I wrote the following regex:

(https?:\/\/)?([da-z\.-]+)\.([a-z]{2,6})(\/(\w|-)*)*\/?

Its behaviour can be seen here: /?34b8m

I wrote the following JavaScript code:

var urlexp = new RegExp(
    '^(https?:\/\/)?([da-z\.-]+)\.([a-z]{2,6})(\/(\w|-)*)*\/?$', 'gi'
);
document.write(urlexp.test("blaaa"))

And it returns true even though the regex was supposed to not allow single words as valid.

What am I doing wrong?

I wrote the following regex:

(https?:\/\/)?([da-z\.-]+)\.([a-z]{2,6})(\/(\w|-)*)*\/?

Its behaviour can be seen here: http://gskinner./RegExr/?34b8m

I wrote the following JavaScript code:

var urlexp = new RegExp(
    '^(https?:\/\/)?([da-z\.-]+)\.([a-z]{2,6})(\/(\w|-)*)*\/?$', 'gi'
);
document.write(urlexp.test("blaaa"))

And it returns true even though the regex was supposed to not allow single words as valid.

What am I doing wrong?

• javascript
• regex

• This is why I hate using the new RegExp Construct for Regular Expression initialization in JS. Every backslash has to be doubled. Try the exact same code but with var urlexp = /^(https?:\/\/)?([da-z\.-]+)\.([a-z]{2,6})(\/(\w|-)*)*\/?$/gi – FrankieTheKneeMan Commented Mar 30, 2013 at 8:53
• 3 Also, when using new RegExp, you don't have to escape your forward slashes - that's exclusively for with you're using /regex/mod notation (like you don't have to escape your single quotes in a double quoted string and vice versa), so var urlexp = new RegExp('^(https?://)?([da-z.-]+)\\.([a-z]{2,6})(/(\\w|-)*)*/?$', 'gi'); will work as well. – FrankieTheKneeMan Commented Mar 30, 2013 at 8:56
• possible duplicate of Why this javascript regex doesn't work? – Jerry Commented Mar 31, 2014 at 7:16

1 Answer 1

Your problem is that JavaScript is viewing all your escape sequences as escapes for the string. So your regex goes to memory looking like this:

^(https?://)?([da-z.-]+).([a-z]{2,6})(/(w|-)*)*/?$

Which you may notice causes a problem in the middle when what you thought was a literal period turns into a regular expressions wildcard. You can solve this in a couple ways. Using the forward slash regular expression syntax JavaScript provides:

var urlexp = /^(https?:\/\/)?([da-z\.-]+)\.([a-z]{2,6})(\/(\w|-)*)*\/?$/gi

Or by escaping your backslashes (and not your forward slashes, as you had been doing - that's exclusively for when you're using /regex/mod notation, just like you don't have to escape your single quotes in a double quoted string and vice versa):

var urlexp = new RegExp('^(https?://)?([da-z.-]+)\\.([a-z]{2,6})(/(\\w|-)*)*/?$', 'gi')

Please note the double backslash before the w - also necessary for matching word characters.

A couple notes on your regular expression itself:

[da-z.-]

d is contained in the a-z range. Unless you meant \d? In that case, the slash is important.

(/(\w|-)*)*/?

My own misgivings about the nested Kleene stars aside, you can whittle that alternation down into a character class, and drop the terminating /? entirely, as a trailing slash will be match by the group as you've given it. I'd rewrite as:

(/[\w-]*)*

Though, maybe you'd just like to catch non space characters?

(/[^/\s]*)*

Anyway, modified this way your regular expression winds up looking more like:

^(https?://)?([\da-z.-]+)\.([a-z]{2,6})(/[\w-]*)*$

Remember, if you're going to use string notation: Double EVERY backslash. If you're going to use native /regex/mod notation (which I highly remend), escape your forward slashes.