javascript regular expression not working

Question

I am trying to validate a textbox to ensure that a URL is written inside (minus the "http://" part). Here is my code:

var isUrl;
var regex = new RegExp("^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)");
var siteUrl = e.target.value;
if (siteUrl.match(regex)) {
   isUrl = true;
}
else {
   isUrl = false;
}

So my regular expression is ^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?). I was under the impression that this would do the following:

NOT match anything beginning with http.. which it does correctly
Allow an optional www. at the start
Accept 1 or more characters from a-z to be typed in
Accept a dot after those characters
Accept 2 or more characters following the dot, and
Allow an optional dot followed by two or more characters again.

In practice, the textbox accepts strings like "ashdiguasdf" and "aksjdnfa:://',~" which it should not do. Can anyone explain why?

Use a regular expression literal or double escape using a constructor. Also, you don't need the Negative Lookahead if you're anchoring your pattern, you can omit that part and make sure you use the end of string $ anchor as well. — hwnd
– hwnd, Commented Jun 24, 2015 at 15:13
I.e. use var regex = /^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)/i; (I think case-insensitive matching is necessary here, thus I added /i). — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jun 24, 2015 at 15:19
Is this for very specific URLs? Are URLs without digits not allowed? Check this out - URL regex is way more complicated than expected: stackoverflow.com/questions/161738/… — Mike Robinson
– Mike Robinson, Commented Jun 24, 2015 at 15:20

freefaller · Accepted Answer · 2015-06-24 15:22:22Z

2

The main problem is that you're using the \ character in a quoted string, which javascript will interpret as the start of a "control character" (such as \n for a newline, etc).

One option is to escape it by replacing \ with \\.

But the easiest solution is to use the following format...

var regex = new RegExp(/^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)/);

This also allows you to make it case insensitive (if you wish) by using the i character at the end, like this...

var regex = new RegExp(/^(?!http)(www\.)?(([a-z])+\.[a-z]{2,}(\.[a-z]{2,})?)/i);

As an extra bit, you're using more capture groups than really necessary. Your expression could also be written like this with the same result...

^(?!http)(?:www\.)?[a-z]+(?:\.[a-z]{2,}){1,2}

answered Jun 24, 2015 at 15:22

freefaller

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