Inspects lines that contain only a character, let's say c, repeated n times. The name of the matching groups will tell you if n is 0, the unit 1, a prime number, or a composite number.
Removing the outermost anchors ^ and $ from the first (and very large) alternate group will do the same but, instead of inspecting per line, it will inspect any repetition of a character (line breaks, obviously, stop repetitions).
^(?: ...... )$|(?<Zero>^$)
to
(?: ...... )|(?<Zero>^$)