JS regular expression basic knowledge learning

Create regular objects
1. Literal var t = /1/;
2. Instantiate objects var t = new RegExp(‘1’); // Slash no Use quotation marks
Matching mode （ Modifier ） Add... After the slash
g – The global matching
i – Ignore case
var t = /a/i; // Match uppercase A and A lowercase letter a
var t = new RegExp(‘a’,‘i’);
Common methods
1. Regular objects provide regular methods
(1) exec(str) // Returns the string matching part No match returned null
(2) test(str) // Matches the specified string return true false

2.String Object
(1)var z = ‘sssscaa555’.search(/a/); // Returns the position subscript of the first occurrence of the matched string
(2)z = ‘sssscaa555’.match(/a/g); // Return the first matching result in the form of an array If added g Return all results as an array
(3)z = ‘sssscaa555’.replace(/a/); // Replace the specified string If added g Replace all matching strings
(4)z = ‘sssscaa555’.split(‘s’); // The matched string is node Division Specified string Global mode is used by default


// Match the first a And replace with *

/g and replaceAll() : Match all a And replace with *

Commonly used search characters ( according to ASCII Code table to find )
[a-z] // matching a To z Characters between
[A-Z]
[0-9]
[a-c0-5A-G]
[adghj]

[^abc] // except abc Any character outside （ In square brackets ^ Representation inversion ）
\d // Match numeric characters
\D // Match non numeric characters
\w // Match alphanumeric underscores
\W // Match non alphanumeric underscores Equivalent to [^\w]
\s // Matching blank character
\S // Match non white space characters Equivalent to [^\s]
. // Match any character except carriage return （\n）
[\u4e00-\u9fa5] // Match any Chinese characters \u On behalf of the use of 16 Base number

subexpression
The expression in parentheses of regular expression is a subexpression /a(d)/ Used to provide values in the cache for subsequent matches
Capture
The subexpression matching content is put into the cache This process is called capture , Every time a matching character is obtained Just put it into a new cache We can label the cache （$1 $2 $3）, The reason why there is capture Because of the data in the cache Subsequent regular matching can call cached data More convenient matching .
backreferences
After the subexpression The remaining regularities of use the characters matched by the front sub expression This process is called reverse reference .
‘dasdsadsaaaa’.match(/(a)\1\1\1/); // return aaaa
‘12221221’.match(/(\d)(\d)\2\1/); // return 1221

qualifiers
(*) asterisk Match the front face expression 0 Times or times
(+) plus Match the front face expression 1 Times or times
? Match the front face expression 0 Time or 1 Time
{n} matching n Time
{n,} At least n Time
{n,m} Match at least n~m Time Between It's OK to exceed


Locator
^ Put it outside square brackets =》 The starting position of the matching string The first character of a string
$ The end of the matching string The last character of the string
\b Matches a word boundary ‘an’.match(/\ban\b/); Means to find a word ：an
\B Match non word boundaries

Special character escape
The backslash Regular expressions are also used , Therefore, if the string matches, it should be escaped
~~ This is just an example Please Baidu to match the regular website

//  Here we escape the backslash  \/\/
/^[a-zA-Z]{
    1,4}:\/\/*.[a-zA-Z]{1,6}/.test('http://www.baidu.com');

perhaps ( | ) Usage of
‘sb’.match(/\b(sb|dsb)\b/g); // Match custom string

Greedy mode （ Match multiple numbers ）
Regular default matches more Less mismatches Hardworking little bee {} The number of matching numbers in the middle Default match 9 individual

Non greedy model ( Fewer matches )
Inertia matching Give priority to those with less matching Add stay {} Back plus ? {} The number of matching numbers in the middle matching 3 individual

Pre check
The specified string has not been matched But first preview the following string
1、 Forward lookahead （ Assertion ）： Check from left to right

// Inquire about   With 136 For the beginning of the   cell-phone number 
' 13658542165 15236578954 13654741254'.match(/\b(?=136)\d+\b/g);
// Inquire about   Don't to 136 For the beginning of the   cell-phone number   hold  =  Change to  !
' 13658542165 15236578954 13654741254'.match(/\b(?!136)\d+\b/g);

2、 Reverse Preview ： Check from the back right to the left

// matching hello word   Look from right to left   It's a match olleh  After matching   Look again  h hinder 3 Then stop 
"123hello123".match(/(?<=3)hello/g);

~~ Video learning recommendations b standing 【 Black horse programmer 】
https://www.bilibili.com/video/BV1uC4y187dF?spm_id_from=333.337.search-card.all.click