How can I check either string is full domain or subdomain using regex.
like
abc.com | Full domain
xyz.abc.com | Subdomain
test.co.uk | Full domain
text.test.co.uk/ | Subdomain
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why do you want to use regex?Steve B– Steve B2012-06-25 14:19:01 +00:00Commented Jun 25, 2012 at 14:19
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1There are domains like xxx.com.hk, which to regex will look the same as xyz.abc.uk. Don't use REGEX!Anirudh Ramanathan– Anirudh Ramanathan2012-06-25 14:20:41 +00:00Commented Jun 25, 2012 at 14:20
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possible duplicate of Regex to extract subdomain from URL?Renatas M.– Renatas M.2012-06-25 14:21:17 +00:00Commented Jun 25, 2012 at 14:21
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then what you suggest how to achveive it?SOF User– SOF User2012-06-25 14:21:30 +00:00Commented Jun 25, 2012 at 14:21
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2 Answers
Recursively do a DNS lookup, while removing one section from the left at a time
subdomain
xyz.abc.com //DNS check PASS
abc.com //DNS check PASS
domain
xxx.com.hk //DNS check PASS
com.hk //DNS check FAIL
If there no nameservers/DNS resolution fails for a different reason, the other way to confirm if it is a subdomain or not is to use IANA's TLD (Top-Level Domain) Registry Here.
(USE CAUTIOUSLY)
There are several cases like com.com, com.google.com, and several other URLs using TLDs in their name.
answered Jun 25, 2012 at 14:23
Anirudh Ramanathan
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8 Comments
hometoast
+1 for DNS Solution. You can't say without a lookup what is or isn't a second level domain.
Ωmega
What if valid domain name has no name servers?
Anirudh Ramanathan
If a DNS check fails, there is only one other way. To check it against all the names in the IANA registry. Updating the answer.
Ωmega
But some tld(s) are actually indirect, such as
us.com, de.com, etc (see centralnic.com/names/domains), and IANA does not recognize them. Very well know such tlds are co.uk, co.jp, or com.br.Anirudh Ramanathan
In my UPDATE, if you see a no-match just once, it is a domain. And if you see a no-match more than once, it is a sub-domain.
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With Regex if not possible to check is a domain or subdomain. The most browser use for this problem the project of Mozilla - PublicSuffix. It is a maintained list of TLD.
You can use the following nuget package. (Install-Package Nager.PublicSuffix) https://www.nuget.org/packages/Nager.PublicSuffix/
Example:
var domainParser = new DomainParser();
var data = await domainParser.LoadDataAsync();
var tldRules = domainParser.ParseRules(data);
domainParser.AddRules(tldRules);
var domainName = domainParser.Get("sub.test.co.uk");
//domainName.Domain = "test";
//domainName.Hostname = "sub.test.co.uk";
//domainName.RegistrableDomain = "test.co.uk";
//domainName.SubDomain = "sub";
//domainName.TLD = "co.uk";
