What is a regular expression which will match a valid domain name without a subdomain

Question

I need to validate a domain name   google com  stackoverflow com  So a domain in its rawest form - not even a subdomain like www    Characters should only be a-z   A-Z   0-9 and period    and dash -  The domain name part should not start or end with dash  -   e g  -google- com  The domain name part should be between 1 and 63 characters long The extension  TLD  can be anything under  1 rules for now  I may validate them against a list later  it should be 1 or more characters though   Edit  TLD is apparently 2-6 chars as it stands  no  4 revised  TLD should actually be labelled  subdomain  as it should include things like  co uk -- I would imagine the only validation possible  apart from checking against a list  would be  after the first dot there should be one or more characters under rules  1  Thanks very much  believe me I did try

User · Answer

I know that this is a bit of an old post, but all of the regular expressions here are missing one very important component: the support for IDN domain names.

IDN domain names start with xn--. They enable extended UTF-8 characters in domain names. For example, did you know "?.com" is a valid domain name? Yeah, "love heart dot com"! To validate the domain name, you need to let http://xn--c6h.com/ pass the validation.

Note, to use this regex, you will need to convert the domain to lower case, and also use an IDN library to ensure you encode domain names to ACE (also known as "ASCII Compatible Encoding"). One good library is GNU-Libidn.

idn(1) is the command line interface to the internationalized domain name library. The following example converts the host name in UTF-8 into ACE encoding. The resulting URL https://nic.xn--flw351e/ can then be used as ACE-encoded equivalent of https://nic.??/.

  $ idn --quiet -a nic.??
  nic.xn--flw351e

This magic regular expression should cover most domains (although, I am sure there are many valid edge cases that I have missed):

^((?!-))(xn--)?[a-z0-9][a-z0-9-_]{0,61}[a-z0-9]{0,1}\.(xn--)?([a-z0-9\-]{1,61}|[a-z0-9-]{1,30}\.[a-z]{2,})$

When choosing a domain validation regex, you should see if the domain matches the following:

xn--stackoverflow.com
stackoverflow.xn--com
stackoverflow.co.uk

If these three domains do not pass, your regular expression may be not allowing legitimate domains!

Check out The Internationalized Domain Names Support page from Oracle's International Language Environment Guide for more information.

Feel free to try out the regex here: http://www.regexr.com/3abjr

ICANN keeps a list of tlds that have been delegated which can be used to see some examples of IDN domains.

Edit:

 ^(((?!-))(xn--|_{1,1})?[a-z0-9-]{0,61}[a-z0-9]{1,1}\.)*(xn--)?([a-z0-9][a-z0-9\-]{0,60}|[a-z0-9-]{1,30}\.[a-z]{2,})$

This regular expression will stop domains that have '-' at the end of a hostname as being marked as being valid. Additionally, it allows unlimited subdomains.

User · Answer

a-zA-Z0-9  a-zA-Z0-9-  1 61  a-zA-Z0-9    a-zA-Z      a-zA-Z

User · Answer

Just a minor correction - the last part should be up to 6  Hence     a-z0-9     -    1  a-z0-9       a-z  2 6     The longest TLD is museum  6 chars  - http   en wikipedia org wiki List of Internet top-level domains

User · Answer

For new gTLDs        -   p L  p N -     lt  -       p L  p N   2    iu

User · Answer

-   xn--   a-z0-9  a-z0-9-   0 61  a-z0-9  0       xn--    a-z0-9 -   1 61   a-z0-9-  0 30     a-z-1-9  2       will validate such domains as           after encoding    https   regex101 com r Hf8wFM 1 - sandbox

User · Answer

a-z0-9     -    1  a-z0-9       a-z  2 7      domain - lower case letters and 0-9 only   can have a hyphen     TLD - lower case only  must be beween 2 and 7 letters long   http   rubular com  is brilliant for testing regular expressions   Edit  Updated TLD maximum to 7 characters for   rentals  as Dan Caddigan pointed out

User · Answer

Not enough rep yet to comment  In response to paka s solution  I found I needed to adjust three items    The dash and underscore were moved due to the dash being interpreted as a range  as in  0-9   Added a full stop for domain names with many subdomains Extended the potential length for the TLDs to 13   Before       a-zA-Z  1     a-zA-Z  1  a-zA-Z  1     a-zA-Z  1  0-9  1     0-9  1  a-zA-Z  1     a-zA-Z0-9  a-zA-Z0-9-   1 61  a-zA-Z0-9       a-zA-Z  2 6   a-zA-Z0-9-  2 30    a-zA-Z  2 3      After       a-zA-Z  1     a-zA-Z  1  a-zA-Z  1     a-zA-Z  1  0-9  1     0-9  1  a-zA-Z  1     a-zA-Z0-9  -   a-zA-Z0-9  1 61  a-zA-Z0-9       a-zA-Z  2 13   a-zA-Z0-9-  2 30    a-zA-Z  2 3

User · Answer

As already pointed out it s not obvious to tell subdomains in the practical sense  e g   co uk domains   We use this regex to validate domains which occur in the wild  It covers all practical use cases I know of  New ones are welcome  According to our guidelines it avoids non-capturing groups and greedy matching                       d w        -  w d   -         w d    -       d w   -          w d       w d   -        w d   -         w d   -  254             w d -       w d -  64          w d   -      lt    w d -         d       lt    w d -  2      lt    w d -  25     Proof  explanation and examples  https   regex101 com r FLA9Bv 9  Note  currently only works in Chrome because the regex uses lookbehinds which are only supported in ECMA2018   There re two approaches to choose from when validating domains   By-the-books FQDN matching  theoretical definition  rarely encountered in practice     max 253 character long  as per RFC-1035 3 1  RFC-2181 11  max 63 character long per label  as per RFC-1035 3 1  RFC-2181 11  any characters are allowed  as per RFC-2181 11  TLDs cannot be all-numeric  as per RFC-3696 2  FQDNs can be written in a complete form  which includes the root zone  the trailing dot    Practical   conservative FQDN matching  practical definition  expected and supported in practice     by-the-books matching with the following exceptions additions valid characters   a-zA-Z0-9 -  labels cannot start or end with hyphens  as per RFC-952 and RFC-1123 2 1  TLD min length is 2 character  max length is 24 character as per currently existing records don t match trailing dot

User · Answer

Well  it s pretty straightforward a little sneakier than it looks  see comments   given your specific requirements      a-zA-Z0-9  a-zA-Z0-9-  1 61  a-zA-Z0-9    a-zA-Z  2       But note this will reject a lot of valid domains

User · Answer

Thank you for pointing right direction in domain name validation solutions in other answers  Domain names could be validated in various ways    If you need to validate IDN domain in it s human readable form  regex  p L  will help  This allows to match any character in any language    Note that last part might contain hyphens too  As punycode encoded Chineese names might have unicode characters in tld   I ve came to solution which will match for example    google com maselkowski pl maselkowski pl m maselkowski pl www maselkowski pl com xn--masekowski-d0b pl               xn--fiqa61au8b7zsevnm8ak20mc4a87e xn--fiqs8s   Regex is     0-9 p L   0-9 p L -    1 61  0-9 p L     0-9 p L    p L -   0-9 p L       Check and tune here  NOTE  This regexp is quite permissive  as is current domain names allowed character set   UPDATE  Even more simplified  as a-aA-Z p L  is same as just  p L   NOTE2  The only problem is that it will match domains with double dots in it      like maselk  owski pl  If anyone know how to fix this please improve

User · Answer

Accepted answer not working for me  try this            -  A-Za-z0-9-  1 63    lt  -      A-Za-z  2 6     Visit this Unit Test Cases for validation

User · Answer

My bet        a-z0-9     a-z0-9-  0 61  a-z0-9        a-z0-9  a-z0-9-  0 61  a-z0-9     Explained   Domain name is built from segments  Here is one segment  except final     a-z0-9     a-z0-9-  0 61  a-z0-9      It can have 1-63 characters  does not start or end with  -    Now append     to it and repeat at least one time       a-z0-9     a-z0-9-  0 61  a-z0-9          Then attach final segment  which is 2-63 characters long    a-z0-9  a-z0-9-  0 61  a-z0-9    Test it here  http   regexr com 3au3g

User · Answer

a-zA-Z0-9  -a-zA-Z0-9   a-zA-Z0-9   a-z  2 3    a-z  2 3      a-z  2 3       Examples that work   stack com sta-ck com sta---ck com 9sta--ck com sta--ck9 com stack99 com 99stack com sta99ck com   It will also work for extensions   com uk  co in  uk edu in   Examples that will not work   -stack com   it will work even with the longest domain extension   versicherung

User · Answer

This answer is for domain names  including service RRs   not host names  like an email hostname          1 253            -         A-Za-z0-9-   1 63    lt  -           2      It is basically mkyong s answer and additionally    Max length of 255 octets including length prefixes and null root  Allow trailing     for explicit dns root  Allow leading     for service domain RRs   bugs  doesn t enforce 15 char max for   labels  nor does it require at least one domain above service RRs  Matches all possible TLDs  Doesn t capture subdomain labels    By Parts  Lookahead  limit max length between    to 253 characters with optional trailing literal           1 253         Lookahead  next character is not a  -  and no     follows any characters before the next      That is to say  enforce that the first character of a label isn t a  -  and only the first character may be a          -           Between 1 and 63 of the allowed characters per label    A-Za-z0-9-   1 63    Lookbehind  previous character not  -   That is to say  enforce that the last character of a label isn t a  -       lt  -    Force a     at the end of every label except the last  where it is optional              Mostly combined from above  this requires at least two domain levels  which is not quite correct  but usually a reasonable assumption  Change from  2   to   if you want to allow TLDs or unqualified relative subdomains through  eg  localhost  myrouter  to          -         A-Za-z0-9-   1 63    lt  -           2     Unit tests for this expression

User · Answer

The following regex extracts the sub  root and tld of a given domain       lt domain gt    lt domain sub gt                s   -             s              lt domain root gt           s     n       lt domain tld gt    xn--    w-  2 7       a-zA-Z-  2 3          Tested for the following domains     stack com   sta-ck com   sta---ck com   9sta--ck com   sta--ck9 com   stack99 com   99stack com   sta99ck com   google com uk   google co in    google com   maselkowski pl   maselkowski pl   m maselkowski pl   www maselkowski pl com   xn--masekowski-d0b pl   xn--fiqa61au8b7zsevnm8ak20mc4a87e xn--fiqs8s    xn--stackoverflow com   stackoverflow xn--com   stackoverflow co uk

User · Answer

localhost      -  A-Za-z0-9-  1 63    lt  -      A-Za-z  2 253      Thank you  mkyong for the basis for my answer  I ve modified it to support longer acceptable labels   Also   localhost  is technically a valid domain name  I will modify this answer to accommodate internationalized domain names

User · Answer

Here is complete code with example    lt  php function is domain  url         parse   parse url  url       if  isset  parse  host                domain    parse  host          else            domain    url             return preg match        -     a-zA-Z d -  0 62  a-zA-Z d     1 126     d   a-zA-Z d  1 63       domain      echo is domain  example com      true echo is domain  https   example com      true echo is domain  https    example com      false echo is domain  https   localhost      false

User · Answer

My RegEx is next     a-zA-Z0-9  a-zA-Z0-9-   0 61  a-zA-Z0-9  0 1     a-zA-Z  1 6   a-zA-Z0-9-  1 30    a-zA-Z  2 3      it s ok for i oh1 me and for wow british-library uk  UPD  Here is updated rule      a-zA-Z  1     a-zA-Z  1  a-zA-Z  1     a-zA-Z  1  0-9  1     0-9  1  a-zA-Z  1     a-zA-Z0-9  a-zA-Z0-9-   1 61  a-zA-Z0-9       a-zA-Z  2 6   a-zA-Z0-9-  2 30    a-zA-Z  2 3        https   www debuggex com r y4Xe hDVO11bv1DV  now it check for - or   in the start or end of domain label

User · Answer

I did the below to simple fetch the domain along with the protocol  Example  https   www facebook com profile user  ftp   182 282 34 337 movies M use the below Regex pattern    a-zA-Z0-9          will get you the output   https   www facebook com  ftp   192 282 34 337

User · Answer

a-zA-Z  1 2     0-9  1 2     a-zA-Z0-9  1 2     a-zA-Z0-9  a-zA-Z0-9-  1 61  a-zA-Z0-9        a-zA-Z  2 6         a-zA-Z  1 2   -  for accepting only two characters    0-9  1 2  -  for accepting two numbers only   if anything exceeds beyond two   a-zA-Z0-9  a-zA-Z0-9-  1 61  a-zA-Z0-9   this regex will take care of that   If we want to do the matching for at least one time   will be used

User · Answer

Quite simple  quite permissive  It will have false positives like -notvalid at-all  but it won t have false negatives      0-9a-z-         i  It makes sure it has a sequence of letters numbers and dashes that could end with a dot  and following it  any number of those kind of sequences  The things I like about this regexp  it s short  maybe the shortest here   easily understandable  and good enough for validating user input errors in the client side

[regex] What is a regular expression which will match a valid domain name without a subdomain?

Examples related to regex

Examples related to validation

Examples related to domain-name