Can I improve this regex check for valid domain names?

Question

So, I have been working on this domain name regular expression. So far, it seems to pick up domain names with SLDs and TLDs (with the optional ccTLD), but there is duplication of the TLD listing. Can this be refactored any further?

params[:domain_name].downcase.strip.match(/^[a-z0-9\-]{2,63}
\.((a[cdefgilmnoqrstuwxz]|aero|arpa)|(b[abdefghijmnorstvwyz]|biz)|
(c[acdfghiklmnorsuvxyz]|cat|com|coop)|d[ejkmoz]|(e[ceghrstu]|edu)|f[ijkmor]|
(g[abdefghilmnpqrstuwy]|gov)|h[kmnrtu]|(i[delmnoqrst]|info|int)|
(j[emop]|jobs)|k[eghimnprwyz]|l[abcikrstuvy]|
(m[acdghklmnopqrstuvwxyz]|me|mil|mobi|museum)|(n[acefgilopruz]|name|net)|(om|org)|
(p[aefghklmnrstwy]|pro)|qa|r[eouw]|s[abcdeghijklmnortvyz]|
(t[cdfghjklmnoprtvwz]|travel)|u[agkmsyz]|v[aceginu]|w[fs]|y[etu]|z[amw])
(\.((a[cdefgilmnoqrstuwxz]|aero|arpa)|(b[abdefghijmnorstvwyz]|biz)|
(c[acdfghiklmnorsuvxyz]|cat|com|coop)|d[ejkmoz]|(e[ceghrstu]|edu)|f[ijkmor]|
(g[abdefghilmnpqrstuwy]|gov)|h[kmnrtu]|(i[delmnoqrst]|info|int)|
(j[emop]|jobs)|k[eghimnprwyz]|l[abcikrstuvy]|
m[acdghklmnopqrstuvwxyz]|mil|mobi|museum)|
(n[acefgilopruz]|name|net)|(om|org)|
(p[aefghklmnrstwy]|pro)|qa|r[eouw]|s[abcdeghijklmnortvyz]|
(t[cdfghjklmnoprtvwz]|travel)|u[agkmsyz]|v[aceginu]|w[fs]|y[etu]|z[amw]))?$/)

Answer 1

Please, please, please don't use a fixed and horribly complicated regex like this to match for known domain names.

The list of TLDs is not static, particularly with ICANN looking at a streamlined process for new gTLDs. Even the list of ccTLDs changes sometimes!

Have a look at the list available from http://publicsuffix.org/ and write some code that's able to download and parse that list instead.

Answer 2

Download this: http://data.iana.org/TLD/tlds-alpha-by-domain.txt

Example usage (in Python):

import re
def validate(domain):
    valid_domains = [ line.upper().replace('.', '\.').strip() 
                      for line in open('domains.txt') 
                      if line[0] != '#' ]
    r = re.compile(r'^[A-Z0-9\-]{2,63}\.(%s)$' % ('|'.join(valid_domains),))
    return True if r.match(domain.upper()) else False


print validate('stackoverflow.com')
print validate('omnom.nom')

You can factor the domain-list-building out of the validate function to help performance.

Answer 3

I don't know enough about domain names probably. But why is domains like "foo.info.com" matched? It seems that the domain name is "info.com" in that particular case.

And you might want to make sure the name starts with [a-z\d]. I don't think you can register a domain that starts with a dash?

Answer 4

Well as you have it written, the TLD part is equivalent but longer than (\.<tldpart>){1,2} but I'm sure it could be fixed for duplication...

edit: yech, no, it would be possible but essentially a very slow brute force list to handle the duplications I think. Simpler and faster to put the possible TLD and SLD+country pairs in a big hashmap and check the substring against that.

Answer 5

I'd recommend starting with the rules laid out in RFC 1035, and then working backwards -- but only if you really really really need to do this from scratch. A domain regex pattern has got to be (arguable second only to email address regex patterns) the most common thing out there. I would check out the site regexlib.com and browse through what other folks have done.

Answer 6

You can build up the regex as a string and then do Regexp.new(string).