Regex Confusion JavaScript

Question

I've been trying to match the following the path below with a regex:

path = "/r/356363/"

that looks like:

regex = RegExp("^\/r\/\d+\/$")

However, whenever I run test, I do

regex.test(path)

which returns

false

Why doesn't the regex match the path?

To me the regex says, find a string that starts with a /r/, has atleast one digit after, and ends in a /.

I checked the regex in http://regex101.com/#javascript and it appears as a match which leads me more confused.

Do this in the console and see why: regex = RegExp("^\/r\/\d+\/$") — Rudie
– Rudie, Commented Oct 28, 2014 at 21:38

David Thomas · Accepted Answer · 2014-10-28 22:18:39Z

4

The problem is you need to escape the escapes; the first \ escapes the following character in the string, you need a second \ to escape the subsequent \ for the RegExp itself:

var path = "/r/356363/",
    regex = RegExp("^\\/r\\/\\d+/$");
console.log(regex.test(path));

var path = "/r/356363/",
    regex = RegExp("^\\/r\\/\\d+\\/$");
console.log(regex.test(path));

edited Oct 28, 2014 at 22:18

answered Oct 28, 2014 at 21:38

David Thomas

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5 Comments

Rudie Over a year ago

Wow, that snippet stuff doesn't make the markup more readable... Or editable.

Gergo Erdosi Over a year ago

There is no need to escape /, it's not a special character. This should be enough: var regex = new RegExp("^/r/\\d+/$").

David Thomas Over a year ago

@Rudie: I'm not sure how much it's meant to, really; but yes: it's slightly awkward to play with it (unlike JS Fiddle, JS Bin et al).

Gergo Erdosi Over a year ago

@DavidThomas Why would it do that? The regex is between quotes, it is treated as a string.

David Thomas Over a year ago

@Gergo: finally edited the answer to remove the unnecessary escapes. Thanks! :)

Greg · Accepted Answer · 2014-10-28 22:35:39Z

2

RegExp("^\/r\/\d+\/$").test('/r/124/') === false

but if you use the regex literal writing:

/^\/r\/\d+\/$/.test('/r/124/') === true

While the forward slashes need to be escaped in the literal version, they don't need to be in the object version. However, backward slashes need to be escaped in the object version (not in the literal version). So you could also have written:

RegExp("^/r/\\d+/$").test('/r/124/') === true

The issue is that you have used a syntax that works only with the literal version.

answered Oct 28, 2014 at 22:35

Greg

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