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JavaScript RegExp Constructor Property - GeeksforGeeks

JavaScript RegExp Constructor Property

Last Updated : 05 Aug, 2025

The constructor property of a JavaScript RegExp object returns a reference to the function that created the regular expression. This property typically points to the built-in RegExp function.

JavaScript


 // Creating a regular expression
let regex = /test/;

// Checking the constructor property
console.log(regex.constructor === RegExp);

Key Points

Default Value: The constructor property of any regular expression points to RegExp by default.
Instance Identification: It can be used to verify if a particular object is an instance of RegExp.
Read-Only Reference: While the constructor property provides a reference, it should not be altered.

Syntax:

regex.constructor

Real-World Examples 1. Verifying the Type of an Object JavaScript


 let regex = /hello/;
if (regex.constructor === RegExp) {
    console.log("This is a RegExp object!"); 
}

2. Creating a New Instance Using the Constructor JavaScript


 let regex1 = /world/;
let regex2 = new regex1.constructor("hello", "gi");

console.log(regex2);

The constructor allows the creation of a new RegExp object with the same constructor function as an existing one.

3. Testing the Constructor of Modified Objects JavaScript


 let regex = /test/;

// Modifying the prototype
regex.__proto__.constructor = function () {
    console.log("Modified constructor");
};

// Checking the constructor
let newRegex = new regex.constructor();

Note: Modifying the constructor is not recommended as it can lead to unexpected behavior.

4. Recreating a Pattern Dynamically JavaScript


 let pattern = "abc";
let flags = "gi";

let regex1 = new RegExp(pattern, flags);
let regex2 = regex1.constructor("123", "g");

console.log(regex2);

Using the constructor, you can dynamically generate new patterns based on existing objects.

Limitations

Prototype Modification: Modifying the prototype's constructor can lead to unreliable behavior and is generally discouraged.
Compatibility: The constructor property is not commonly used for everyday tasks, so its usage may not always be intuitive.

Why Use the Constructor Property?

Dynamic Pattern Creation: Helps in generating new RegExp objects programmatically.
Type Validation: Ensures an object is a regular expression before performing regex-specific operations.
Advanced Use Cases: Useful in scenarios involving prototype manipulation or debugging.

Conclusion

The constructor property is a utility feature in JavaScript’s RegExp objects, enabling dynamic and programmatically controlled regular expression handling. While not a frequently used property, it provides flexibility for advanced tasks in JavaScript programming.

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