JS uses regular rules to determine date of birth

This time I will bring you JS's use of regular expressions to determine the date of birth. What are the precautions for using JS's regular expressions to determine the date of birth? The following is a practical case, let's take a look.

In brief

In form validation, regular expressions are often used to verify birth date. This article divides the date of birth into several parts and introduces step by step the complete process of implementing a date of birth verification. I believe that after understanding the content of this article, you will have a deeper understanding and stronger confidence in writing regular expressions.

We divide a birth date in the form of 2018-06-15 into three components: year, month and date, and write the corresponding regular rules respectively.

1 Year regular expression

First give the rule definition of year regular expression:

• The year consists of 4 digits

• Only accepts years starting with 19 and 20

According to the above rules, it is easy to write the regular expression of the year Formula:

var pattern = /^(19|20)\d{2}$/;
//输出 true
console.log(pattern.test("2008"));

where / / the two slashes and the characters between them are the definition of the regular expression literal; ^ means matching the beginning of the string , $ means matching the string End; ^(19|20) means matching a string starting with 19 or 20, a pair of parentheses is to combine several items into a unit; and \d{2} means matching any ASCII number twice, \d, etc. The value is [0-9], and {2} means matching the previous item 2 times.

The above regular expression can match the years from 1900 to 2099. If you want to limit the range of years, add the following rules:

• The year starts in 1920

• The year ends in 2018

According to the above rules, change the regular expression as follows:

var pattern =/^(19[2-9]\d{1})|(20((0[0-9])|(1[0-8])))$/;
//输出 false
console.log(pattern.test("1916"));
//输出 true
console.log(pattern.test("2008"));
//输出 false
console.log(pattern.test("2022"));

2 Month regular expression

First give the rule definition of the month regular expression:

• The month can be 1-12

• If the month is 1-9, you can add 0

Based on the above rules, the following regular rules and simple tests are given:

var pattern = /^((0?[1-9])|(1[0-2]))$/;
//输出 false
console.log(pattern.test("19"));
//输出 true
console.log(pattern.test("02"));
//输出 true
console.log(pattern.test("2"));
//输出 true
console.log(pattern.test("11"));

3 Date regular expression

First give the rule definition of date regular expression:

• The date can be 1-31

• If the date is 1-9, you can add 0 in front

Based on the above rules, the following regular rules and simple tests are given:

var pattern = /^((0?[1-9])|([1-2][0-9])|30|31)$/;
//输出 false
console.log(pattern.test("32"));
//输出 true
console.log(pattern.test("02"));
//输出 true
console.log(pattern.test("2"));

4 Combination verification

According to the above regular expression of year, month and date, the regular expression of date of birth is formed:

var pattern = /^((19[2-9]\d{1})|(20((0[0-9])|(1[0-8]))))\-((0?[1-9])|(1[0-2]))\-((0?[1-9])|([1-2][0-9])|30|31)$/;
//输出 true
console.log(pattern.test("1923-3-18"));
//输出 true
console.log(pattern.test("1923-4-31"));
//输出 true
console.log(pattern.test("1923-2-29"));
//输出 true
console.log(pattern.test("2016-2-29"));

It can be seen from the above test results that the above regular verification is not perfect, mainly due to the number of days in February, April, June, September and November.

5 Improvement

According to the question in step 4, add the limiting rules as follows:

• There is no 31st day in April, June, September and November

• February has 28 days in ordinary years

• ##February has 29 days in leap years

var checkBirth = function (val) {
  var pattern = /^((?:19[2-9]\d{1})|(?:20(?:(?:0[0-9])|(?:1[0-8]))))\-((?:0?[1-9])|(?:1[0-2]))\-((?:0?[1-9])|(?:[1-2][0-9])|30|31)$/;
  var result = val.match(pattern);
  if(result != null) {
    var iYear = parseInt(result[1]);
    var month = result[2];
    var date = result[3];
    if(/^((0?[469])|11)$/.test(month) &&　date == '31') {
      return false;
    } else if(parseInt(month) == 2){
      if((iYear % 4 ==0 && iYear % 100 != 0) || (iYear % 400 == 0)) {
        if(date == '29') {
          return true;
        }
      }
      if(parseInt(date) > 28) {
        return false;
      }
    }
    return true;
  }
  return false;
}
//输出 true
console.log(checkBirth("1923-3-18"));
//输出 false 4月份没有31日
console.log(checkBirth("1923-4-31"));
//输出 false 平年
console.log(checkBirth("1923-2-29"));
//输出 true 闰年
console.log(checkBirth("2016-2-29"));

根据上述分析与测试，我们不但实现了年月日的正则的一般判定，还实现了日期范围及2，4，6，9，11月等特殊月份天数的处理，测验结果达到了我们设定的目标。

根据上述讲解和分析，我们可以调整相应的限定规则，使其满足于特定场景下的项目需要。

延伸

根据 V2EX网友 xiangyuecn 的意见，上述checkBirth的逻辑代码太多，确实有点 low。现将上述代码更新如下：

var checkBirth = function (val) {
  var pattern = /^((19[2-9]\d{1})|(20((0[0-9])|(1[0-8]))))\-((0?[1-9])|(1[0-2]))\-((0?[1-9])|([1-2][0-9])|30|31)$/;
  if(pattern.test(val)) {
    var date = new Date(val);
    var month = val.substring(val.indexOf("-")+1,val.lastIndexOf("-"));
    return date && (date.getMonth()+1 == parseInt(month));
  }
  return false;
}

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