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Re: Perl Parsing Power

by BrowserUk (Pope)
on Sep 24, 2012 at 10:09 UTC ( #995338=note: print w/ replies, xml ) Need Help??


in reply to Perl Parsing Power

He seemed astonished when I showed him one line of Perl that demonstrated recursive descent parsing.

Recursive descent parsing in one line of Perl; I'd like to see that?


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

RIP Neil Armstrong


Comment on Re: Perl Parsing Power
Re^2: Perl Parsing Power
by protist (Monk) on Sep 24, 2012 at 10:21 UTC
    Admittedly, it is a basic example, although powerful in its implications.
    print "good string\n" if $_~~ /^({(?:[^{}]++|(?1))*+})/;
    It tests for proper nesting of brackets, anchored to the beginning of the line. You can try it out:
    perl -ne 'print "good string\n" if $_~~ /^({(?:[^{}]++|(?1))*+})/;'

      You can do that like this:

      C:\test>perl -nE"my$t; /{/&&++$t or /}/&&--$t for split''; say $t ? $t + : 'good string'" {} good string {}} -1 {{}} good string {{}{}} good string {{}{}}} -1 {{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{}}}}}}}}}}}}}}}}}}}}}}}}}}}}} 11 {{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{}}}}}}}}}}}}}}}}}}}}}}}}}} 14 {{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{}}}}}}}}}}}}}}}}}}}}}}}}}}}}}} +}}}} 6 {{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{}}}}}}}}}}}}}}}}}}}}}}}}}}}}}} +}}}}}}}}}} good string

      But I wouldn't call it RDP :)

      The problem with doing RDP with regex is that you have to match the entire stream with one (possibly (hopefully :) composite) regex. Which is fine for yes/no booleans, but it makes both debugging the regex and reporting where and why parsing errors occur extremely messy if not impossible.


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

      RIP Neil Armstrong

        Unless I am mistaken, your oneliner will fail if given

        }}}{{{

        as input. It will report a false positive. It is also not RDP, although you know this ;).

        While I will not claim that regex RDP is pretty (complex parsing rarely is), it is possible, and Programming Perl discusses it in some minimal detail.

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