import re


test_string = 'go'   #Initialize flag to exit loop
while test_string != 'stop':

#	Get an input phone number: nnn-nnnn
	test_string = input('Enter a 10-digit phone number in the format xxx-xxx-xxxx: ')
	
	#Perform comparison
	if re.match(r'^\d{3}-\d{3}-\d{4}$', test_string):
		print (test_string, 'is a valid US phone number')
	else:
		if test_string != 'stop':
			print (test_string, 'Rejected. Invalid phone number.')