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Regular Expressions

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

Reg and Regex is a common shorthand for a regular expression.

JavaScript RexExp is an Object for storing Regular Expressions.

RegExp are be used for:

  • Text searching
  • Text replacing
  • Text validation

Example

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

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

Try it Yourself »

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 Expression Syntax

/pattern/modifier flags;

Using String Methods

Regular expressions are often used with the string methods:

MethodDescription
match(regex) Returns an Array of results
replace(regex) Returns a new String
search(regex) Returns the index of the first match

Using String match()

Search for "W3schools" in a string:

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

Try it Yourself »

Using String replace()

Replace Microsoft with W3Schools in a string:

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

Using String search()

Search for "W3Schools" in a string:

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

Try it Yourself »


RexExp Alteration (OR)

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

An alteration matches any of the alternatives separated with |.

Example

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);

Try it Yourself »


RegExp Modifier Flags

Regular expression (regex) flags are optional parameters that can modify how a regex pattern is interpreted and applied.

They can change the behavior of a regex, such as making it case-insensitive or global (matching all occurrences), or enabling multiline seaches.


The /g Flag

Example

A global search for "is" in a string:

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

let result = text.match(/is/g);

Try it Yourself »


The /i Flag

Example

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

let text = "Visit W3Schools";

let result = text.match(/w3schools/i);

Try it Yourself »


The /m Flag

Example

A multiline search for "is" at the beginning of each line in a string:

let text = "\nIs th\nis it?";

let result = text.match(/^is/m);

Try it Yourself »



Full Flag Reference

Revised July 2025

Flag Description
/d Performs substring matches (new 2022)
/g Performs a global match (find all)
/i Performs case-insensitive matching
/m Performs multiline matching
/s Allows . (dot) to match line terminators (new 2018)
/u Enables Unicode support (new 2015)
/v An upgrade to the /u flag for better Unicode support (new 2025)
/y Performs a "sticky" search (new 2015)

Regular Expression Methods

Regular Expression Search and Replace can be done with different methods.

These are the most common:

String Methods

MethodDescription
match(regex) Returns an Array of results
matchAll(regex) Returns an Iterator of results
replace(regex) Returns a new String
replaceAll(regex) Returns a new String
search(regex) Returns the index of the first match
split(regex) Returns an Array of results

RegExp Methods

MethodDescription
regex.exec() Returns an Iterator of results
regex.test() Returns true or false

Exercise?What is this?
Test your skills by answering a few questions about the topics of this page

What does the i modifier represent in Regular Expressions?




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