Question

I want to pull a number out the middle of a string in JavaScript. In Ruby (my main language) I would do this:

Ruby:

name = "users[107][teacher_type]"

num = name.scan(/\d+/).first

But in JavaScript I have to do this, which seems a bit clunky.

JavaScript:

var name = "users[107][teacher_type]"

var regexp = new RegExp(/\d+/)

var num = regexp.exec(name)[0]

Is there way to pull out the matching parts without building a RegExp object? I.e. a one-liner equivalent of Ruby’s String#scan?

Also, as a side note, since this string will always have the same format, I could potentially do it using .replace. This isn’t as clever a solution but again I have problems in JavaScript.

In Ruby:

num = name.gsub(/users\[|\]\[teacher_type\]/,"")

But when I try this in JavaScript it doesn’t like the or (|) in the middle of the regex:

In JavaScript:

//works

num = name.replace(/users\[/, "").replace(/\]\[teacher_type\]/,"")

//doesn't work

num = name.gsub(/users\[|\]\[teacher_type\]/,"")

Can anyone set me straight?

Answer 1

You only need to use the new RegExp() part when creating dynamic regular expressions. You can use literals at other times. /\d+/ is the equivalent of new RegExp("\\d+"). Note that you have to escape special characters when using the latter.

Also noteworthy is that String#match returns null or an array. This is not apparent based on the supplied solutions (parseInt(name.match(/\d+/), 10)). It just so happens that it is converted to a string when passed to parseInt. ParseInt converts string values to integers (when possible).

name.match(/\d+/)[0]
/\d+/.exec(name)[0]

Those two are functionally identical in this case.

The other match you were referring to (the negative matching) requires a special flag. To duplicate the functionality of gsub you need to tell the regex to be applied more than once with the g flag.

'users[107][teacher_type]'.replace(/users\[|\]\[teacher_type\]/g,'')

Or if you had to use new RegExp for some reason you’d accomplish the same as above like so:

‘users[107][teacher_type]’.replace(new RegExp(‘users\[|\]\[teacher_type\]’, ‘g’),”)

Notice again how I had to escape all the backslashes. Mozilla’s Developer Center is a good reference to familiarize yourself with regex in javascript.