How Can I Dynamically Create Regular Expressions with Variables in JavaScript?

Dynamic Regular Expressions with Variables in JavaScript

JavaScript's regular expressions provide a powerful tool for pattern matching and manipulation. However, inserting variables into these patterns can be a challenge.

Consider the following code snippet:

function(input) {
    var testVar = input;
    string = ...
    string.replace(/ReGeX + testVar + ReGeX/, "replacement")
}

This code attempts to replace all occurrences of a specific pattern in a string with a provided replacement string. However, it will fail, as you cannot directly concatenate a variable within a regular expression.

Solution: Using Template Strings

In ES6 JavaScript, you can utilize template strings to dynamically create a regular expression with a variable. Here's how you would do it:

const regex = new RegExp(`ReGeX${testVar}ReGeX`);
...
string.replace(regex, "replacement");

In this example, the RegExp constructor accepts a template string as the first argument. The template string contains the original regular expression pattern with the interpolated variable testVar. The backticks (``) indicate a template string.

ES6 vs. Pre-ES6

Before ES6, you had to use a more verbose method to achieve the same result:

var regex = new RegExp("ReGeX" + testVar + "ReGeX");
...
string.replace(regex, "replacement");

The older syntax still works, but the template string method is preferred for its simplicity and readability.

Caution: Escaping Variables

It's crucial to note that if the variable you're inserting has potential malicious content (e.g., user input), you should escape it to prevent injection attacks. The escape() or encodeURIComponent() functions can be used for escaping.

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