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.