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Re: Perl regex in real life

by blazar (Canon)
on Nov 01, 2008 at 10:46 UTC ( #720838=note: print w/ replies, xml ) Need Help??


in reply to Perl regex in real life

I personally believe that we may have different interpretations of the "real life" expression: for example, in real life a problem I've often had to face is that of dating with girls. In this respect, none of the "Perl-specific" regex features ever helped me, and neither did [Pp ]erl as a whole although I can imagine some convoluted and highly improbable situations in which they would. Now, due to my severe health conditions, I am coping with intense pains and precisely, in the very instants I'm writing this, especially at my left leg: (to the point that it's very hard to write at all,) needless to say, those things do not help in any way either.

So, my point is that there is not such a thing as a "real life" IT/CS problem, which appears to be what you're referring to. There are actual problems, period. And there's that Turing thingie which I'm not repeating here, which states that if your programming language is powerful enough, then it will be equally able as any other one to solve them. Then there are tools, and there are tools which are similar to others, but slightly more or slightly less powerful than those others. Thus you may have a situation in which some particular Perl's enhanced regex feature would not be "indispensable" but the absence of which would make your regex say ten time longer and several times more error prone: how indispensable would that feature be for you?

To only take into account the simplest of examples, since you talk about non-greedy matching, see my recent Insane (?) Regexp-based jpeg (JFIF) extractor... which uses the extremely simple /(\xFF\xD8 .*? \xFF\xD9)/xsg regex: how would you have done that without non-greedy matching? Please note that however ridiculous, the problem I was trying to solve with that program was a real world one in your sense, namely that of "extracting those images from that file ASAP."

On to the future: one oft repeated mantra at the Monastery and elsewhere is that regexen are not well suited to parse HTML and XML, which is perfectly true. But under Perl 6 they will be so enhanced as to be promoted to "rules" which in turn, it is said, will be perfectly apt at implementing real parsers (although I'm not an expert, and not even a beginner, in parsing theory and I don't know which kind of parsers...) Ain't this a real-life problem?

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Re^2: Perl regex in real life
by Anonymous Monk on Nov 01, 2008 at 15:08 UTC
Re^2: Perl regex in real life
by RezaRob (Initiate) on Nov 01, 2008 at 19:07 UTC
    /(\xFF\xD8 .*? \xFF\xD9)/
    Like a great friend of mine, you do talk a lot off track!

    But your example is absolutely correct :-) This _is_ backtracking. However, it is exactly equivalent to
    /(\xFF\xD8 .*? (?>\xFF\xD9))/
    So, in a sense, it isn't _actually_ backtracking. Complicated backtracking would be when you cannot do that.

    On to the future: one oft repeated mantra at the Monastery and elsewhere is that regexen are not well suited to parse HTML and XML, which is perfectly true. But under Perl 6 they will be so enhanced as to be promoted to "rules" which in turn, it is said, will be perfectly apt at implementing real parsers (although I'm not an expert, and not even a beginner, in parsing theory and I don't know which kind of parsers...) Ain't this a real-life problem?
    Indeed, parsers are real life problems. And I did realize that in PerlFAQ there is an entry:
    Can I use Perl regular expressions to match balanced text?

    That's "real life" enough.

    You'll realize though, that when you're parsing (complicated) languages, you'll need to do things like error checking. So, in practice, either you'll need to make the regex un-humanly complex, or it still must contain "outlets" in form of function callbacks etc. in order to handle special conditions and the logic and semantics of the language that you're parsing.

    I can see that the regex features mentioned above are useful, but I still don't feel very cozy about their beauty and simplicity. It _is_ however clear, that Perl will be at least as useful (and better) than traditional parsers (yet probably not quite as fast, unless you really understand internal implementation issues.

    Thanks a lot for bringing up these thoughts.

    Reza.

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