Would:

  1. 12 .13. 14
  089.23.45.67

Both match that? (Your general point is made though - RegExes look fine to the person that just crafted them, but are opaque to the casual observer)

I think you forgot to verify that an octet doesn't have leading zeros (unless its value actually is zero).

I didn't forget: the (o.lstrip(0) or '0') expression does that.

Actually, that should be o.lstrip('0')...

Wrong.

  >>> is_ipv4_addr("01.0.0.0")
  True

It should reject that (i.e. return False) because the first octet contains a leading zero. But you're just stripping the zero away, ignoring its existence. For no effect, because converting with int() already ignores them for you.

Your code is also ok with bizarre inputs like "0..." :-)

Regexes really do have their strengths -- they compactly express a state machine, and you can always break the expression into parts which'll show exactly what the state machine will accept. They could also be much more readable if people bothered to break them into parts instead of typing it out all inside a long string that becomes really difficult to parse visually. There are other notations to improve readability, for example rx in emacs: http://www.emacswiki.org/emacs/rx

A seemingly simple regex can be implemented in imperative code and it might look clean and pretty until you get the logic exactly right and amend it to handle all the corner cases that are not obvious at first sight. For comparison I did the exercise in old-fashioned C (and the indentation got messed up along the way, sigh).

https://pastebin.mozilla.org/3171656

A state machine would be more appropriate in my opinion.

You're right. My mistake.

I like automata and I think regexes are good for some things, but I definitely agree about the crappy syntax. When working in CL, I loved Edi Weitz' CLPPCRE package which allowed you to specify regexes using either the traditional broken string form or an s-exp syntax. Much cleaner.