Python Email Slicer Project

"""Split a reasonably formed email address into useful components."""

from __future__ import annotations

import re


EMAIL_PATTERN = re.compile(
    r"^(?=.{1,254}$)(?P<username>[A-Za-z0-9.!#$%&'*+/=?^_`{|}~-]{1,64})@"
    r"(?P<domain>(?:[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?\.)+"
    r"[A-Za-z]{2,63})$"
)


def slice_email(address: str) -> dict[str, str]:
    """Return email components after practical (not deliverability) validation."""
    match = EMAIL_PATTERN.fullmatch(address.strip())
    if match is None:
        raise ValueError("Enter an address such as name@example.com.")

    username = match.group("username")
    if username.startswith(".") or username.endswith(".") or ".." in username:
        raise ValueError("Dots cannot begin, end, or repeat in the username.")

    domain = match.group("domain").lower()
    name, top_level_domain = domain.rsplit(".", 1)
    return {
        "username": username,
        "domain": domain,
        "domain_name": name,
        "top_level_domain": top_level_domain,
    }


def main() -> None:
    print("Python Email Slicer")
    address = input("Email address: ").strip()
    try:
        parts = slice_email(address)
    except ValueError as error:
        print(f"Invalid input: {error}")
        return

    print(f"Username: {parts['username']}")
    print(f"Domain: {parts['domain']}")
    print(f"Domain name: {parts['domain_name']}")
    print(f"Top-level domain: {parts['top_level_domain']}")
    print("Note: valid-looking syntax does not prove that the mailbox exists.")


if __name__ == "__main__":
    main()