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JS regex to get domain name from an email [duplicate]

How can I extract only top-level and second-level domain from a URL using regex? I want to skip all lower level domains. Any ideas?

Here's my idea,
Match anything that isn't a dot, three times, from the end of the line using the $ anchor.
The last match from the end of the string should be optional to allow for .com.au or .co.nz type of domains.
Both the last and second last matches will only match 2-3 characters, so that it doesn't confuse it with a second-level domain name.
Regex:
[^.]*\.[^.]{2,3}(?:\.[^.]{2,3})?$
Demonstration:
Regex101 Example

Updated 2019
This is an old question, and the challenge here is a lot more complicated as we start adding new vanity TLDs and more ccTLD second level domains (e.g. .co.uk, .org.uk). So much so, that a regular expression is almost guaranteed to return false positives or negatives.
The only way to reliably get the primary host is to call out to a service that knows about them, like the Public Suffix List.
There are several open-source libraries out there that you can use, like psl, or you can write your own.
Usage for psl is quite intuitive. From their docs:
var psl = require('psl');
// Parse domain without subdomain
var parsed = psl.parse('google.com');
console.log(parsed.tld); // 'com'
console.log(parsed.sld); // 'google'
console.log(parsed.domain); // 'google.com'
console.log(parsed.subdomain); // null
// Parse domain with subdomain
var parsed = psl.parse('www.google.com');
console.log(parsed.tld); // 'com'
console.log(parsed.sld); // 'google'
console.log(parsed.domain); // 'google.com'
console.log(parsed.subdomain); // 'www'
// Parse domain with nested subdomains
var parsed = psl.parse('a.b.c.d.foo.com');
console.log(parsed.tld); // 'com'
console.log(parsed.sld); // 'foo'
console.log(parsed.domain); // 'foo.com'
console.log(parsed.subdomain); // 'a.b.c.d'
Old answer
You could use this:
(\w+\.\w+)$
Without more details (a sample file, the language you're using), it's hard to discern exactly whether this will work.
Example: http://regex101.com/r/wD8eP2

Also, you can likely do that with some expression similar to,
^(?:https?:\/\/)(?:w{3}\.)?.*?([^.\r\n\/]+\.)([^.\r\n\/]+\.[^.\r\n\/]{2,6}(?:\.[^.\r\n\/]{2,6})?).*$
and add as much as capturing groups that you want to capture the components of a URL.
Demo
If you wish to simplify/modify/explore the expression, it's been explained on the top right panel of regex101.com. If you'd like, you can also watch in this link, how it would match against some sample inputs.
RegEx Circuit
jex.im visualizes regular expressions:

For anyone using JavaScript and wanting a simple way to extract the top and second level domains, I ended up doing this:
'example.aus.com'.match(/\.\w{2,3}\b/g).join('')
This matches anything with a period followed by two or three characters and then a word boundary.
Here's some example outputs:
'example.aus.com' // .aus.com
'example.austin.com' // .austin.com
'example.aus.com/howdy' // .aus.com
'example.co.uk/howdy' // .co.uk
Some people might need something a bit cleverer, but this was enough for me with my particular dataset.
Edit
I've realised there are actually quite a few second-level domains which are longer than 3 characters (and allowed). So, again for simplicity, I just removed the character counting element of my regex:
'example.aus.com'.match(/\.\w*\b/g).join('')

Since TLDs now include things with more than three-characters like .wang and .travel, here's a regex that satisfies these new TLDs:
([^.\s]+\.[^.\s]+)$
Strategy: starting at the end of the string, look for one or more characters that aren't periods or whitespace, followed by a single period, followed by one or more characters that aren't periods or whitespace.
http://regexr.com/3bmb3

With capturing groups you can achieve some magix.
For example, consider the following javascript:
let hostname = 'test.something.else.be';
let domain = hostname.replace(/^.+\.([^\.]+\.[^\.]+)$/, '$1');
document.write(domain);
This will result in a string containing 'else.com'. This is because the regex itself will match the complete string and the capturing group will be mapped to $1. So it replaces the complete string 'test.something.else.com' with '$1' which is actually 'else.com'.
The regex isn't pretty and can probably be made more dynamic with things like {3} for defining how many levels deep you want to look for subdomains, but this is just an illustration.

if you want all specific Top Level Domain name then you can write regular expression like this:
[RegularExpression("^(https?:\\/\\/)?(([\\w]+)?\\.?(\\w+\\.((za|zappos|zara|zero|zip|zippo|zm|zone|zuerich|zw))))\\/?$", ErrorMessage = "Is not a valid fully-qualified URL.")]
You can also put more domain name from this link:
https://www.icann.org/resources/pages/tlds-2012-02-25-en

The following regex matches a domain with root and tld extractions (named capture groups) from a url or domain string:
(?:\w+:\/{2})?(?<cs_domain>(?<cs_domain_sub>(?:[\w\-]+\.)*?)(?<cs_domain_root>[\w\-]+(?<cs_domain_tld>(?:\.\w{2})?(?:\.\w{2,3}|\.xn-+\w+|\.site|\.club))))\|
It's hard to say if it is perfect, but it works on all the test data sets that I have put it against including .club, .xn-1234, .co.uk, and other odd endings. And it does it in 5556 steps against 40k chars of logs, so the efficiency seems reasonable too.

If you need to be more specific:
/\.(?:nl|se|no|es|milru|fr|es|uk|ca|de|jp|au|us|ch|it|io|org|com|net|int|edu|mil|arpa)/
Based on http://www.seobythesea.com/2006/01/googles-most-popular-and-least-popular-top-level-domains/

Javascript RegEx to match URL's but exclude images

I need to replace all text links in a string of HTML text by actual clickable links. Works fine with the following RegEx:
/\b(https?|ftp|file):\/\/[-A-Z0-9+&##\/%?=~_|!:,.;]*[-A-Z0-9+&##\/%=~_|])/gi
I then noticed it also replaces images and already formatted links. Figures I need to exclude links preceded by src" and > ... I searched a bit and read a lot on negative lookahead in many questions answered here. I tried this (added something right after the first /):
/(^(?!src="|>)\b(https?|ftp|file):\/\/[-A-Z0-9+&##\/%?=~_|!:,.;]*[-A-Z0-9+&##\/%=~_|])/gi
But this doesn't match any link anymore. I tried several similar statements, without the ^, changing some brackets, etc etc, but simply nothing seems to work. I tried putting .{0} in between the part I added and \b, to make sure he would only look at stuff right in front of the url and not consider anything farther away.

EDIT: The discussion was getting long, so I decided to update the answer instead.
Trusting that your original regex works, I'm just going to refer to a simplified version through the rest of this answer:
/\b(https?|ftp|file)/gi
Now, you attempted this:
/^(?!src="|>)\b(https?|ftp|file)/gi
^
The main error here is marked by a caret: the caret. That forces your regex to match from the beginning of the line, which is why it matched nothing. Let's remove that and move on:
/(?!src="|>)\b(https?|ftp|file)/gi
The main error, this time, is in your conception of lookahead assertions. As I explained in the comments, this assertion is redundant, because you are saying, "Match http or https or ftp or file, as long as none of these are src=" or >." It's almost so redundant that the sentence doesn't even make sense to us! What you want, instead, is a lookbehind assertion:
/(?<!src="|>)\b(https?|ftp|file)/gi
^
Why? Because you wish to find src=" or > behind the string you potentially wish to match. The problem? JavaScript doesn't support lookbehind assertions. So, I suggested an alternative. Admittedly, it was flawed (although not the cause of the HTML breaking, as you brought up). Here it is, fixed:
/(.[^>"]|[^=]")\b(https?|ftp|file)/gi
^^^^^^^^^^^^
This is indeed a non-intuitive regex, and warrants explanation. It splits our cases into two. Say we have a two-character set. If the set doesn't end in > or ", then we're not suspicious of it; we're good to go; match any URL that might follow. However, if it does end in > or ", well, the only "forgivable" case is where the first character is not an =. So you see, a bit of logic trickery here.
Now, as for why this might break your HTML. Be sure to use JavaScript's replace, and substitute the first captured group back into the page! If you simply substitute each match with nothingness, you end up "eating up" the two-character sets, which we only meant to investigate, not destroy.
html.replace(/(.[^>"]|[^=]")\b(https?|ftp|file)/gi,
function(match, $1, offset, original) {
return $1;
});

I have to go home and haven't tested yet, but I'd feel more comfortable dealing with the easier task of isolating HTML you don't want out first.
Do a match to get an array of the stuff you don't want to deal with.
Rip it all out with a split.
Iterate the split array and replace URLs and then splice matched items back in
Join and return
The only assumption is that you don't end on an anchor or img tag in your text
function zipperParse(htmlText,matcher){
var zipBackInArray = htmlText.match(matcher),
workingArray = htmlText.split(matcher),
i = workingArray.length;
while(i--){
buildAnchorTagIfURLPresent(workingArray[i]); //You got this one covered
workingArray.splice(i,0,zipBackInArray.pop());
//working backwards makes splice much easier to use here
}
return workingArray.join('');
}
var toExclude = /<a[^>]*>[^>]*>|<img[^>]*>/g;
// is supposed to match all img and anchor pairs but not handling tags inside anchors yet
zipperParse(yourHtmlText,toExclude);

this code works for me... just change the Google Api KEY to exclude..=> XXXXXXXXXXXXXXXXXXXXXX i just put it in my functions.php theme of my wordpress. The first thing is to see, how your google maps code appears on your site, and then it is to match it to what is replaced.
function remove_script_version( $src ) {
$parts1 = explode( '?', $src );
$parts2 = str_replace('//maps.googleapis.com/maps/api/js', '//maps.googleapis.com/maps/api/js?language=es&v=3.31&libraries=places&key=XXXXXXXXXXXXXXXXXXXXXX&ver=3.31', $parts1);
return $parts2[0]; }
add_filter( 'script_loader_src', 'remove_script_version', 15, 1 );
add_filter( 'style_loader_src', 'remove_script_version', 15, 1 );

Stop matching when a certain character is reached

I've the following regex which needs to stop matching when it encounters a hash.
Regex:
/[?&]+([^=&]+)=([^&]*)/gi
URL Sample:
http://website.com/1068?page=4&taco=cat#tasty
The above regex will capture cat#tasty instead of just cat in the last capture group. I attempted the following which works ONLY if a hash is present.
Regex Test:
/[?&]+([^=&]+)=([^&]*)#/gi
If the url doesn't have a hash, it won't match. making the hash optional — #? — doesn't work either as the greedy * of the last capture group still grabs cat#tasty.

A little-known way to parse URLs in JavaScript is to simply create an a element and give it the url as the href attribute!
var link=document.createElement('a')
link.href="http://website.com/1068?page=4&taco=cat#tasty"
alert(link.search) //?page=4&taco=cat
alert(link.hash) //#tasty
Just tossing this out there. If you do your regex on just link.search (or perhaps link.search.substr(1)) you won't have to worry about ever matching anything but parameters.

/[?&]+([^=&]+)=([^&#]*)/gi
Although as Ray pointed out, there are many url parsers available.

Regular expression for detecting hyperlinks

I've got this regex pattern from WMD showdown.js file.
/<((https?|ftp|dict):[^'">\s]+)>/gi
and the code is:
text = text.replace(/<((https?|ftp|dict):[^'">\s]+)>/gi,"$1");
But when I set text to http://www.google.com, it does not anchor it, it returns the original text value as is (http://www.google.com).
P.S: I've tested it with RegexPal and it does not match.

Your code is searching for a url wrapped in <> like: <http://www.google.com>: RegexPal.
Just change it to /((https?|ftp|dict):[^'">\s]+)/gi if you don't want it to search for the <>: RegexPal

As long as you know your url's start with http:// or https:// or whatever you can use:
/((https?|s?ftp|dict|www)(://)?)[A-Za-z0-9.\-]+)/gi
The expression will match till it encounters a character not allowed in the URL i.e. is not A-Za-z\.\-. It will not however detect anything of the form google.com or anything that comes after the domain name like parameters or sub directory paths etc. If that is your requirement that you can simply choose to terminate the terminating condition as you have above in your regex.
I know it seems pointless but it may be useful if you want the display name to be something abbreviated rather than the whole url in case of complex urls.

You could use:
var re = /(http|https|ftp|dict)(:\/\/\S+?)(\.?\s|\.?$)/gi;
with:
el.innerHTML = el.innerHTML.replace(re, '<a href=\'$1$2\'>$1$2<\/a>$3');
to also match URLs at the end of sentences.
But you need to be very careful with this technique, make sure the content of the element is more or less plain text and not complex markup. Regular expressions are not meant for, nor are they good at, processing or parsing HTML.

Building a Hashtag in Javascript without matching Anchor Names, BBCode or Escaped Characters

I would like to convert any instances of a hashtag in a String into a linked URL:
#hashtag -> should have "#hashtag" linked.
This is a #hashtag -> should have "#hashtag" linked.
This is a [url=http://www.mysite.com/#name]named anchor[/url] -> should not be linked.
This isn't a pretty way to use quotes -> should not be linked.
Here is my current code:
String.prototype.parseHashtag = function() {
return this.replace(/[^&][#]+[A-Za-z0-9-_]+(?!])/, function(t) {
var tag = t.replace("#","")
return t.link("http://www.mysite.com/tag/"+tag);
});
};
Currently, this appears to fix escaped characters (by excluding matches with the amperstand), handles named anchors, but it doesn't link the #hashtag if it's the first thing in the message, and it seems to grab include the 1-2 characters prior to the "#" in the link.
Halp!

How about the following:
/(^|[^&])#([A-Za-z0-9_-]+)(?![A-Za-z0-9_\]-])/g
matches the hashtags in your example. Since JavaScript doesn't support lookbehind, it tries to either match the start of the string or any character except & before the hashtag. It captures the latter so it can later be replaced. It also captures the name of the hashtag.
So, for example:
subject.replace(/(^|[^&])#([A-Za-z0-9_-]+)(?![A-Za-z0-9_\]-])/g, "$1http://www.mysite.com/tag/$2");
will transform
#hashtag
This is a #hashtag and this one #too.
This is a [url=http://www.mysite.com/#name]named anchor[/url]
This isn't a pretty way to use quotes
into
http://www.mysite.com/tag/hashtag
This is a http://www.mysite.com/tag/hashtag and this one http://www.mysite.com/tag/too.
This is a [url=http://www.mysite.com/#name]named anchor[/url]
This isn't a pretty way to use quotes
This probably isn't what t.link() (which I don't know) would have returned, but I hope it's a good starting point.

There is an open-source Ruby gem to do this sort of thing (hashtags and #usernames) called twitter-text. You might get some ideas and regexes from that, or try out this JavaScript port.
Using the JavaScript port, you'll want to just do:
var linked = TwitterText.auto_link_hashtags(text, {hashtag_url_base: "http://www.mysite.come/tag/"});

Tim, your solution was almost perfect. Here's what I ended up using:
subject.replace(/(^| )#([A-Za-z0-9_-]+)(?![A-Za-z0-9_\]-])/g, "$1#$2");
The only change is the first conditional, changed it to match the beginning of the string or a space character. (I tried \s, but that didn't work at all.)