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How to Validate an Email Address

The following function checks whether a string represents a well-formed email address. It is typically useful in registration forms to validate information entered by the user.

The code was taken and briefly modified from the web2py source code (gluon/validators.py), which at the same time was based on the solution proposed in this article. The rules that constitute a valid email address are defined using regular expressions with the re standard module.

import re

body_regex = re.compile('''
    ^(?!\.)                            # name may not begin with a dot
    (
      [-a-z0-9!\#$%&'*+/=?^_`{|}~]     # all legal characters except dot
      |
      (?<!\.)\.                        # single dots only
    )+
    (?<!\.)$                            # name may not end with a dot
''', re.VERBOSE | re.IGNORECASE)
domain_regex = re.compile('''
    (
      localhost
      |
      (
        [a-z0-9]
            # [sub]domain begins with alphanumeric
        (
          [-\w]*                         # alphanumeric, underscore, dot, hyphen
          [a-z0-9]                       # ending alphanumeric
        )?
      \.                               # ending dot
      )+
      [a-z]{2,}                        # TLD alpha-only
   )$
''', re.VERBOSE | re.IGNORECASE)

def is_valid_email(email):
    if not isinstance(email, str) or not email or '@' not in email:
        return False
    
    body, domain = email.rsplit('@', 1)

    match_body = body_regex.match(body)
    match_domain = domain_regex.match(domain)

    if not match_domain:
        # check for Internationalized Domain Names
        # see https://docs.python.org/2/library/codecs.html#module-encodings.idna
        try:
            domain_encoded = domain.encode('idna').decode('ascii')
        except UnicodeError:
            return False
        match_domain = domain_regex.match(domain_encoded)

    return (match_body is not None) and (match_domain is not None)

Examples:

print(is_valid_email('a@b.com'))  # True.
print(is_valid_email('abc@def.com'))  # True.
print(is_valid_email('abc@3def.com'))  # True.
print(is_valid_email('abc@def.us'))  # True.
print(is_valid_email('abc@d_-f.us'))  # True.
print(is_valid_email('@def.com'))  # False.
print(is_valid_email('"abc@def".com'))  # False.
print(is_valid_email('abc+def.com'))  # False.
print(is_valid_email('abc@def.x'))  # False.
print(is_valid_email('abc@def.12'))  # False.
print(is_valid_email('abc@def..com'))  # False.

re email