5 ms·

What is it with the obsession with regular expressions? They are useful things, sure, but I just use them in connection with grep or if I search for strings and

What is it with the obsession with regular expressions? They are useful things, sure, but I just use them in connection with grep or if I search for strings and normally they are pretty basic, e.g. $ grep -r -n --color "foo*bar" src If I want to validate input data with the machine I just use a parser.

deleted 13y ago
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hnriot 13y ago
I don't think your example does what you think it does, match fo followed by o zero or more times, followed by bar. when you do understand regex, you'll be amazed at the myriad of things you can do with it.
- helloTree 13y ago
  You are absolutely right, I forgot the '.'.
ghshephard 13y ago
Regexes are an elegant and very powerful way to validate data in scripts in a concise (and if they aren't abused) easy to read fashion. There are almost infinite number of examples, but let's say I want to verify that a field is a 64 Bit hexadecimal MAC address $mac =~ ^[A-Fa-f0-9]{16}$ Gets the job done. How else, but a regular expression so concisely? 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.
- ghshephard 13y ago
  Here is another one I just did tonight - I wanted to match IPv4 addresses, but didn't want to validate anything with a leading 0 (specifies octal format, which 99.9999% of the time is not what people want), but I do want to accept a leading 0 if it's the only value (I.E. 3.0.2.1, 0.0.0.0, etc...) 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.
  - mjhoy 13y ago
    > How else would you do it? 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 https://gist.github.com/mjhoy/6751909
  - MichaelSalib 13y ago
    Maybe something like this: 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 It is longer; on the other hand, it is easier to read and more importantly easier to verify correctness.
    - clarry 13y ago
      I think you forgot to verify that an octet doesn't have leading zeros (unless its value actually is zero).
      - MichaelSalib 13y ago
        I didn't forget: the (o.lstrip(0) or '0') expression does that. Actually, that should be o.lstrip('0')...
        clarry 13y ago
        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 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 https://pastebin.mozilla.org/3171656 A state machine would be more appropriate in my opinion.
- helloTree 13y ago
  Ok then maybe it's just a matter of taste. I like the parser approach more: import Text.ParserCombinators.ReadP -- or the parser lib of your choice import Data.Char ... macP = count 16 (satisfy isHexDigit)

deleted 13y ago

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hnriot 13y ago

I don't think your example does what you think it does, match fo followed by o zero or more times, followed by bar. when you do understand regex, you'll be amazed at the myriad of things you can do with it.

helloTree 13y ago
You are absolutely right, I forgot the '.'.

ghshephard 13y ago

Regexes are an elegant and very powerful way to validate data in scripts in a concise (and if they aren't abused) easy to read fashion. There are almost infinite number of examples, but let's say I want to verify that a field is a 64 Bit hexadecimal MAC address $mac =~ ^[A-Fa-f0-9]{16}$ Gets the job done. How else, but a regular expression so concisely? 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.

ghshephard 13y ago
Here is another one I just did tonight - I wanted to match IPv4 addresses, but didn't want to validate anything with a leading 0 (specifies octal format, which 99.9999% of the time is not what people want), but I do want to accept a leading 0 if it's the only value (I.E. 3.0.2.1, 0.0.0.0, etc...) 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.
- mjhoy 13y ago
  > How else would you do it? 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 https://gist.github.com/mjhoy/6751909
- MichaelSalib 13y ago
  Maybe something like this: 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 It is longer; on the other hand, it is easier to read and more importantly easier to verify correctness.
  - clarry 13y ago
    I think you forgot to verify that an octet doesn't have leading zeros (unless its value actually is zero).
    - MichaelSalib 13y ago
      I didn't forget: the (o.lstrip(0) or '0') expression does that. Actually, that should be o.lstrip('0')...
      - clarry 13y ago
        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 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 https://pastebin.mozilla.org/3171656 A state machine would be more appropriate in my opinion.
helloTree 13y ago
Ok then maybe it's just a matter of taste. I like the parser approach more: import Text.ParserCombinators.ReadP -- or the parser lib of your choice import Data.Char ... macP = count 16 (satisfy isHexDigit)