Multi-line Regex In Javascript with Comments

Regular Expressions are great, but not easy to look at!

Let's take for example a regular expression in Javascript for parsing an email adddress.

^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$

To a trained eye this is likely as least readable enough, that they can guess that this parses an email address. But most people I work with, and myself included aren't going to be able to tell if there are any obvious bugs in this.

Coffeescript did something really well

While Coffeescript is frequently greeted with a grunt and moan around the people I know, it actually did many things better than javascript, before ES6 and all the new toys in JS started showing up a few years ago.

It allowed regular expressions to be broken up over multiple lines, AND it allowed comments. Take a look at the example from their docs.

Now it does come with a downside, and that is that it handles spaces in the regular expression special, which made some regular expressions hard to work with. But it was still a lot better than the example above.

Javascript can actually do this too (with a tiny bit of reusable code)

This example illustrates how to do this, by creating and using a method called multilineRegex, to make a new RegExp for parsing an AWS secret ARN.

const multilineRegex = (...parts) =>

new RegExp(parts.map(x => (x instanceof RegExp) ? x.source : x).join(''));

// a variable I want to use as part of the regular expression

const partition = "aws-cn";

const re = multilineRegex(

/aws:/, // I can COMMENT!!!!

partition, // this is being pased in as a variable

/:secretsmanager:cn-north-1:111111111111:/

);

console.log({

a: re.test('aws:aws:secretsmanager:cn-north-1:111111111111:'),

b: re.test('aws:aws-cn:secretsmanager:cn-north-1:111111111111:')

});

The RegExp object in Javascript has an often neglected property called source, which returns the string version of a regular expression. What the multilineRegex method does, is loop through all the parameters passed into it, and combines them into a long regex string, which is then used to create a new RegExp object.

Since multilineRegex is a method call, it can have each part broken up over multiple lines, and that naturally allows it to also have comments.