Check for a valid domain name in a string?

Question

I am using python and would like a simple api or regex to check for a domain name's validity. By validity I am the syntactical validity and not whether the domain name actually exists on the Internet or not.

Answer 1

Note that while you can do something with regular expressions, the most reliable way to test for valid domain names is to actually try to resolve the name (with socket.getaddrinfo):

from socket import getaddrinfo

result = getaddrinfo("www.google.com", None)
print result[0][4]

Note that technically this can leave you open to DoS (if someone submits thousands of invalid domain names, it can take a while to resolve invalid names) but you could simply rate-limit someone who tries this.

The advantage of this is that it'll catch "hotmail.con" as invalid (instead of "hotmail.com", say) whereas a regex would say "hotmail.con" is valid.

Answer 2

Any domain name is (syntactically) valid if it's a dot-separated list of identifiers, each no longer than 63 characters, and made up of letters, digits and dashes (no underscores).

So:

r'[a-zA-Z\d-]{,63}(\.[a-zA-Z\d-]{,63})*'

would be a start. Of course, these days some non-Ascii characters may be allowed (a very recent development) which changes the parameters a lot -- do you need to deal with that?