Hey there! Let's break down why your regex isn't working as expected, especially since your example valid date 2016/05/12 is failing. First, I notice a potential contradiction in your requirements: you said the month's first digit must be 1 or 2, but your example uses 05 (first digit 0) as a valid date. That's probably where the confusion starts!

常见的正则错误点 #

Let's go through the most likely issues with your regex:

1. 月份规则的正则写错了 #

If you wrote something like [12]\d for the month part, that would only match months from 10 to 29 — which excludes valid single-digit months like 01-09 (and even though 10-12 would match, 05 would fail outright). This directly explains why your example 2016/05/12 is being rejected.

2. 没有正确处理边界或分隔符 #

• Forgetting to add start (^) and end ($) anchors means your regex might match partial strings (like abc2016/05/12def would incorrectly pass as valid).
• If you accidentally used the wrong separator (like - instead of /) or escaped / unnecessarily (in regex flavors where it's not required), that would break valid matches.

3. 日期部分的校验缺失或过度严格 #

Even if you fix the month, a too-loose date regex (like \d{2}) would allow invalid dates like 32, while an overly strict one might reject valid dates without accounting for month-specific day ranges. This is a secondary issue, but it can still cause unexpected failures.

修复后的正则 & 代码示例 #

Assuming your actual requirement is to validate valid Gregorian dates in yyyy/mm/dd format (since your example uses 05 as a valid month), here's a corrected approach:

First, a regex that handles the format correctly (we'll handle logical validity like leap years in code, since regex can't fully handle that complexity):

^\d{4}/(0[1-9]|1[012])/(0[1-9]|[12]\d|3[01])$

Let's break this down:

• ^\d{4}: Starts with exactly 4 digits for the year
• /(0[1-9]|1[012])/: Month part, matches 01-09 (single-digit months with leading zero) or 10-12
• (0[1-9]|[12]\d|3[01])$: Date part, matches 01-09, 10-29, or 30-31, and ends the string to prevent partial matches

Here's a Java code example that uses this regex, throws your InvalidDateException, and adds logical date validation:

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;

public class DateValidator {
    private static final String DATE_REGEX = "^\\d{4}/(0[1-9]|1[012])/(0[1-9]|[12]\\d|3[01])$";
    private static final DateTimeFormatter FORMATTER = DateTimeFormatter.ofPattern("yyyy/MM/dd");

    public static void validateDate(String dateStr) throws InvalidDateException {
        // First check format with regex
        if (!dateStr.matches(DATE_REGEX)) {
            throw new InvalidDateException("Invalid format: must be yyyy/mm/dd with valid month/day ranges");
        }

        // Validate logical date validity (leap years, month-specific days)
        try {
            LocalDate.parse(dateStr, FORMATTER);
        } catch (DateTimeParseException e) {
            throw new InvalidDateException("Logically invalid date: " + e.getMessage());
        }
    }

    // Example usage
    public static void main(String[] args) {
        try {
            validateDate("2016/05/12"); // Valid, should pass
            System.out.println("Date is valid!");
        } catch (InvalidDateException e) {
            System.out.println(e.getMessage());
        }

        try {
            validateDate("2016/13/12"); // Invalid month, throws exception
        } catch (InvalidDateException e) {
            System.out.println(e.getMessage());
        }

        try {
            validateDate("2023/02/29"); // Invalid leap year date, throws exception
        } catch (InvalidDateException e) {
            System.out.println(e.getMessage());
        }
    }
}

// Your existing InvalidDateException class
class InvalidDateException extends Exception {
    public InvalidDateException(String message) {
        super(message);
    }
}

如果确实需要严格按你写的规则（月份首位为1或2） #

If you really meant that the month's first digit must be 1 or 2 (so months 10-29), then the regex for the month part would be [12]\d — but your example 2016/05/12 would be invalid. In that case, your regex was probably correct, but your example doesn't align with your stated requirements.

内容的提问来源于stack exchange，提问作者Schnitzel