Sorting out the usage of expression in javascript_javascript skills

Brother, I have been learning JavaScript recently. When I learned regular expression, I found that the knowledge points were a bit confusing, so I wrote a blog to summarize it.

Definition

There are two ways to define reg exp in JavaScript:

1) Use new exp: var exp1 = new exp("abc");

2) Place pattern directly between the two /s: var exp2 = /abc/; //Note. . There are no double quotation marks. If you add them, it becomes a string.

Special characters

Visual inspection shows that the special characters are the same as those in Perl. . Just use it directly

d Digit characters
w Alphanumeric characters (“word characters”)
s Whitespace characters (space, tab, newline, and similar)
D Characters that are not digits
W Non-alphanumeric characters
S Non-whitespace characters
. A period matches all characters except newlines
There is a very simple way to remember:

d = digit So it’s numbers

w = word So it’s letters

s = space So it’s spaces

All uppercase letters are reversed. .

Brackets []

Putting pattern in the brackets means that it is true as long as it matches any character. (Same as java or Perl)

For example,

Copy the code The code is as follows:

console.log(/[01]/.test("023424")); // true

console.log(/[01]/.test("13424")) ; // true

console.log(/[01]/.test("23424")); // false

brackets ()

means It must match everything in the brackets to be true

For example,

Copy the code The code is as follows:

console.log(/[01]/.test("013424")); // true

console.log(/[01]/.test("13424")); // false

console.log(/[01]/.test("230424")); // false

console.log(/[01]/.test("230142401 ")); // true

Quantifiers

are the same as java. . This watch is very good. . Brother, I always like to use

Greedy	Reluctant	Possessive	Meaning
X?	X??	X?	X, once or not at all
X*	X*?	X*	X, zero or more times
X	X ?	X	X, one or more times
X{n}	X{n}?	X{n}	X, exactly n times
X{n,}	X{n,}?	X{n,}	X, at least n times
X{n,m}	X{n,m}?	X{n,m}	X, at least n but not more thanm times

expression object functions

1) test This is very simple, just put the string to be tested in test(...), this function will return true/false Represents match/unmatch

2) exec, this function returns null if the matching string is not found.. If found, it will return an array. This contains the matching strings in order

3 ) String.replace(expression1, string1) This function replaces the match part in expression with string1. In string1, the parenthesized group

in the previous expression can be used to replace a certain part of it. For example, "co-ol".replace(/[w] -[w] /,"$2-$1"); //"ol-co" can be used until $9

4)String.replace (expression, function) This is an enhanced version, and it is very powerful. You can define any output you want through function. The specific usage is not listed here, please refer to

Click to open the link

Dynamic generation of reg expression
When the things you want to use in reg exp are only known by runtime , this method can be applied
to generate reg exp. In fact, you only need to use string to build the appearance of reg exp, and then use the constructor of Exp. (Mentioned at the beginning of the article)

For example:

Copy the code The code is as follows:

var name = "dear"

"oh, my dear".replace(new Exp(name), "god"); // oh, my god

But if there are special characters in the name that may be used in regular expressions, the above method will often go wrong.
So, in that case, we can add a backslash in front of each character of the input string, for example:

Copy code The code is as follows:

var name = df[]vxv;
var expName = name.replace("/[^/w/s]/g"," \$&");
"my name is df[]vxv".replace(new Exp(name), "Bob"); // my name is Bob