About regular expressions:
A regular expression is a logical formula that operates on strings (including ordinary characters (for example, letters between a and z) and special characters (called "metacharacters")), that is, using some pre-defined specific characters , and the combination of these specific characters to form a "rule string", this "rule string" is used to express a filtering logic for strings. A regular expression is a text pattern that describes one or more strings to match when searching for text.
First, the declaration of regular expressions
1. Literal declaration:
/regex rules/match patterns
例：var reg = /^abc$/i;
2. New keyword declaration:
var reg = new RegExp("^abc$",i);
Second, the description of regular expression rules
Rule description:
| symbol | describe |
| /.../ | Represents the beginning and end of a pattern |
| ^ | matches the beginning of the string |
| $ | Matches the end of the string |
| \s | any whitespace characters |
| \S | any non-whitespace character |
| \d | Match a digit character, equivalent to [0-9] |
| \D | Any character except numbers, equivalent to [^0-9] |
| \w | Matches a digit, underscore or alphabetic character, equivalent to [A-Za-z0-9] |
| \W | Any non-word character, equivalent to [^A-Za-z0-9_] |
| . | any character except newline |
Quantifier description:
| symbol | describe |
| {n} | matches the previous item n times |
| {n,} | Match the previous item n times, or more times |
| {n,m} | Match the previous item at least n times, but not more than m times |
| * | Matches the previous item 0 or more times, equivalent to {0,} |
| + | Matches the previous item 1 or more times, equivalent to {1,} |
| ? | Matches the previous item 0 or 1 times, that is, the previous item is optional |
symbol:
| symbol | describe |
| | | Match any one of the selected characters, such as p|q, can match q or p |
| () | grouping |
| [] | data to match |
| ^ | Negate |
3. Common patterns of regular expressions
1. g means global match . Without g, it means that only the first string that meets the requirements is matched;
"aaa".replace(/a/,"*"); --> "*aa"
"aaa".replace(/a/g,"*"); --> "***"
2、i 表示忽略大小写匹配，默认要求大小写也必须符合正则要求。
"aAa".replace(/A/,"#"); --> "a#a"
"aAa".replace(/A/i,"#"); --> "#Aa"
"aAa".replace(/A/gi,"#"); --> "###"
3、m 表示多行匹配模式：
如果不带m，表示一个字符串只有一个开头一个结尾；
如果带m，那么对于多行字符串，可以有多个开头多个结尾。
例一:
var str="This is an\n antzone good";
var reg=/an$/;
console.log(str.match(reg));
不能够匹配字符串"an"，尽管"an"后面已经换行了，但是并没有采用多行匹配，所以不是字符串行的结尾。
例二:
var str="This is an\n antzone good";
var reg=/an$/m;
console.log(str.match(reg));
可以匹配字符串"an"，因为采用了多行匹配。
例三：
var reg = /^b/;
var str = 'test\nbbs';
execReg(reg,str);
匹配失败，因为字符串的开头没有b字符。但是加上m修饰符之后：
例四：
var reg = /^b/m;
var str = 'test\nbbs';
execReg(reg,str);
匹配到b，因为加了m修饰符之后，^已经表示行首，由于bbs在字符串第二行的行首，所以可以成功地匹配。
[多行字符串]
① 字符串中用\n表示换行："abc\nabc".replace(/^a/gm,"*");
② ES6中可以用反引号``表示字符串，这种字符串，支持直接换行。
4、检测方法
（1）reg.test(str)：检测一个字符串是否符合正则要求，返回true或false。
（2）reg.exec(str)：检测一个字符串是否符合正则要求，符合返回数组，不符合返回null。
返回数组的格式：
① index属性： 表示字符串中，第几个字符，开始匹配正则。
② input属性： 表示完整的被检索字符串。
③ 下标第0个： 表示 符合正则要求的 字符串子串。
④ 下标从1往后： 表示 匹配正则中()包裹的字符串子串。也就是说，正则中有几个()， 返回的数组中就有几个下标。
例：
/12(3)(\d)56/.exec("aaa123056bbb");
[
0:"123056",
1:"3",
2:"0",
index:3,
input:"aaa123056bbb",
length:3
]
四、正则表达式常用验证练习
1、验证邮箱
var reg = /^[a-zA-Z0-9]+@[a-zA-Z0-9]+\.[a-zA-Z]+(\.[a-zA-Z]{2,3})?$/;
var mail = "[email protected]";
console.log(reg.test(mail));
2、验证邮政编码（6位）
var reg = /^\d{5}$/;
var mailno = "251400";
console.log(reg.test(mailno));
3、验证手机号（第一位为1，第二位为3/5/7/8，共11位）
var reg = /^1(3|5|7|8)\d{9}$/;
var phone = "17862009622";
console.log(reg.test(phone));
4、验证年龄（1~120）
var reg = /^(\d|[1-9]\d|1[01]\d|120)$/;
// var reg = /^((1[0-1]|[1-9])?\d|120)$/;
var age = "121";
console.log(reg.test(age));