Regular Expression to find a pangram
Assuming you're using a flavor of regex that include look-aheads, detecting a pangram is straightforward, but tedious.  Using Python syntax:

r"(?=.*a)(?=.*b)(?=.*c)(?=.*d)(?=.*e)(?=.*f)(?=.*g)(?=.*h)(?=.*i)(?=.*j)(?=.*k)(?=.*l)(?=.*m)(?=.*n)(?=.*o)(?=.*p)(?=.*q)(?=.*r)(?=.*s)(?=.*t)(?=.*u)(?=.*v)(?=.*w)(?=.*x)(?=.*y)(?=.*z).*"

https://regex101.com/r/Z7NjEQ/2

The (?=.*a) block tests if what follows after that point matches .*a; another way to say that is, is this expression followed by any characters and then an a, or just, does this include an a?  Then repeat that check for every letter of the alphabet.

Without lookaheads, it's still possible, but even more tedious, because you have to account for every possible ordering of the alphabet.  If you were limited to the letters abc, here's what it might look like:

r".*(a.*b.*c|a.*c.*b|b.*a.*c|b.*c.*a|c.*a.*b|c.*b.*a).*"

https://regex101.com/r/OnXQe7/1/

This checks for an a, any set of characters, a b, any set of characters, then a c, OR an a, any set of characters, a c, any set of characters, then a b, OR...you get the idea.

You could use the same approach to check for the standard meaning of perfect pangram (every character exactly once), but not the modified version used here.