Find it with class

I recently covered the very basics of regular expression syntax. Now I’ll describe a little bit about how regular expressions work in JavaScript.

The simplest method on the RegExp object is test which returns true if the string contains the specified regular expression.

/[A-Z]+/.test('Foo') // true

The exec method returns an array with the first match or null if there was no match.

/[a-z]+.exec('foo bar') // returns Array['foo']

That array also contains the index of where the match was found (0 in this case) and the input string (‘foo bar’).

To do the match more than once, you can use the g flag and call exec repeatedly.

let lower = /[a-z]+/g
lower.exec('foo bar') // null
lower.exec('foo bar') // bar
lower.exec('foo bar') // foo

An easier way to get all of the matches is the use the match method on the string class. If you use the g flag, all of the matches are returned in an array.

'foo bar'.match(/a-z]+/g) // Array['foo', 'bar']

The matchAll method on the String class returns an iterable which can be very nice.

for (a of 'foo bar'.matchAll(/[a-z]+/g)) {
  console.log(a)
}

A good property of the matchAll method is that it uses lazy evaluation so that it can be very efficient if only a few initial matches are needed.

The replace method replaces the first match with a replacement string. If you set the g flag, all of the matches will be replaced.

'foo bar'.replace(/[a-z]+/g, 'x') // 'x x'

There are some more subtleties about the RegExp and String objects but this covers the very basics.

javascript