A regular expression (RegExp) in JavaScript is an object that describes a pattern of characters. It can contain the alphabetical, numeric, and special characters. Also, the regular expression pattern can have single or multiple characters.

The JavaScript RegExp class represents regular expressions, and both String and RegExp define methods that use regular expressions to perform powerful pattern-matching and search-and-replace functions on text.

The regular expression is used to search for the particular pattern in the string or replace the pattern with a new string.

There are two ways to construct the regular expression in JavaScript.

• Using the RegExp() constructor.

• Using the regular expression literal.

Syntax

A regular expression could be defined with the RegExp () constructor, as follows −

var pattern = new RegExp(pattern, attributes);
or simply
var pattern = /pattern/attributes;

Parameters

Here is the description of the parameters −

• pattern − A string that specifies the pattern of the regular expression or another regular expression.

• attributes − An optional string containing any of the “g”, “i”, and “m” attributes that specify global, case-insensitive, and multi-line matches, respectively.

Before we learn examples of regular expression, let”s learn about regular expression modifiers, Quantifiers, literal characters, etc.

Modifiers

Several modifiers are available that can simplify the way you work with regexps, like case sensitivity, searching in multiple lines, etc.

Brackets

Brackets ([]) have a special meaning when used in the context of regular expressions. They are used to find a range of characters.

The ranges shown above are general; you could also use the range [0-3] to match any decimal digit ranging from 0 through 3, or the range [b-v] to match any lowercase character ranging from b through v.

Quantifiers

The frequency or position of bracketed character sequences and single characters can be denoted by a special character. Each special character has a specific connotation. The +, *, ?, and $ flags all follow a character sequence.

Examples

Following examples explain more about matching characters.

Literal characters

The literal characters can be used with a backslash () in the regular expression. They are used to insert special characters, such as tab, null, Unicode, etc., in the regular expression.

Metacharacters

A metacharacter is simply an alphabetical character preceded by a backslash that acts to give the combination a special meaning.

For instance, you can search for a large sum of money using the ”d” metacharacter: /([d]+)000/, Here d will search for any string of numerical character.

The following table lists a set of metacharacters which can be used in PERL Style Regular Expressions.

Let”s learn to create regular expressions below.

let exp = /tutorialspoint/i 

• /tutorialspoint/ – It finds a match for the ”tutorialspoint” string.

• i – It ignores the case of the characters while matching the pattern with the string. So, it matches with ”TutoiralsPoint”, or ”TUTORIALSpoint”, etc.

let exp = /d+/

• d – It matches 0 to 9 digits.

• + – It matches one or more numeric digits.

let exp = /^Hi/

• ^ – It matches the start of the text.

• Hi – It checks whether the text contains ”Hi” at the start.

Let exp = /^[a-zA-Z0-9]+@[a-zA-Z]+.[a-zA-Z]{2,3}$/

The above regular expression validates the email. It looks complex, but it is very easy to understand.

• ^ – Start of the email address.

• [a-zA-Z0-9] – It should contain the alphanumeric characters in the start.

• + – It should contain at least one alphanumeric character.

• @ – It must have the ”@” character after the alphanumeric characters.

• [a-zA-Z]+ – After the ”@” character, it must contain at least 1 alphanumeric character.

• . – It must contain a dot after that.

• [a-zA-Z] – After the dot, the email should contain alphabetical characters.

• {2, 3} – After the dot, it should contain only 2 or 3 alphabetical characters. It specifies the length.

• $ – It represents the end of the pattern.

Now, the question is whether we can use the search() or replace() method to search or replace text in the string by passing the string as an argument; then what is the need for the regular expression?

The question is obvious. Let”s understand it via the example below.

Example

In the below example, we used the regular expression literal to define the regular expression. The pattern matches the ”tutorialspoint” string without comparing the case of characters.

In the first case, the string search() method searches for the ”tutorialspoint” string, which performs the case-sensitive match. So, it returns -1.

In the second case, we passed the regular expression as an argument of the search() method. It performs the case-insensitive match. So, it returns 11, the index of the required pattern.

<html>
<head>
   <title> JavaScript - Regular Expression </title>
</head>
<body>
   <p id = "output"> </p>
   <script>
      const output = document.getElementById("output");
      let pattern = /tutorialspoint/i;
      let str = "Welcome to TuTorialsPoint! It is a good website!";
      let res = str.search(''tutorialspoint'');
      output.innerHTML += "Searching using the string : " + res + "<br>";
      res = str.search(pattern);
      output.innerHTML += "Searching using the regular expression : " + res;
   </script>
</body>
</html>

Execute the program to see the desired results.

Example

In the example below, we used the replace() method to match the pattern and replace it with the ”100” string.

Here, the pattern matches the pair of digits. The output shows that each number is replaced with ”100” in the string. You may also add alphabetical characters in the string.

<html>
<head>
   <title> JavaScript - Regular expression </title>
</head>
<body>
   <p id = "output"> </p>
   <script>
      let pattern = /d+/g; // Matches pair of digits
      let str = "10, 20, 30, 40, 50";

      let res = str.replace(pattern, "100");
      document.getElementById("output").innerHTML = 
		"String after replacement : " + res;
   </script>
</body>
</html>

Execute the program to see the desired results.

Example (Email validation)

In the example below, we used the RegExp() constructor function with a ”new” keyword to create a regular expression. Also, we have passed the pattern in the string format as an argument of the constructor.

Here, we validate the email using the regular expression. In the first case, email is valid. In the second case, the email doesn”t contain the ‘@’ character, so the test() method returns false.

<html>
<body>
   <p id = "output"> </p>
   <script>
      const pattern = new RegExp(''^[a-zA-Z0-9]+@[a-zA-Z]+.[a-zA-Z]{2,3}$'');
      document.getElementById("output").innerHTML = 
		"[email protected] is valid? : " + pattern.test(''[email protected]'') + "<br>" +
      "abcdgmail.com is valid? : " + pattern.test(''abcdgmail.com'');
</script>
</body>
</html>

So, the regular expression can be used to find a particular pattern in the text and perform operations like replace.

RegExp Properties

Here is a list of the properties associated with RegExp and their description.

In the following sections, we will have a few examples to demonstrate the usage of RegExp properties.

RegExp Methods

Here is a list of the methods associated with RegExp along with their description.

In the following sections, we will have a few examples to demonstrate the usage of RegExp methods.