100x100 - OK
100%x100 - OK
99x99 - OK
99%x100 - not OK
99x99% - not OK

s = '100%x99%' # this should be failed
match = re.match(r'(100%|d+(?!%))x(100%|d+(?!%))', s)
if match:
  x = match.group(1)
  y = match.group(2)

re.match(r'(100%|d+(?!%))x(100%|d+(?!%))$', s)

(100%|d+)x(100%|(d+(?!%|d)))