feat(strings): Create is_valid_email_address algorithm

strings/is_valid_email_address.py

@@ -4,6 +4,8 @@ Implements an is valid email address algorithm
 @ https://en.wikipedia.org/wiki/Email_address
 """
 
+import re
+import string
 
 email_tests: tuple[tuple[str, bool], ...] = (
     ("simple@example.com", True),
@@ -16,16 +18,17 @@ email_tests: tuple[tuple[str, bool], ...] = (
     ("example-indeed@strange-example.com", True),
     ("test/test@test.com", True),
     (
-        "1234567890123456789012345678901234567890123456789012345678901234567890@example.com",
+        "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),
     (
-        "12345678901234567890123456789012345678901234567890123456789012345678901@example.com",
+        "12345678901234567890123456789012345678901234567890123456789012345@example.com",
         False,
     ),
     ("i.like.underscores@but_its_not_allowed_in_this_part", False),
@@ -45,19 +48,65 @@ def is_valid_email_address(email: str) -> bool:
     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) is valid
     """
 
-    # Make sure that there is only one @ symbol in the email address
+    # (1.) Make sure that there is only one @ symbol in the email address
     if email.count("@") != 1:
         return False
 
     local_part, domain = email.split("@")
-    # Check octet length of the local part and domain
+    # (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
+    if (
+        local_part.startswith(".")
+        or local_part.endswith(".")
+        or re.search(r"\.\.+", 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 re.search(r"\.\.+", domain):
+        return False
     return True
 
 
@@ -66,6 +115,7 @@ if __name__ == "__main__":
 
     doctest.testmod()
 
-    for email, _ in email_tests:
+    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 is_valid is False else ''}valid")