannot have curved lines (circles are curved). Therefore, a circle is not a polygon. You *might* think that a circle is just made up of a gazillion squares, but no. It's simply just a curved shape.