Accept the IPv6 tag of a domain literal in any case

CHANGELOG.md

@@ -4,6 +4,7 @@ In Development
 * Python 3.8 and 3.9 are no longer supported.
 * The `original` field of the returned validated email object incorrectly un-escaped the local part of the email address. Now it returns the original address, without any display name, as passed to validate_email.
 * add support python3.15
+* The IPv6 tag of a domain literal is now case-insensitive, as in the RFCs, so `me@[ipv6:::1]` is accepted (and normalized to `me@[IPv6:::1]`) when `allow_domain_literal=True`.
 
 2.3.0 (August 26, 2025)
 -----------------------

email_validator/syntax.py

@@ -784,8 +784,9 @@ def validate_email_domain_literal(domain_literal: str) -> DomainLiteralValidatio
             "domain": f"[{addr}]",
         }
 
-    # If it begins with "IPv6:" it's an IPv6 address.
-    if domain_literal.startswith("IPv6:"):
+    # If it begins with "IPv6:" it's an IPv6 address. The tag is case-insensitive
+    # like all ABNF literal strings (RFC 5234 2.3), so "ipv6:" is accepted too.
+    if domain_literal[:5].lower() == "ipv6:":
         try:
             addr = ipaddress.IPv6Address(domain_literal[5:])
         except ValueError as e:

tests/test_syntax.py

@@ -407,6 +407,12 @@ def test_domain_literal() -> None:
     assert validated.domain == "[IPv6:::1]"
     assert repr(validated.domain_address) == "IPv6Address('::1')"
 
+    # Check that the IPv6 tag is case-insensitive and normalized.
+    for email_input in ("me@[ipv6:::1]", "me@[IPV6:::1]"):
+        validated = validate_email(email_input, allow_domain_literal=True)
+        assert validated.domain == "[IPv6:::1]"
+        assert repr(validated.domain_address) == "IPv6Address('::1')"
+
     # Check that an IPv4 address in an IPv6 address is accepted.
     validated = validate_email("me@[IPv6:::ffff:1.2.3.4]", allow_domain_literal=True)
     assert validated.domain == "[IPv6:::ffff:1.2.3.4]"