in reply to Re: s/// with an empty pattern uses the previous pattern of a s///
in thread s/// with an empty pattern uses the previous pattern of a s///

Oh! Another to fix the problem is to add a no-op to the regexp. 

$\ = "\n";
$pat = 'b'; $_='abc'; s/$pat/!/g;     print;
$pat = '';  $_='abc'; s/$pat/@/g;     print;
$pat = '';  $_='abc'; s/(?:$pat)/#/g; print;

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outputs 

a!c
a@c
#a#b#c#

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Replies are listed 'Best First'.
Re^3: s/// with an empty pattern uses the previous pattern of a s///
by ckeith100 (Initiate) on May 17, 2007 at 16:25 UTC

    Thank you for these tips. I'm not looking to replace the empty scalar, I'm looking to replace whatever is in that scalar with the value of a different scalar. Those values are determined dynamically so the best answer is to use, as you suggested, something like:

     $pat && ($msg =~ s/$pat/$replace/mg);

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    I was doing some more searching and discovered this exact quote in the perlop man page, as you said in reply #1, however in my copy of the man page it is listed in the m// operator rather than the substitute operator I wasn't aware that it applied there also. It does make sense that it would do, but I have to note this to try to scrape together a little dignity after having just posted a question on something in the man pages like a newbie coder :)

    Thanks again for your help! Colin

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[d/l]

      That won't work if $pat is zero. Fix: 

      length($pat) && ($msg =~ s/$pat/$replace/mg);

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      I have to note this to try to scrape together a little dignity after having just posted a question on something in the man pages like a newbie coder :)

      It's not well known, and it's easy to forget, because it's not a situation that's usually encountered. No dignity lost.

      By the way compiling the pattern is not enough. I ran a test that showed $pat = qr//; /$pat/ is the same thing as m// even though the stringification of $pat is not an empty string.

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[d/l]
[select]

      I think even many old-timers rarely see this bug. You may want to change your expression to something like length( $path ) && ($msg =~ s/$pat/$replace/mg); because the expression 0 is false but non-empty and potentially meaningful to you.

      ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊

[reply]
[d/l]
      I'm looking to replace whatever is in that scalar with the value of a different scalar.

      Maybe I'm being dense, but why not just 

      $msg =~ s/.*/$replace/s;

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      A word spoken in Mind will reach its own level, in the objective world, by its own weight
[reply]
[d/l]

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