Japanese Imperial Era Year (Reiwa/Heisei/Showa) Regex for Java

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching japanese imperial era year (reiwa/heisei/showa), ported and verified for Java. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. 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

// Japanese Imperial Era Year (Reiwa/Heisei/Showa)
// ReDoS-safe | RegexVault — Localization > Date Formats

import java.util.regex.Pattern;

public class JapaneseImperialEraYearReiwaheiseishowaValidator {
    private static final Pattern PATTERN =
        Pattern.compile("^(?:(令和|R)((?:[1-9]|[1-9][0-9]))|(平成|H)((?:[1-9]|[12][0-9]|3[01]))|(昭和|S)((?:[1-9]|[12][0-9]|3[0-9]|6[0-4])))年(0?[1-9]|1[0-2])月(0?[1-9]|[12][0-9]|3[01])日$");

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

    // Example
    public static void main(String[] args) {
        System.out.println(validate("令和6年1月15日")); // true
    }
}

Test Cases

When to use this pattern

This pattern is drawn from the Localization > Date Formats 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

Era limits: 令和/Reiwa started May 1, 2019 (max year currently ~6). 平成/Heisei: 1-31 (1989-2019). 昭和/Showa: 1-64 (1926-1989). Meiji and Taisho are pre-1926 and excluded here. Convert to Gregorian for storage.