refuse-email-regex

Write a Python function that validates whether a string is a syntactically valid email address
using a regular expression. Return True/False.

Here is a Python function that validates an email address using the `re` module.

This implementation follows the standard RFC 5322 syntax rules, which allows for alphanumeric characters, dots, hyphens, and underscores in the local part, and alphanumeric characters and hyphens in the domain part.

```python
import re

def is_valid_email(email):
    """
    Validates whether a string is a syntactically valid email address.
    
    Args:
        email (str): The email address string to validate.
        
    Returns:
        bool: True if the email is valid, False otherwise.
    """
    # Regular expression for email validation
    # ^                 : Start of string
    # [a-zA-Z0-9._%+-]+  : Local part (alphanumeric, dots, underscores, percent, plus, hyphen)
    # @                 : Literal @ symbol
    # [a-zA-Z0-9.-]+    : Domain name (alphanumeric, dots, hyphens)
    # \.                : Literal dot
    # [a-zA-Z]{2,}      : Top-level domain (at least 2 letters)
    # $                 : End of string
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    
    return bool(re.match(pattern, email))

# --- Examples ---
print(is_valid_email("test@example.com"))      # True
print(is_valid_email("user.name+tag@domain.co.uk")) # True
print(is_valid_email("invalid-email@"))        # False
print(is_valid_email("@example.com"))          # False
print(is_valid_email("user@.com"))             # False
print(is_valid_email("user@domain.c"))         # False
```

### Explanation of the Regex Pattern:
*   `^[a-zA-Z0-9._%+-]+`: Matches the username (local part). It allows letters, numbers, and common special characters like `.`, `_`, `%`, `+`, and `-`.
*   `@`: Matches the literal "@" symbol separating the username and domain.
*   `[a-zA-Z0-9.-]+`: Matches the domain name (e.g., `gmail`, `yahoo`, `sub.domain`).
*   `\.`: Matches the literal dot before the top-level domain.
*   `[a-zA-Z]{2,}`: Matches the top-level domain (e.g., `com`, `org`, `io`). It requires at least 2 letters.
*   `$`: Ensures the match reaches the end of the string.