https://medium.com/@yeukhon/non-capturing-group-in-pythons-regular-expression-75c4a828a9eb

re_problem.py

import re
text1 = "alphanumeric@example.org"
text2 = "alphanumeric@long.subdomain.more.subdomain.domain.org"

r1 = re.compile(r'[a-zA-Z0-9]+@[a-zA-Z]+\.[a-zA-Z]+(\.[a-zA-Z]+)*')

print(r1.match(text1).group())
print(r1.match(text2).group())

print("part 2 - compare with and without (?:")
r2 = re.compile(r'[a-zA-Z0-9]+@[a-zA-Z]+\.[a-zA-Z]+(?:\.[a-zA-Z]+)*')

print(r1.findall(text2))
print(r2.findall(text2))

print("part 3 - udacity assignment")

def addresses(haystack): 
    emails = re.findall(r'\w+@[a-zA-Z]+\.[a-zA-Z]+(?:\.[a-zA-Z]+)*', haystack)
    return [re.sub("NOSPAM", "", email) for email in emails]

input1 = """louiseNOSPAMaston@germany.de (1814-1871) was an advocate for
democracy. irmgardNOSPAMkeun@NOSPAMweimar.NOSPAMde (1905-1982) wrote about
the early nazi era. rahelNOSPAMvarnhagen@berlin.de was honored with a 1994
deutsche bundespost stamp. seti@home is not actually an email address."""

output1 = ['louiseaston@germany.de', 'irmgardkeun@weimar.de', 'rahelvarnhagen@berlin.de']

print addresses(input1) == output1