Using regular expressions in js

(the figure above comes from Baidu Encyclopedia) regular expression_ Baidu Encyclopedia

  Next, let's learn about the use of regular expressions:

  1. We first create a new regular expression by literal (the format of the regular expression is / * / * which is the content of the regular expression. The content in the regular expression does not need to be quoted, whether it is numeric or string)

var rg =/123/;

    2. Next, we use the test method of regular expression to detect whether the string meets the specification required by regular expression (the return value of regular expression is boolean type)

console.log(rg.test(123)); console.log(rg.test('abc'));

  The following figure shows the results printed in the console

  3. Next, we introduce the boundary operators in regular expressions

  The following figure shows the regular expression without boundary and the results printed in the console

var rg = /abc/; console.log(rg.test('abc')); console.log(rg.test('abcd')); console.log(rg.test('aabcd'));

The following figure shows the regular expression with only the start boundary character and the result printed in the console (the symbol bit of the start boundary character ^)

var reg = /^abc/; console.log(reg.test('abc')); console.log(reg.test('abcd')); console.log(reg.test('aabcd'));

  The following figure shows the regular expression with start boundary symbol and end boundary symbol and the result printed in the console (the symbol of end boundary symbol is $)

var reg1 =/^abc$/; console.log(reg1.test('abc')); console.log(reg1.test('abcd')); console.log(reg1.test('aabcd')); console.log(reg1.test('abcabc'));

  4. Next, we introduce the character set in regular expressions

  [] indicates that there are a series of characters to choose from. Just match one of them

var rg = /[abc]/; // It returns true as long as it contains a, b or c console.log(rg.test('andy')); //true console.log(rg.test('bady')); //true console.log(rg.test('color')); //true console.log(rg.test('red')); //false

Boundary characters need to be written outside the character set

var rg1 =/^[abc]$/;//Choose one from three. Only the letters a, b or c return true console.log(rg1.test('aa')); //false console.log(rg1.test('a')); //true console.log(rg1.test('b')); //true console.log(rg1.test('c')); //true console.log(rg1.test('abc')); //false

var reg = /^[a-z]$/; //26 English letters. Any letter returns true - indicating the range from a to z console.log(reg.test('a')); //true console.log(reg.test('z')); //true console.log(reg.test('1')); //false console.log(reg.test('A')); //false

Character combinations are described below

var reg1 =/^[a-zA-Z0-9_-]$/; //26 English letters (both uppercase and lowercase) any letter returns true //Any number from 0 to 9 returns true - indicating the range of a-z and the range of 0-9 console.log(reg1.test('a')); console.log(reg1.test('B')); console.log(reg1.test(9)); console.log(reg1.test('_')); console.log(reg1.test('-')); console.log(reg1.test('!'));

 ^ When a symbol is written into a character set, it means reverse, which means that all symbols except uppercase and lowercase letters, numbers from 0 to 9 and underlined lines return true

var reg2 = /^[^a-zA-Z0-9_-]$/; console.log(reg2.test('a')); console.log(reg2.test('B')); console.log(reg2.test(9)); console.log(reg2.test('_')); console.log(reg2.test('-')); console.log(reg2.test('!'));

  5. The next two introduce quantifiers in regular expressions

//Quantifier: used to set the number of occurrences of a pattern //Simple understanding: how many times does the following character a repeat var reg =/^a$/; // *Equivalent to > = 0, it can occur 0 or many times var reg =/^a*$/; console.log(reg.test('a')); //true console.log(reg.test('')); //true console.log(reg.test('aaaa')); //true // +Equivalent to > = 1, it can occur once or many times var reg =/^a+$/; console.log(reg.test('a')); //true console.log(reg.test('')); //false console.log(reg.test('aaaa')); //true // ? Equivalent to 1 | 0 can only occur once or 0 times var reg =/^a?$/; console.log(reg.test('a')); //true console.log(reg.test('')); //true console.log(reg.test('aaaa')); //false // Just repeat it three times var reg =/^a$/; console.log(reg.test('a')); //false console.log(reg.test('')); //false console.log(reg.test('aaaa')); //false console.log(reg.test('aaa'));//true // is greater than or equal to 3 var reg=/^a$/; console.log(reg.test('a')); //false console.log(reg.test('')); //false console.log(reg.test('aaaa')); //true console.log(reg.test('aaa'));//true // greater than or equal to 3 and less than or equal to 16 var reg =/^a$/; console.log(reg.test('a')); //false console.log(reg.test('')); //false console.log(reg.test('aaaa')); //true console.log(reg.test('aaa'));//true console.log(reg.test('aaaaaaa'));//false

  The above is a brief introduction to regular expressions  

30 November 2021, 16:53 | Views: 8837