JavaScript Regex

JavaScript Regex

In this tutorial, you will learn about JavaScript regular expressions (Regex) with the help of examples.

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.

Expression String Matched?
/^a...s$/ abs No match
alias Match
abyss Match
Alias No match
An abacus No match

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,
    cost 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 reguarExp = 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 other several methods available to use with JavaScript RegEx. Before we explore that, 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.

Expression String Matched?
[abc] a 1 match
ac 2 matches
Hey Jude No match
abc de ca 5 matches

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').

Expression String Matched?
.. a No match
ac 1 match
acd 1 match
acde 2 matches (contains 4 characters)

^ - Caret

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

Expression String Matched?
^a a 1 match
abc 1 match
bac No match
^ab abc 1 match
acb No match (starts with a but not followed by b)

$ - Dollar

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

Expression String Matched?
a$ a 1 match
formula 1 match
cab No match

* - Star

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

Expression String Matched?
ma*n mn 1 match
man 1 match
mann 1 match
main No match (a is not followed by n)
woman 1 match

+ - Plus

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

Expression String Matched?
ma+n mn No match (no a character)
man 1 match
mann 1 match
main No match (a is not followed by n)
woman 1 match

? - Question Mark

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

Expression String Matched?
ma?n mn 1 match
man 1 match
mann No match (more than one a character)
main No match (a is not followed by n)
woman 1 match

{} - Braces

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

Expression String Matched?
a{2,3} abc dat No match
abc daat 1 match (at daat)
aabc daaat 2 matches (at aabc and daaat)
aabc daaaat 2 matches (at aabc and daaaat)

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

Expression String Matched?
[0-9]{2,4} ab123csde 1 match (match at ab123csde)
12 and 345673 3 matches (12, 3456, 73)
1 and 2 No match

| - Alternation

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

Expression String Matched?
a|b cde No match
ade 1 match (match at ade)
acdbea 3 matches (at acdbea)

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

Expression String Matched?
(a|b|c)xz ab xz No match
abxz 1 match (match at abxz)
axz cabxz 2 matches (at axzbc cabxz)

\ - Backslash

Backlash \ 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.

Expression String Matched?
\Athe the sun Match
In the sun No match

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

Expression String Matched?
\bfoo football Match
a football Match
foo\b a football No match
the foo Match
the afoo test Match
the afootest No match

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

Expression String Matched?
\Bfoo football No match
a football No match
foo\B a football Match
the foo No match
the afoo test No match
the afootest Match

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

Expression String Matched?
\d 12abc3 3 matches (at 12abc3)
JavaScript No match

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

Expression String Matched?
\D 1ab34"50 3 matches (at 1ab34"50)
1345 No match

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

Expression String Matched?
\s JavaScript RegEx 1 match
JavaScriptRegEx No match

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

Expression String Matched?
\S a b 2 matches (at a b)
  No match

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

Expression String Matched?
\w 12&": ;c 3 matches (at 12&": ;c)
%"> ! No match

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

Expression String Matched?
\W 1a2%c 1 match (at 1a2%c)
JavaScript No match

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

Expression String Matched?
JavaScript\Z I like JavaScript 1 match
I like JavaScript Programming No match
JavaScript is fun No match

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 use either RegExp() or regular expression literal to create a RegEx in JavaScript.

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

Method Description
exec() Executes a search for a match in a string and returns an array of information. It returns null on a mismatch.
test() Tests for a match in a string and returns true or false.
match() Returns an array containing all the matches. It returns null on a mismatch.
matchAll() Returns an iterator containing all of the matches.
search() Tests for a match in a string and returns the index of the match. It returns -1 if the search fails.
replace() Searches for a match in a string and replaces the matched substring with a replacement substring.
split() Break a string into an array of substrings.

Example 1: Regular Expressions

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

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

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

// splitting strings into array elements
const re = /[\s,]+/;
let result = 'Hello world! '.split(re);
console.log(result); // ["I", "am", "learning", "JavaScript", "RegEx"]

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

Flags Description
g Performs a global match (find all matches)
m Performs multiline match
i Performs case-insensitive matching

Example 2: Regular Expression Modifier

let string = 'Hello hello hello';

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

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

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

// performing global case-insensitive replacement
let 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.