JS string: Replace with index in regex

Question

I have little to no experience with regex, and I was wondering how you would go about to replace a section in a string identified by regex, where the index is part of the identified section?

Here is my example string:

let exampleStr = "How do I {0} the {n} with the {1} in my array?";

This is my data array:

let arr = ["replace", "items"];

Now, with replace and regex, I would like to match the index in the {#} section with the array element matching the index.

Resulting string:

let result = "How do I replace the {n} with the items in my array?";

Notice how it would ignore the {n}, as it does not contain a numeric value.

I can do this with Array.indexOf, Number.isNaN, typeof etc, but regex seems to be the "right" and cleaner way to do it, while a bit harder to read :)

Thanks in advance.

Why do you need ajax for that if you know the exact strings you want to replace? — Dekel
– Dekel, Commented Dec 29, 2016 at 12:51
As a programmer, I really like a generic solution. But generic examples are some times hard to grasp, and might confuse the reader. So, while I'm looking for a generic solution, I find it more appropriate to give an example in a more specific manner. — David Gustavsson
– David Gustavsson, Commented Dec 29, 2016 at 13:07

Kobi · Accepted Answer · 2016-12-29 12:52:47Z

5

You can use replace with a callback:

let exampleStr = "How do I {0} the {n} with the {1} in my array?";
let arr = ["replace", "items"];

let result = exampleStr.replace(/\{(\d+)\}/g, (g0, g1)=>arr[parseInt(g1,10)]);
console.log(result);

The pattern is simple - it matches a number inside curly braces, and captures the number to group number 1.
The callback function parses the number (which is not strictly required, but arr["1"] is not pretty), and then return the proper element from the array.
The callback could use some more error checking.

answered Dec 29, 2016 at 12:52

Kobi

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3 Comments

Kobi Over a year ago

@torazaburo - You are right, but that's a style choice. { and } are metacharacters, and I usually like to escape them. This is even more justifiable here, in the context of numbers. {\d+} might become {2}, for example in a test or modified code, resulting in a syntax error.

Ian Y. Over a year ago

Thanks. This solution seems to require that you know in advance that there are two strings to be replaced ({0} and {1}). What if the number of strings to be replaced is unknown, meaning there could be {2}, {3}, {4}, ...?

Kobi Over a year ago

@IanY. - this should work for any number as long as you supply the right arr - which could be create dynamically. The first two lines here are copied from the question so I'll have runnable code.

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