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Re: qr// and user provided regex patterns...

by ikegami (Pope)
on Jul 30, 2009 at 06:27 UTC ( #784503=note: print w/ replies, xml ) Need Help??


in reply to qr// and user provided regex patterns...

The function of my program is to evaluate user provided patterns

/Bill Clinton/i is NOT a regex pattern. It's a match operator. If you want to execute arbitrary Perl code such as the match operator, you need eval EXPR (and all the attendant validation and security problems) or parse the input yourself (which could be done simply enough in this case).

If you wish to continue use qr//, you'll need to pass an actual regex pattern such as (?i:Bill Clinton)

By the way, I'd avoid using $& since simply using it can negatively affect distant parts of your program. You could change

if($text =~ $pattern){ print $&; #prints entire match
to
if($text =~ /($pattern)/){ print $1; #prints entire match
but I'd avoid globals entirely and use
if(my ($match) = $text =~ /($pattern)/){ print $match; #prints entire match


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Re^2: qr// and user provided regex patterns...
by misterMatt (Novice) on Jul 30, 2009 at 06:57 UTC
    Okay, I decided to go the eval route. Because as you pointed out - I'm only using the match operator - which I thought was a regex pattern (I'm really still not clear on the difference.) anyway, I'm also going that route because as another monk pointed out - the use of qr// messes with the user pattern that's already wrapped in //. (the interpolation issue) right now I'm looking at this:
    if($options{c}){ #variable grab my $pattern = eval "qr$options{c}" or die $@; print "Please enter the text you wish to run the pattern on: "; my $text = <STDIN>; chomp $text; if($text =~ $pattern){ print $&; #prints entire match print " " . $text; } else{ print "$pattern on $text Failed. "; } }
    It works, as long as what the user provides doesn't have any spaces in it. (ex: /Bill Clinton/i causes the program to fail, but /BillClinton/i doesn't.) How do I fix that?

      I'm really still not clear on the difference.

      A multiplier tells the multiplication operator by how much it should multiply. The multiplier is data, the operator is Perl code.

      A regex pattern tells the match operator what it should be match. The pattern is data to Perl, the operator is Perl code.

      You don't expect $c = '4 * 5'; 3 + $c to execute the contents of $c, so why would you expect differently from $c = '/foo/'; $x =~ $c?

      It works, as long as what the user provides doesn't have any spaces in it. (ex: /Bill Clinton/i causes the program to fail, but /BillClinton/i doesn't.) How do I fix that?

      I'm guessing the problem is that the user did NOT provide an argument with spaces in it, but rather passed two arguments. Quote the argument appropriately for your shell.

        Wrapping the argument in quotes solved it - good catch. Thanks.
Re^2: qr// and user provided regex patterns...
by misterMatt (Novice) on Jul 31, 2009 at 19:58 UTC
    Using the eval works great for matches : $pattern = eval "qr$pattern" or die $@;
    But of course, when I do a 'substitution pattern' s///, it fails because it looks like eval "qrs/stuff/morestuff/. So I tried adding a bang, to wrap around the interpolated variable in the original : $pattern = eval "qr!$pattern!" or die $@; But now my patterns fail to match anything. Example:
    s/Bill Clinton/The ex-President/gi on Bill Clinton is the ex-President +. bill clinton. results in Failure.

      Using the eval works great for matches : $pattern = eval "qr$pattern" or die $@;

      No, it doesn't.

      First of all, it doesn't work at all if $pattern actually contains a pattern.

      And then there are issues with improper escaping. If you pass /a\+/, it won't match "a" followed by "+" as desired.

Re^2: qr// and user provided regex patterns...
by misterMatt (Novice) on Aug 02, 2009 at 07:33 UTC
    Okay, I've managed to get this to work perfectly with a regular match (//) - but when I try to do a substitution(s///), the pattern fails to match anything - and no substitution is done. I'm calling the program like this: -r "s/matt/matthew/" -i -g, with the text 'matt, Matt'. Here is the code that processes the substitution bit:
    if($options{r}){ #variable grab, add flags to pattern if they exist. my $pattern = $options{r}; $pattern .= 'g' if $options{g}; $pattern .= 'i' if $options{i}; $pattern .= 's' if $options{s}; #compile that stuff with eval my $compd_pattern = eval "qr($pattern)" or die $@; print "Please enter the text you wish to run the pattern on: "; my $text = <STDIN>; chomp $text; #do work and display if($text =~ $compd_pattern){ print $text; } else{ print "$pattern on \n\t{$text} Failed. "; } } #end R FLAG
      I suspect that qr was not intended to be used on an entire subsitution operator like that. I believe it should only be used on the regular expression portion (left side) of the subsitution. See perlop.

      Here is an example script which uses s/// as a command-line option:
      http://www.cpan.org/authors/id/F/FO/FORMAN/ren-regexp-1.5

        so I pretty much will have to split it and evaluate it like this: eval("$text =~ s/$pa/$tx/$md"); It might be difficult splitting a regex that could contain multiple /'s. I'll have to adjust the program so the user enters the pattern, and replacement separately I guess.
      As toolic points out, qr// is meant to quote a pattern, not an entire substitution. If you want to pass in literal substitutions, rather than just patterns, then just eval at a different point:
      eval "\$text =~ $pattern";

      By the way, ikegami suggested using (?i:) in place of a string-based eval. Since such evals are always a cause for concern, and should be a cause for terror when run on user-provided input, are you sure that you considered the alternative properly before making your decision?

      If you're not happy with the inability of a (?i:)-type solution to handle substitutions, then you can probably just offer --search and --replace --by flags so that the user can specify directly what he or she wants.

      • It makes no sense to let the user specify /g for a match. You can't even use it on qr// because it makes no sense. ixsm are the four modifiers that apply to the pattern as opposed to the operator.

      • The followign doesn't make much sense:

        my $compd_pattern = eval "qr($pattern)" or die $@;

        It removes the ability of qr// to quote, which is what you want. The code should be

        my $compd_pattern = qr($pattern);
      • You say you have problems doing substitutions, but you didn't show us your attempt (despite your claim). You should have no problems using a qr// pattern in a substitution.

      my $pat = ...; my $repl = ...; my $mods = ''; $mods .= 'i' if ...; $mods .= 's' if ...; $mods .= 'm' if ...; $mods .= 'x' if ...; my $re = qr/(?$mods:$pat)/; if (...) { s/$re/$repl/g; } else { s/$re/$repl/; }
        You say you have problems doing substitutions, but you didn't show us your attempt (despite your claim). You should have no problems using a qr// pattern in a substitution.
        I think that misterMatt's problem was that he was trying to stuff a substitution into a qr//, like:
        my $substitution = 's/out/in/'; my $pattern = qr/$substitution/;
        which, naturally, doesn't work (well, doesn't cause a substitution, anyway).

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