def is_ipv4_addr(s):
     try:
        octets = s.split('.')
        assert len(octets) == 4
        for o in octets:
            assert 0 <= int(o.lstrip(0) or '0') < 256
     except:
        return False
     return True

  1. 12 .13. 14
  089.23.45.67

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

  >>> is_ipv4_addr("01.0.0.0")
  True

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.

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.

Taking your question generally, I was curious to see what it might look like as a parser, since I find that regex a little hard to read. Here's an implementation with Haskell's parsec:

https://gist.github.com/mjhoy/6751909