Email Local Part Only Regex for Java

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching email local part only, ported and verified for Java. 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 Java project — whether you're validating in a Spring Boot controller, a Jakarta EE service, or a standalone utility class.

Java Implementation

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

import java.util.regex.Pattern;

public class EmailLocalPartOnlyValidator {
    private static final Pattern PATTERN =
        Pattern.compile("^(?![^@]*\\.\\.)[a-zA-Z0-9][a-zA-Z0-9._%+\\-]{0,62}[a-zA-Z0-9]$|^[a-zA-Z0-9]$");

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

    // Example
    public static void main(String[] args) {
        System.out.println(validate("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 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

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.