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JavaScript. W3Schools in English. Lessons for beginners

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JavaScript Regular Expressions


A regular expression is a sequence of characters that forms a search pattern.

The search pattern can be used for text search and text replace operations.


What Is a Regular Expression?

A regular expression is a sequence of characters that forms a search pattern.

When you search for data in a text, you can use this search pattern to describe what you are searching for.

A regular expression can be a single character, or a more complicated pattern.

Regular expressions can be used to perform all types of text search and text replace operations.

Syntax

/pattern/modifiers;

Example

/w3schools/i;

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).


Using String Methods

In JavaScript, regular expressions are often used with the two string methods: search() and replace().

The search() method uses an expression to search for a match, and returns the position of the match.

The replace() method returns a modified string where the pattern is replaced.


Using String search() With a String

The search() method searches a string for a specified value and returns the position of the match:

Example

Use a string to do a search for "W3schools" in a string:

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

The result in n will be:

6
Try it Yourself »

Using String search() With a Regular Expression

Example

Use a regular expression to do a case-insensitive search for "w3schools" in a string:

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

The result in n will be:

6
Try it Yourself »

Using String replace() With a String

The replace() method replaces a specified value with another value in a string:

let text = "Visit Microsoft!";
let result = text.replace("Microsoft", "W3Schools");
Try it Yourself »

Use String replace() With a Regular Expression

Example

Use a case-insensitive regular expression to replace Microsoft with W3Schools in a string:

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

The result in res will be:

Visit W3Schools!
Try it Yourself »

Did You Notice?

Regular expression arguments (instead of string arguments) can be used in the methods above.
Regular expressions can make your search much more powerful (case-insensitive for example).


Regular Expression Modifiers

Modifiers can be used to perform case-insensitive more global searches:

Modifier Description Try it
i Perform case-insensitive matching Try it »
g Perform a global match (find all matches rather than stopping after the first match) Try it »
m Perform multiline matching Try it »

Regular Expression Patterns

Brackets are used to find a range of characters:

Expression Description Try it
[abc] Find any of the characters between the brackets Try it »
[0-9] Find any of the digits between the brackets Try it »
(x|y) Find any of the alternatives separated with | Try it »

Metacharacters are characters with a special meaning:

Metacharacter Description Try it
\d Find a digit Try it »
\s Find a whitespace character Try it »
\b Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b Try it »
Try it »
\uxxxx Find the Unicode character specified by the hexadecimal number xxxx Try it »

Quantifiers define quantities:

Quantifier Description Try it
n+ Matches any string that contains at least one n Try it »
n* Matches any string that contains zero or more occurrences of n Try it »
n? Matches any string that contains zero or one occurrences of n Try it »

Using the RegExp Object

In JavaScript, the RegExp object is a regular expression object with predefined properties and methods.


Using test()

The test() method is a RegExp expression method.

It searches a string for a pattern, and returns true or false, depending on the result.

The following example searches a string for the character "e":

Example

const pattern = /e/;
pattern.test("The best things in life are free!");

Since there is an "e" in the string, the output of the code above will be:

true
Try it Yourself »

You don't have to put the regular expression in a variable first. The two lines above can be shortened to one:

/e/.test("The best things in life are free!");

Using exec()

The exec() method is a RegExp expression method.

It searches a string for a specified pattern, and returns the found text as an object.

If no match is found, it returns an empty (null) object.

The following example searches a string for the character "e":

Example

/e/.exec("The best things in life are free!");
Try it Yourself »

Complete RegExp Reference

For a complete reference, go to our Complete JavaScript RegExp Reference.

The reference contains descriptions and examples of all RegExp properties and methods.