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JavaScript Regular Expressions

A Regular Expression is a sequence of characters that forms a search pattern.

Regex is a common shorthand for a regular expression.

JavaScript RexExp is an Object for handling Regular Expressions.

RegExp are be used for:

• Text searching
• Text replacing
• Text validation

Do a case-insensitive search for "w3schools" in a string:

let text = "Visit W3Schools";
let n = text.search(/w3schools/i);

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Example explained:

/w3schools/i  is a regular expression.

w3schools  is a pattern (to be used in a search).

i  is a modifier (modifies the search to be case-insensitive).

Regular expressions are often used with the string methods:

Search for "W3schools" in a string:

let text = "Visit W3Schools";
let n = text.match(/W3schools/);

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Replace Microsoft with W3Schools in a string:

let text = "Visit Microsoft!";
let result = text.replace(/Microsoft/i, "W3Schools");

Search for "W3Schools" in a string:

let text = "Visit W3Schools";
let n = text.search(/W3Schools/);

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In a regular expression an alteration is denoted with a vertical line character |.

An alteration matches any of the alternatives separated with |.

A global search for the alternatives (red|green|blue):

let text = "Black, white, red, green, blue, yellow.";

let result = text.match(/red|green|blue/g);

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Regular expression flags are parameters that can modify how a pattern is used, such as making it case-insensitive or global.

These are the most common:

The /g flag matches all occurrences of the pattern, rather than just the first one.

A global search for "is" in a string:

let text = "Is this all there is?";

const pattern = /is/g;

let result = text.match(pattern);

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The /i flag makes a match case-insensitive: /abc/i matches "abc", "AbC", "ABC".

A case-insensitive search for "w3schools" in a string:

let text = "Visit W3Schools";
const pattern = /w3schools/i;
let result = text.match(pattern);

Try it Yourself »

// Match words
const pattern = /\w/;

Metacharacters are characters with a special meaning.

They can be used to match digts, words, spaces, and more.

These are the most common:

The \d metacharacter matches digits.

A global search for digits in a string:

let text = "Give 100%!";

const pattern = /\d/g;

let result = text.match(pattern);

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The \w metacharacter matches word characters.

A word character is a character a-z, A-Z, 0-9, including _ (underscore).

A global search for word characters:

let text = "Give 100%!";

const pattern = /\w/g;

let result = text.match(pattern);

// Match at least one zero
const pattern = /0+/;

Quantifiers define the numbers of characters or expressions to match.

These are the most common:

x? matches zero or one occurrences of x.

A global search for "1", followed by zero or more "0" characters:

let text = "1, 100 or 1000?";

const pattern = /10?/g;

let result = text.match(pattern);

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// Match beginning of string

const pattern = /^W3Schools/;

// Match end of string

const pattern = /W3Schools$/;

Assertions matches Boundaries and Lookarounds:

String Boundaries and Word Boundaries.

Lookarounds: Lookaheads and Lookbehinds.

These are the most common:

The ^ metacharacter matches the beginning of a string.

Test if a string starts with W3Schools:

const pattern = /^W3Schools/;

let text = "W3Schools Tutorial";

let result = pattern.test(text); // true

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const pattern = /^W3Schools/;

let text = "Hello W3Schools";

let result = pattern.test(text); // false

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The $ metacharacter matches the end of a string.

Test if a string ends with W3Schools:

const pattern = /W3Schools$/;

let text = "Hello W3Schools";

let result = pattern.test(text); // true

const pattern = /W3Schools$/;

let text = "W3Schools tutorial";

let result = pattern.test(text); // false

// Match Digits
const pattern = /[0-9]/;

Character Classes are characters enclosed in square brackets [].

A character class matches any character from a set within brackets.

These are the most common:

A global search for the characters "0" to "9" in a string:

let text = "More than 1000 times";

const pattern = /[0-9]/g;

let result = text.match(pattern);

Try it Yourself »

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