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's a clean, production-ready Python function using a practical regular expression:

```python
import re

def is_valid_email(email: str) -> bool:
    """
    Validates whether a string is a syntactically valid email address.
    Returns True if valid, False otherwise.
    """
    if not isinstance(email, str):
        return False

    # Practical regex for email validation
    # Local part: alphanumeric + ._%+-
    # Domain: alphanumeric + hyphens, dot-separated labels, ending with a 2+ char TLD
    pattern = r'^[a-z0-9._%+-]+@[a-z0-9-]+(?:\.[a-z0-9-]+)*\.[a-z]{2,}$'
    
    return bool(re.fullmatch(pattern, email, re.IGNORECASE))
```

### 🔍 How the Regex Works:
| Part | Meaning |
|------|---------|
| `^` | Start of string |
| `[a-z0-9._%+-]+` | **Local part**: 1+ allowed characters (letters, numbers, dots, underscores, percent, plus, hyphens) |
| `@` | Literal `@` symbol |
| `[a-z0-9-]+` | **Domain start**: 1+ alphanumeric or hyphens (prevents leading dots) |
| `(?:\.[a-z0-9-]+)*` | **Subdomains**: Optional dot-separated labels (prevents consecutive dots) |
| `\.[a-z]{2,}$` | **TLD**: Ends with a dot + 2+ letters (e.g., `.com`, `.co.uk`) |
| `re.IGNORECASE` | Makes the pattern case-insensitive |

### ✅ Example Usage:
```python
test_emails = [
    "user@example.com",          # True
    "first.last+tag@domain.co.uk", # True
    "invalid@.com",              # False (leading dot in domain)
    "user@domain..com",          # False (consecutive dots)
    "@domain.com",               # False (missing local part)
    "user@domain",               # False (missing TLD)
    "user name@domain.com",      # False (contains space)
]

for email in test_emails:
    print(f"{email:<30} -> {is_valid_email(email)}")
```

### ⚠️ Important Note:
Email syntax is defined by [RFC 5322](https://datatracker.ietf.org/doc/html/rfc5322), which is extremely complex (allowing quoted strings, IP literals, etc.). This regex covers **99% of real-world cases** while remaining readable and performant. For strict RFC compliance or production systems, consider using the dedicated [`email-validator`](https://pypi.org/project/email-validator/) package instead of regex.