YAML Key (Unquoted Scalar) Regex for PHP

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching yaml key (unquoted scalar), ported and verified for PHP. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your PHP project — whether you're validating in a Laravel validator, a WordPress plugin, or a standalone PHP script.

Php Implementation

<?php
// YAML Key (Unquoted Scalar)
// ReDoS-safe | RegexVault — Dev & Systems > Config

define('YAML_KEY_UNQUOTED_SCALAR_PATTERN', '/^[a-zA-Z][a-zA-Z0-9_\-.]{0,127}(?=\s*:)/');

function validate_yaml_key_unquoted_scalar(string $input): bool {
    return (bool) preg_match(YAML_KEY_UNQUOTED_SCALAR_PATTERN, $input);
}

// Example
var_dump(validate_yaml_key_unquoted_scalar("key: value")); // bool(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 PHP developers because especially relevant in PHP where PCRE backtracking limits can trigger silent failures on malicious input. 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.