How to clone a given regular expression in JavaScript?

JavaScript's regular expression support is widely considered to be among the best in the world, but there's one area where it's lacking: there's no built-in way to clone a regular expression. This can be a problem when you need to create a new regular expression that is similar to an existing one but with a few small changes.

The problem is that regular expressions are objects, and as such, they cannot be copied by simply assigning one to another. Consider the following code −

var regex1 = /foo/;
var regex2 = regex1;
regex2 === regex1; // true

In this code, we've created two regular expressions that are identical. But what if we want to make a small change to one of them?

regex2 = /bar/;
regex1 === regex2; // false

Now the two regular expressions are different, even though they started out as copies of each other. This is because regular expressions are mutable: they can be changed after they're created.

Cloning using a regular expression literal

One way to clone a regular expression is to create a new regular expression literal that is similar to the existing one. For example, if we have a regular expression like this −

var regex1 = /foo/;

We can create a new regular expression that is identical to it using a regular expression literal −

var regex2 = /foo/;
regex1 === regex2; // true

This method is simple and straightforward, but it only works if the regular expression is literal (i.e. it's not created using the new RegExp() constructor).

Cloning regular expressions with the RegExp() constructor

Another way to clone a regular expression is to use the RegExp() constructor. This allows us to create a new regular expression that is based on an existing one but with some changes. For example, if we have a regular expression like this −

var regex1 = /foo/;

We can create a new regular expression that is identical to it using the RegExp() constructor −

var regex2 = new RegExp(regex1);
regex1 === regex2; // true

This method is a bit more complex than the previous one, but it has the advantage of being able to clone regular expressions that are not literal.

Why do we clone regular expressions?

There are a few benefits to cloning regular expressions. First, it can save you time if you need to create a lot of similar regular expressions. Second, it can help you avoid mistakes if you need to make a small change to an existing regular expression. Finally, it can make your code more readable by giving your regular expressions descriptive names.

Disadvantage of cloning regular expressions

One disadvantage of cloning regular expressions is that it can make your code more complex. For example, if you have a regular expression like this −

var regex1 = /foo/;

And you want to create a new regular expression that is similar to it, but with the case insensitive flag set, you would need to do this −

var regex2 = new RegExp(regex1, 'i');

This is more complex than simply creating a new regular expression with the case insensitive flag set −

var regex2 = /foo/i;

In conclusion, cloning regular expressions can be a useful tool, but it comes with a few trade-offs. It can make your code more complex, but it can also save you time and help you avoid mistakes.