Email Syntax Check in Python
3 May 2008
Sometimes you may want to check that an email address is not syntactically invalid, i.e. it looks like a recognisable email address. I use this approach in my zetact contact form processor.
Of course, it does not mean the address actually leads anywhere, but at least you know are dealing with an email address that could exist.
This is the code I have been using, albeit I have changed it from a class method to a simple function to make this post simpler.
"""Email check using regex."""
def invalidreg(emailkey):
"""Email validation, checks for syntactically invalid email
courtesy of Mark Nenadov.
See
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/65215"""
import re
emailregex =
"^.+\\@(\\[?)[a-zA-Z0-9\\-\\.]+\\.([a-zA-Z]{2,3}|[0-9]{1,3\
})(\\]?)$"
if len(emailkey) > 7:
if re.match(emailregex, emailkey) != None:
return False
return True
else:
return True
I decided it would be more Pythonic to try to do this using the built-in string methods, rather than importing the re module and using a monster regular expression. Here was my first attempt.
"""Email checks using string methods - simple version."""
def invalidemail(emailaddress):
"""Checks for a syntactically invalid email address."""
try:
emailitems = emailaddress.rsplit('@', 1)
emailitems.extend(emailitems[1].rsplit('.', 1))
except IndexError:
return True
if [x for x in emailitems if not x.replace(".","").isalnum()] \
and emailaddress >= 7:
return True
else:
return False
After a bit of testing and playing with this, a friend pointed me towards the relevant RFC on restrictions of email addresses. While the standard allows the use of many different special characters, in practice email addresses have to be much stricter if you actually want people in the real world to be able to send email to you.
For example, if we allow the email address []@commandline.org.uk, will whatever receives the output of this function be able to use it? As pointed out by Jan Goyvaerts, most software won't actually be able to handle obscure special characters.
We also don't want to water down the syntax check and allow junk for the sake of theoretical but non-existent addresses.
My compromise is to allow these special symbols -_.%+. in the local-part of the email address, and -_. in the domain name. I also do sanity checking on the top-level domain, it needs to be either a generic name or two characters long (country codes are all two letters).
So below is my current version, I added lots of comments and white space to make it easy to read.
"""Ditch nonsense email addresses."""
GENERIC_DOMAINS = "aero", "asia", "biz", "cat", "com", "coop", \
"edu", "gov", "info", "int", "jobs", "mil", "mobi", "museum", \
"name", "net", "org", "pro", "tel", "travel"
def invalid(emailaddress, domains = GENERIC_DOMAINS):
"""Checks for a syntactically invalid email address."""
# Email address must be 7 characters in total.
if len(emailaddress) < 7:
return True # Address too short.
# Split up email address into parts.
try:
localpart, domainname = emailaddress.rsplit('@', 1)
host, toplevel = domainname.rsplit('.', 1)
except ValueError:
return True # Address does not have enough parts.
# Check for Country code or Generic Domain.
if len(toplevel) != 2 and toplevel not in domains:
return True # Not a domain name.
for i in '-_.%+.':
localpart = localpart.replace(i, "")
for i in '-_.':
host = host.replace(i, "")
if localpart.isalnum() and host.isalnum():
return False # Email address is fine.
else:
return True # Email address has funny characters.
# Start the ball rolling.
if __name__ == "__main__":
print invalid("warrior@example.com")
1 dbr says...
There's a better, if utterly horrible to read way of doing this using regex's.
http://emailverification.pastecode.com/?show=f76a41a8b
This way isn't too bad, it allows blah+thesethingys@example.com which a lot of websites invalidate (Which is incredibly annoying).. One thing I find a little weird - a return of False means the email is valid? I would have though if valid(mail): print "Valid email" would be a more sensible way of doing things? That way: if not valid(email): print "Wrong" # would work
Posted at 4:33 p.m. on May 3, 2008
2 Ted Hosmann says...
I like the idea in your last example to check that the Domain is valid - problem is...what about users with subdomain email addresses (ted@mail.example.com) or users with country email domains (ted@example.co.uk)
Posted at 7:43 a.m. on May 4, 2008
3 Zeth says...
@dbr,
Checking for syntactically invalid email addresses is what the function does, so:
Otherwise the program can just carry on, no else clause required. Maybe my programming style is just different, you can easily change it to be the other way if you want.
Ted, If you read the code more carefully or try it out, you will see that both of your examples will pass the test.
subdomains are not a problem because I allow dots in the hostname: for i in '-_.':
Country code domains are catered for by if len(toplevel) != 2
Posted at 10:06 a.m. on May 4, 2008
4 Zeth says...
@dbr
On regular expressions, the aim of this post is to use Python built-in string methods instead of regular expressions. Your example, blah+thesethingys@example.com will be considered valid by my function as I allow the plus sign: for i in '-_.%+.'
Posted at 10:10 a.m. on May 4, 2008
5 Zeth says...
Here is dbr's regular expression (the pastebin is only temporary).
Posted at 10:33 a.m. on May 4, 2008
6 John Reese says...
Just as an FYI, I get an 'XML Parsing Error: not well-formed' message in my newsreader (Liferea) for this entry. Line number 94, Column 98.
This is the first (mostly/enough) valid email checker I've seen that doesn't use a monster regex. I definitely like it.
Posted at 7:56 p.m. on May 4, 2008
7 Ted Hosmann says...
@Zeth
ARGH - I feel like such a n00b. You, my friend, are absolutely correct. Thanks for clearing that up for me.
Posted at 8:59 p.m. on May 5, 2008
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