There's nothing wrong with regex syntax. But there _is_ something wrong with the formatting of your example: it's not readable. Perhaps you _should_ write (assuming Java-syntax):

  (?x:              #standard token is an uppercase letter, digit, dot, or hyphen
    \b
    [A-Z0-9._%-]+   #1 or more of standard token, underscore, or percent sign
    @               #at-sign
    [A-Z0-9.-]+     #1 or more standard tokens
    \.              #dot
    [A-Z]{2,4}      #2 to 4 uppercase letters
    \b
  )

We can easily optimize for readability with regex syntax.

You can say that about pretty much anything if you aren't familiar with the syntax. That looks pretty readable to me - certainly far more portable and readable than equivalent code.

[It's also perfectly obvious that if this is an attempt to match email addresses that it's not a very good one - but I don't know the context where it's supposed to be used, it might be good enough for whatever the author intended].

I find that perfectly readable, except for the \b which I hadn't seen before. It's matching an all-uppercase email address.

There's plenty of upper case e-mail addresses that won't match that expression.

Well, if you want to be fully compliant you can go for this 6kb monster

http://ex-parrot.com/~pdw/Mail-RFC822-Address.html

/i

There's characters missing[1] and the tld is too short[2]. And even if that's fixed, we still don't match internationalized addresses or actually validate that the e-mail address exists. You're probably better off with something like...

   if "@" in email and "." in email.split("@")[1]:
       send_verification(email)

...but you should probably also check for common misspellings like "gmial.com" etc.

[1] http://en.wikipedia.org/wiki/Email_address#Syntax [2] http://en.wikipedia.org/wiki/List_of_Internet_top-level_doma...

Call me weird, but I find that very readable.