JWT Header (Decoded Algorithm Field) Regex for Java

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching jwt header (decoded algorithm field), ported and verified for Java. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. The snippet below is ready to drop into your Java project — whether you're validating in a Spring Boot controller, a Jakarta EE service, or a standalone utility class.

Java Implementation

// JWT Header (Decoded Algorithm Field)
// ReDoS-safe | RegexVault — Security > API Keys & Tokens

import java.util.regex.Pattern;

public class JwtHeaderDecodedAlgorithmFieldValidator {
    private static final Pattern PATTERN =
        Pattern.compile("^\\{\"alg\":\"(HS256|HS384|HS512|RS256|RS384|RS512|ES256|ES384|ES512|PS256|PS384|PS512|EdDSA)\",\"typ\":\"JWT\"(?:,\"kid\":\"[^\"]{1,100}\")?\\}$");

    public static boolean validate(String input) {
        return PATTERN.matcher(input).matches();
    }

    // Example
    public static void main(String[] args) {
        System.out.println(validate("{"alg":"HS256","typ":"JWT"}")); // true
    }
}

Test Cases

When to use this pattern

This pattern is drawn from the Security > API Keys & Tokens category and carries a ReDoS-safe certification. That matters for Java developers because critical in Java applications since the JVM regex engine uses backtracking and is susceptible to ReDoS without careful pattern design. 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

Explicitly excludes 'none' and weak algorithms (HS1, MD5). Approved algorithms: HMAC-SHA (HS256/384/512), RSA PKCS#1 v1.5 (RS*), ECDSA (ES*), RSA-PSS (PS*), EdDSA. The 'kid' (key ID) claim is optional. JSON key order is enforced — real parsers should be order-agnostic.