$ grep -r -n --color "foo*bar" src
If I want to validate input data with the machine I just use a parser.
$mac =~ ^[A-Fa-f0-9]{16}$
And, when you say, "If I want to validate input data with the machine I just use a parser." - that's pretty much what a regex engine is - a sophisticated parser, and the regular expression is the "commands" that you feed to it to parse the input text.
regex_ipv4='^((25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])\.){3}(25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])$'
Gets the job done.
How else would you do it?
You can then build up a library of these, and use them on other projects.
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:
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
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.
1. 12 .13. 14
089.23.45.67
import Text.ParserCombinators.ReadP -- or the parser lib of your choice
import Data.Char
...
macP = count 16 (satisfy isHexDigit)
when you do understand regex, you'll be amazed at the myriad of things you can do with it.