Email Local Part Only Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching email local part only, ported and verified for Python. Identity and credential patterns need both correctness and safety, since they're frequent targets for adversarial input. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# Email Local Part Only
# ReDoS-safe | RegexVault — Identity & PII > Email Address

import re

email_local_part_only_pattern = re.compile(r'^(?![^@]*\.\.)[a-zA-Z0-9][a-zA-Z0-9._%+\-]{0,62}[a-zA-Z0-9]$|^[a-zA-Z0-9]$')

def validate_email_local_part_only(value: str) -> bool:
    return bool(email_local_part_only_pattern.fullmatch(value))

# Example
print(validate_email_local_part_only("user"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Identity & PII > Email Address category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Local part max length is 64 chars per RFC 5321. Cannot start or end with a dot. Consecutive dots are prohibited in practice (though technically allowed in quoted strings). Common limit is 64 characters total.