Regular Expression: Character Classes

Summary: in this tutorial, you’ll learn about character classes in regular expressions to match a set of characters including digits, whitespace, and word characters.

Introduction to the character classes

A character class allows you to match any symbol from a certain character set. A character class is also called a character set. Suppose that you have a phone number like this:

+1-(408)-555-0105Code language: JavaScript (javascript)

And you want to turn it into a plain number:

14085550105Code language: JavaScript (javascript)

Character classes in regular expressions can help you to do this.

Let’s explore the digit character class first. The digit character class is denoted by \d which matches any single digit:

\dCode language: JavaScript (javascript)

The following example uses the \d to match the first number in the phone number:

let phone = '+1-(408)-555-0105';
let re = /\d/;

console.log(phone.match(re));Code language: JavaScript (javascript)

Output:

["1"]Code language: JavaScript (javascript)

When you add the global flag (g), the regular expression will search for all numbers, not the first one:

let phone = '+1-(408)-555-0105';
let re = /\d/g;

console.log(phone.match(re));Code language: JavaScript (javascript)

Output:

["1", "4", "0", "8", "5", "5", "5", "0", "1", "0", "5"]
Code language: JavaScript (javascript)

Now, you can turn the phone number into a plain number as follows:

• Use the match() method to return an array containing numbers.
• Use the join() method to concatenate elements of the array into a string.

For example:

let phone = '+1-(408)-555-0105';
let re = /\d/g;

let numbers = phone.match(re);
let phoneNo = numbers.join('');

console.log(phoneNo);
Code language: JavaScript (javascript)

Output:

14085550105
Code language: JavaScript (javascript)

To make it short, you can chain the match() and join() methods like this:

console.log('+1-(408)-555-0105'.match(/\d/g).join(''));
Code language: JavaScript (javascript)

Besides the digit character class (\d), regular expressions support other character classes.

The most commonly used character classes are:

• \d – match a single digit or a character from 0 to 9.
• \s – match a single whitespace symbol such a space, a tab (\t), a newline (\n).
• \w – w stands for word character. It matches the ASCII character [A-Za-z0-9_] including Latin alphabets, digits, and the underscore (_)

In practice, you often combine the character classes to form a more powerful match.

For example \w\d matches any word followed by a digit like O2:

let str = 'O2 is oxygen';
let re = /\w\d/g

console.log(str.match(re));Code language: JavaScript (javascript)

Output:

O2Code language: JavaScript (javascript)

A pattern may contain both regular characters and character classes. For example, the ES\d regular expression matches ES followed by a digit like ES6:

let str = 'ES6 Tutorial';
let re = /ES\d/g

console.log(str.match(re));Code language: JavaScript (javascript)

Output:

["ES6"]
Code language: JavaScript (javascript)

Inverse Classes

A character class has an inverse class with the same letter but in the uppercase e.g., \D is the inverse of \d.

The inverse class matches all the other characters. For example, the \D match any character except a digit (or \d). The following are the inverse classes:

• \D – matches any character except a digit e.g., a letter.
• \S – matches any character except a whitespace e.g., a letter
• \W – matches any character except a word character e.g., non-Latin letter or space.

Back to the phone number example, you can use the \d with the global flag (g):

let phone = '+1-(408)-555-0105';
let re = /\d/g;

console.log(phone.match(re).join(''));Code language: JavaScript (javascript)

Output:

14085550105
Code language: JavaScript (javascript)

Or you can remove the non-digit using the \D inverse class and replace all non-digit characters with blank, like this:

let phone = '+1-(408)-555-0105';
let re = /\D/g;

console.log(phone.replace(re,''));Code language: JavaScript (javascript)

Output:

14085550105Code language: JavaScript (javascript)

The dot (.) character class

The dot (.) is a special character class that matches any character except a newline:

let re = /E.6/
console.log('ES6'.match(re)); Code language: JavaScript (javascript)

Output:

["ES6", index: 0, input: "ES6", groups: undefined]
Code language: JavaScript (javascript)

However, the following example returns null:

let re = /ES.6/
console.log('ES\n6'.match(re));Code language: JavaScript (javascript)

If you want to use the dot (.) character class to match any character including the newline, you can use the s flag:

let re = /ES.6/s
console.log('ES\n6'.match(re));Code language: JavaScript (javascript)

Output:

["ES
6"]Code language: PHP (php)

Summary

• Character classes match any symbol from certain character sets e.g., \d, \s, and \w.
• The character classes \d, \s, and \w have the inverse classes \D, \S and \W that match other characters except \d, \s and \w.
• The dot(.) matches any character except the newline character. Use the s flag to make the dot (.) character class matches any character including the newline.