Character Classes

Character Classes （字符类）

Character classes are used for distinguishing characters like distinguishing between digits and letters. （字符类用于区分字符，例如区分数字和字母。）

Let’s start from a practical case. Imagine you have a phone number like +3(522) -865-42-76, and wish to turn it into pure numbers (35228654276). To meet that goal, it is necessary to find and remove everything that’s not a number.Character classes are there to help you with that. （让我们从一个实际案例开始。假设您有一个电话号码，如+3 (522) -865-42-76 ，并希望将其转换为纯数字(35228654276)。为了实现这一目标，有必要查找并删除非数字的所有内容。字符类可以帮助您实现这一目标。）

So, a character class can be described as a specific notation that corresponds to any symbol from a certain set. （因此，字符类可以被描述为对应于某个集合中的任何符号的特定符号。）

We will start from the “digit” class. It should be written as \d and matches any single digit. （我们将从“数字”类开始。它应写为\ d ，并与任何单位数字匹配。）

In the example below, let’s find the first digit:

let str = "+3(522)865-42-76";
let regexp = /\d/;
console.log(str.match(regexp)); // 3

With no flag g, the regular expression searches for the first match, which is the first \d. （在没有标志g的情况下，正则表达式搜索第一个匹配项，即第一个\ d。）

Adding the g flag will enable finding all the digits, like this:

let str = "+3(522)865-42-76";let regexp = /\d/g; 
console.log(str.match(regexp)); // array of matches: 3,5,2,2,8,6,5,4,2,7,6
// the digits-only phone number of them:
console.log(str.match(regexp).join('')); // 35228654276

So, it is a character class for digits. But there exist other character classes, too. （因此，它是数字的字符类。但也有其他字符类。）

The most used character classes are as follows:

• \d ( comes from digit): a digit (a character from 0 to 9).

• \s ( comes from space): a space symbol. It contains \t (tabs),\n (newlines), and other characters (\v, \f,\r ).

• \w (comes from word): it is either a letter of the Latin alphabet, a digit, or an underscore (_). Non-latin letters don’t belong to this class.

A regular expression can include regular symbols, as well as character classes. （正则表达式可以包括正则符号以及字符类。）

Let’s see an example where CSS\d corresponds to a string CSS with a digit following it:

let str = "It is CSS3?";
let regexp = /CSS\d/
console.log(str.match(regexp)); // CSS3

Multiple character classes can be used, like this:

console.log("It is HTML5!".match(/\s\w\w\w\w\d/)); // ' HTML5'

There is an “inverse class” for every character class, denoted with the same but uppercase letter. （每个字符类都有一个“逆类” ，用相同但大写的字母表示。）

“Inverse” means that it corresponds to all other characters:

• \D - non-digit. It accepts any character, except \d (for instance, a letter). （-\ D -非数字。它接受除\ d （例如，字母）之外的任何字符。）

• \S - non-space. Accepts any character, except \s (for instance, a letter). （-\ S -非空格。接受任何字符，\ s除外（例如，字母）。）

• \W- non-wordly character. Accepts anything , except \w ( non-latin letter or a space). （-\ W-非单词字符。接受除\ w （非拉丁字母或空格）以外的任何内容。）

A Dot #

A Dot （一个点）

A dot (.) is considered a special character class corresponding to “any character except a newline”. （点（. ）被视为对应于“除换行符以外的任何字符”的特殊字符类。）

The example will look like this:

console.log("W".match(/./)); // W

In the example below, the dot is in the middle of a regexp:

let regexp = /HTM.5/;
console.log("HTML5".match(regexp)); // HTML5
console.log("HTM-5".match(regexp)); // HTM-5
console.log("HTM 5".match(regexp)); // HTM 5(space is a character, too)

So, the dot is considered “any character”, but not the “absense of a character”. （因此，点被认为是“任何字符” ，而不是“字符的缺失”。）

There should be a character for matching it, like here:

console.log("HTM5".match(/HTM.5/)); // null, no match, as there's no character for the dot

A dot doesn’t correspond to the newline character \n by default. （默认情况下，点与换行符\ n不对应。）

For example, the regexp A.B corresponds to A, and then B with any character between them, except for an \n newline, like this:

console.log("W\nD".match(/W.D/)); // null (no match)

There are circumstances when one wants a dot to mean “any character”, including newline. （在某些情况下，人们希望点表示“任何字符” ，包括换行符。）

The flag s is used for that. In case a regexp has it, then a dot corresponds literally to any character, like this:

console.log("W\nD".match(/W.D/s)); //W\nD (match!)

It is important to pay special attention to spaces. For example, the strings 1-5 and 1 - 5 are similar to each other. But, in case a regexp doesn’t take spaces into account, it might not work. （请务必特别注意房源空间。例如，字符串1-5和1-5彼此相似。但是，如果正则表达式不考虑空格，则可能不起作用。）

For finding the digits, separated by a hyphen, you can act like this:

console.log("1 - 5".match(/\d-\d/)); // null, no match!

Now, let’s fix it by adding spaces in the regular expression \d - \d, like here:

console.log("1 - 5".match(/\d - \d/)); // 1 - 5, now it works
// or we can use \s class:
console.log("1 - 5".match(/\d\s-\s\d/)); // 1 - 5, also works

A space is considered a character. In importance, it is equal to any other character. You can add or remove spaces from a regexp, expecting to work the same way. That is, in a regexp all the characters matter. （空格被视为一个字符。在重要性上，它等同于任何其他字符。您可以在正则表达式中添加或删除空格，期望以相同的方式工作。也就是说，在正则表达式中，所有字符都很重要。）