Create is valid email address algorithm

DIRECTORY.md

@@ -1168,6 +1168,7 @@
   * [Is Pangram](strings/is_pangram.py)
   * [Is Spain National Id](strings/is_spain_national_id.py)
   * [Is Srilankan Phone Number](strings/is_srilankan_phone_number.py)
+  * [Is Valid Email Address](strings/is_valid_email_address.py)
   * [Jaro Winkler](strings/jaro_winkler.py)
   * [Join](strings/join.py)
   * [Knuth Morris Pratt](strings/knuth_morris_pratt.py)

strings/is_valid_email_address.py

@@ -0,0 +1,117 @@
+"""
+Implements an is valid email address algorithm
+
+@ https://en.wikipedia.org/wiki/Email_address
+"""
+
+import string
+
+email_tests: tuple[tuple[str, bool], ...] = (
+    ("simple@example.com", True),
+    ("very.common@example.com", True),
+    ("disposable.style.email.with+symbol@example.com", True),
+    ("other-email-with-hyphen@and.subdomains.example.com", True),
+    ("fully-qualified-domain@example.com", True),
+    ("user.name+tag+sorting@example.com", True),
+    ("x@example.com", True),
+    ("example-indeed@strange-example.com", True),
+    ("test/test@test.com", True),
+    (
+        "123456789012345678901234567890123456789012345678901234567890123@example.com",
+        True,
+    ),
+    ("admin@mailserver1", True),
+    ("example@s.example", True),
+    ("Abc.example.com", False),
+    ("A@b@c@example.com", False),
+    ("abc@example..com", False),
+    ("a(c)d,e:f;g<h>i[j\\k]l@example.com", False),
+    (
+        "12345678901234567890123456789012345678901234567890123456789012345@example.com",
+        False,
+    ),
+    ("i.like.underscores@but_its_not_allowed_in_this_part", False),
+    ("", False),
+)
+
+# The maximum octets (one character as a standard unicode character is one byte)
+# that the local part and the domain part can have
+MAX_LOCAL_PART_OCTETS = 64
+MAX_DOMAIN_OCTETS = 255
+
+
+def is_valid_email_address(email: str) -> bool:
+    """
+    Returns True if the passed email address is valid.
+
+    The local part of the email precedes the singular @ symbol and
+    is associated with a display-name. For example, "john.smith"
+    The domain is stricter than the local part and follows the @ symbol.
+
+    Global email checks:
+     1. There can only be one @ symbol in the email address. Technically if the
+        @ symbol is quoted in the local-part, then it is valid, however this
+        implementation ignores "" for now.
+        (See https://en.wikipedia.org/wiki/Email_address#:~:text=If%20quoted,)
+     2. The local-part and the domain are limited to a certain number of octets. With
+        unicode storing a single character in one byte, each octet is equivalent to
+        a character. Hence, we can just check the length of the string.
+    Checks for the local-part:
+     3. The local-part may contain: upper and lowercase latin letters, digits 0 to 9,
+        and printable characters (!#$%&'*+-/=?^_`{|}~)
+     4. The local-part may also contain a "." in any place that is not the first or
+        last character, and may not have more than one "." consecutively.
+
+    Checks for the domain:
+     5. The domain may contain: upper and lowercase latin letters and digits 0 to 9
+     6. Hyphen "-", provided that it is not the first or last character
+     7. The domain may also contain a "." in any place that is not the first or
+        last character, and may not have more than one "." consecutively.
+
+    >>> for email, valid in email_tests:
+    ...     assert is_valid_email_address(email) == valid
+    """
+
+    # (1.) Make sure that there is only one @ symbol in the email address
+    if email.count("@") != 1:
+        return False
+
+    local_part, domain = email.split("@")
+    # (2.) Check octet length of the local part and domain
+    if len(local_part) > MAX_LOCAL_PART_OCTETS or len(domain) > MAX_DOMAIN_OCTETS:
+        return False
+
+    # (3.) Validate the characters in the local-part
+    if any(
+        char not in string.ascii_letters + string.digits + ".(!#$%&'*+-/=?^_`{|}~)"
+        for char in local_part
+    ):
+        return False
+
+    # (4.) Validate the placement of "." characters in the local-part
+    if local_part.startswith(".") or local_part.endswith(".") or ".." in local_part:
+        return False
+
+    # (5.) Validate the characters in the domain
+    if any(char not in string.ascii_letters + string.digits + ".-" for char in domain):
+        return False
+
+    # (6.) Validate the placement of "-" characters
+    if domain.startswith("-") or domain.endswith("."):
+        return False
+
+    # (7.) Validate the placement of "." characters
+    if domain.startswith(".") or domain.endswith(".") or ".." in domain:
+        return False
+    return True
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    for email, valid in email_tests:
+        is_valid = is_valid_email_address(email)
+        assert is_valid == valid, f"{email} is {is_valid}"
+        print(f"Email address {email} is {'not ' if not is_valid else ''}valid")