In-depth understanding of the analysis of predefined classes and boundaries of JS regular expressions

This article mainly introduces the analysis of predefined classes and boundaries for in-depth understanding of JS regular expressions. It has a certain reference value. Now I share it with you. Friends in need can refer to it.

Preliminary Define classes

Regular expressions provide predefined classes to match common character classes

##\d[0-9]Numeric characters\D[^0-9]Non-numeric characters\s[\t\n\x0B\f\r] White space character\S[^\t\n\x0B\f\r]Non-white space character \w[a-zA-Z_0-9]Word characters (letters, numbers, underscores)\W[^a-zA-Z_0-9]Non-word characters##Use pre Defining classes can quickly match targets, such as matching an ab number and any character. Using character classes and range classes requires writing a lot of code, but using predefined classes only requires writing like this:

Characters	Equivalent classes	Meaning
.	[^\r\n]	Except carriage return and line feed characters All characters

/ab\d/

Boundary

Regular expressions also provide several commonly used boundary matching characters

Characters ^$ \b##\BNon-word boundariesWhen we just want Boundaries are often useful when matching part of a word in a paragraph rather than letters in a word

Meaning
Start with xxx
End with xxx
Word boundaries

Word Boundary vs. Non-Word Boundary

Sometimes, I want to match the is word in a sentence, And I don’t want to match the is letter in the word. At this time, using word boundaries can easily solve this problem:

let text = 'This is a boy'

let reg1 = /is/g

let reg2 = /\bis\b/g

text.replace(reg1, 'IS')    // 没有使用单词边界\b区分，结果为：ThIS IS a boy

text.replace(reg2, 'IS')    // 使用了单词边界进行区分，结果为：This IS a boy

And what should I do when I only want to match the is that is the end of the word? At this time, you can mix word boundaries and non-word boundaries wisely:

let text = 'This is a boy'

let reg3 = /\Bis\b/g

text.replace(reg3, 'IS')    // ThIS is a boy

^ and $ - the beginning and end of

are often used. We will want to match the beginning or end characters, use

^

and

$ can perfectly solve this problem:

let text = '@123@abc@'

let reg1 = /@/g

text2.replace(reg1, 'Q')    // 没有使用^和$，匹配了所有的@，结果为：Q123QabcQ

let reg2 = /^@/g

text.replace(reg2, 'Q')     // 使用^匹配开头的@，结果为：Q123@abc@

let reg3 = /@$/g

text.replace(reg3, 'Q')     // 使用$匹配结尾的@，结果为：@123@abcQ

tips: In actual use, ^

needs to be written in front of the matching item, while

$ Need to unload the rear of the match In the case of multiple lines, use ^ and $

In the case of multiple lines, use

m

to enter multiple The line pattern can match the matching characters at the beginning and end of each line:

let text = '@123\n@456\n@789'

let reg1 = /^@\d/g

text.replace(reg1, 'Q')      
/*
    由于换行实际上只是一个换行符字符，在正常模式下，依然看做一段字符
    结果为：
    Q23
    @456
    @789
*/

let reg2 = /^@\d/gm

text.replace(reg2, 'Q')
/*
    添加了m进入多行模式：
    结果为：
    Q23
    Q56
    Q89
*/

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