JavaScript Regex

In JavaScript, a Regular Expression (RegEx) is an object that describes a sequence of characters used for defining a search pattern. For example,

/^a...s$/

The above code defines a RegEx pattern. The pattern is: any five letter string starting with a and ending with s.

A pattern defined using RegEx can be used to match against a string.

Create a RegEx

There are two ways you can create a regular expression in JavaScript.

1. Using a regular expression literal:
   The regular expression consists of a pattern enclosed between slashes /. For example,

   const regularExp = /abc/;

   Here, /abc/ is a regular expression.
2. Using the RegExp() constructor function:
   You can also create a regular expression by calling the RegExp() constructor function. For example,

   const regularExp = new RegExp('abc');

For example,

const regex = new RegExp(/^a...s$/);
console.log(regex.test('alias')); // true

In the above example, the string alias matches with the RegEx pattern /^a...s$/. Here, the test() method is used to check if the string matches the pattern.

There are several other methods available to use with JavaScript RegEx. Before we explore them, let's learn about regular expressions themselves.

If you already know the basics of RegEx, jump to JavaScript RegEx Methods.

Specify Pattern Using RegEx

To specify regular expressions, metacharacters are used. In the above example (/^a...s$/), ^ and $ are metacharacters.

MetaCharacters

Metacharacters are characters that are interpreted in a special way by a RegEx engine. Here's a list of metacharacters:

[] . ^ $ * + ? {} () \ |

[] - Square brackets

Square brackets specify a set of characters you wish to match.

Here, [abc] will match if the string you are trying to match contains any of the a, b or c.

You can also specify a range of characters using - inside square brackets.

[a-e] is the same as [abcde].

[1-4] is the same as [1234].

[0-39] is the same as [01239].

You can complement (invert) the character set by using caret ^ symbol at the start of a square-bracket.

[^abc] means any character except a or b or c.

[^0-9] means any non-digit character.

. - Period

A period matches any single character (except newline '\n').

^ - Caret

The caret symbol ^ is used to check if a string starts with a certain character.

$ - Dollar

The dollar symbol $ is used to check if a string ends with a certain character.

* - Star

The star symbol * matches zero or more occurrences of the pattern left to it.

+ - Plus

The plus symbol + matches one or more occurrences of the pattern left to it.

? - Question Mark

The question mark symbol ? matches zero or one occurrence of the pattern left to it.

{} - Braces

Consider this code: {n,m}. This means at least n, and at most m repetitions of the pattern left to it.

Let's try one more example. This RegEx [0-9]{2, 4} matches at least 2 digits but not more than 4 digits.

| - Alternation

Vertical bar | is used for alternation (or operator).

Here, a|b match any string that contains either a or b

() - Group

Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that matches either a or b or c followed by xz

\ - Backslash

Backslash \ is used to escape various characters including all metacharacters. For example,

\$a match if a string contains $ followed by a. Here, $ is not interpreted by a RegEx engine in a special way.

If you are unsure if a character has special meaning or not, you can put \ in front of it. This makes sure the character is not treated in a special way.

Special Sequences

Special sequences make commonly used patterns easier to write. Here's a list of special sequences:

\A - Matches if the specified characters are at the start of a string.

\b - Matches if the specified characters are at the beginning or end of a word.

\B - Opposite of \b. Matches if the specified characters are not at the beginning or end of a word.

\d - Matches any decimal digit. Equivalent to [0-9]

\D - Matches any non-decimal digit. Equivalent to [^0-9]

\s - Matches where a string contains any whitespace character. Equivalent to [ \t\n\r\f\v].

\S - Matches where a string contains any non-whitespace character. Equivalent to [^ \t\n\r\f\v].

\w - Matches any alphanumeric character (digits and alphabets). Equivalent to [a-zA-Z0-9_]. By the way, underscore _ is also considered an alphanumeric character.

\W - Matches any non-alphanumeric character. Equivalent to [^a-zA-Z0-9_]

\Z - Matches if the specified characters are at the end of a string.

Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101. This tool not only helps you in creating regular expressions, but it also helps you learn it.

Now you understand the basics of RegEx, let's discuss how to use RegEx in your JavaScript code.

JavaScript Regular Expression Methods

As mentioned above, you can either use RegExp() or regular expression literal to create a RegEx in JavaScript.

const regex1 = /^ab/;
const regex2 = new Regexp('/^ab/');

In JavaScript, you can use regular expressions with RegExp() methods: test() and exec().

There are also some string methods that allow you to pass RegEx as its parameter. They are: match(), replace(), search(), and split().

Example 1: Regular Expressions

const string = 'Find me';
const pattern = /me/;

// search if the pattern is in string variable
const result1 = string.search(pattern);
console.log(result1); // 5

// replace the character with another character
const string1 = 'Find me';
string1.replace(pattern, 'found you'); // Find found you

// splitting strings into array elements
const regex1 = /[\s,]+/;
const result2 = 'Hello world! '.split(regex1);
console.log(result2); // ['Hello', 'world!', '']

// searching the phone number pattern
const regex2 = /(\d{3})\D(\d{3})-(\d{4})/g;
const result3 = regex2.exec('My phone number is: 555 123-4567.');
console.log(result3); // ["555 123-4567", "555", "123", "4567"]

Regular Expression Flags

Flags are used with regular expressions that allow various options such as global search, case-insensitive search, etc. They can be used separately or together.

Example 2: Regular Expression Modifier

const string = 'Hello hello hello';

// performing a replacement
const result1 = string.replace(/hello/, 'world');
console.log(result1); // Hello world hello

// performing global replacement
const result2 = string.replace(/hello/g, 'world');
console.log(result2); // Hello world world

// performing case-insensitive replacement
const result3 = string.replace(/hello/i, 'world');
console.log(result3); // world hello hello

// performing global case-insensitive replacement
const result4 = string.replace(/hello/gi, 'world');
console.log(result4); // world world world

Example 3: Validating the Phone Number

// program to validate the phone number

function validatePhone(num) {

    // regex pattern for phone number
    const re = /^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/g;

    // check if the phone number is valid
    let result = num.match(re);
    if (result) {
        console.log('The number is valid.');
    }
    else {
        let num = prompt('Enter number in XXX-XXX-XXXX format:');
        validatePhone(num);
    }
}

// take input
let number = prompt('Enter a number XXX-XXX-XXXX');

validatePhone(number);

Output

Enter a number XXX-XXX-XXXX: 2343223432
Enter number in XXX-XXX-XXXX format: 234-322-3432
The number is valid

Example 4: Validating the Email Address

// program to validate the email address

function validateEmail(email) {

    // regex pattern for email
    const re = /\S+@\S+\.\S+/g;

    // check if the email is valid
    let result = re.test(email);
    if (result) {
        console.log('The email is valid.');
    }
    else {
        let newEmail = prompt('Enter a valid email:');
        validateEmail(newEmail);
    }
}

// take input
let email = prompt('Enter an email: ');

validateEmail(email);

Output

Enter an email: hellohello
Enter a valid email: learningJS@gmail.com
The email is valid.