How to clone a given regular expression in JavaScript?

JavaScript’s regular expression support is widely considered to be one of the best in the world, but it has one drawback: there’s no built-in way to clone a regular expression. This can become a problem when you need to create a new regular expression that is similar to an existing regular expression but with some minor changes.

The problem is that regular expressions are objects, so they cannot be copied by simply assigning one object to another. Consider the following code -

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

In this code, we create two identical regular expressions. 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 are created.

Clone using regular expressions

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

var regex1 = /foo/;

We can use a regular expression literal to create a new regular expression that is identical to -

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

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

Use the RegExp() constructor to clone a regular expression

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

var regex1 = /foo/;

We can use the RegExp() constructor to create a new regular expression identical to it -

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

This method is slightly more complex than the previous one, but it has the advantage of being able to clone non-literal regular expressions.

Why do we clone regular expressions?

There are some benefits to cloning regular expressions. First, it saves you time if you need to create a lot of similar regular expressions. Second, it helps you avoid errors if you need to make minor changes to an existing regular expression. Finally, it can make your code more readable by giving the regular expression a descriptive name.

Disadvantages of cloning regular expressions

One disadvantage of cloning a regular expression is that it makes the 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 similar to it but with the case insensitive flag set, you 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 summary, cloning a regular expression can be a useful tool, but it comes with some trade-offs. It can make your code more complex, but it can also save you time and help you avoid errors.

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