YAML Key (Unquoted Scalar) Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching yaml key (unquoted scalar), ported and verified for Python. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. 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

# YAML Key (Unquoted Scalar)
# ReDoS-safe | RegexVault — Dev & Systems > Config

import re

yaml_key_unquoted_scalar_pattern = re.compile(r'^[a-zA-Z][a-zA-Z0-9_\-.]{0,127}(?=\s*:)')

def validate_yaml_key_unquoted_scalar(value: str) -> bool:
    return bool(yaml_key_unquoted_scalar_pattern.fullmatch(value))

# Example
print(validate_yaml_key_unquoted_scalar("key: value"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Dev & Systems > Config 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

The lookahead (?=\s*:) ensures only the key is matched. YAML keys can technically be any node, but this covers the common identifier case. Use a YAML parser for full key extraction.