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Re: Changes in m//m in 5.10

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December 23, 2007 03:21
Re: Changes in m//m in 5.10
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On 23/12/2007, Chris Dolan <> wrote:
> On Dec 22, 2007, at 8:35 AM, Ævar Arnfjörð Bjarmason wrote:
> > On Dec 21, 2007 12:08 PM, demerphq <> wrote:
> >> No it wont actually. The warning would only be raised when the $qr
> >> variable is the only thing in the pattern. The diagnostics should
> >> read
> >> (added an 'only'):
> >>
> >> +(W regexp) Your pattern contains only a variable containing a
> >> precompiled
> >> +regular expression pattern whose behavior will not be changed by the
> >> +pattern modifiers provided.
> >>
> >> $qr=/^foo/;
> >> $x=~/$qr/m; # warns;
> >> $x=~/$qr|foo/m; #does not warn
> >> $x=~/$qr/x;  # does not warn (/x doesnt change match semantics)
> >
> > In the current implementation yes, but we might potentially want to
> > warn in the future on useless modifiers in general, eg:
> >
> > $x =~ /(?-msix:^foo)/msi;
> I would like to make a few points in opposition to this new warning.
> Before I do so, I'd like to acknowledge that such a warning would
> certainly be useful in alerting 5.8 -> 5.10 upgraders about the bug
> fix.  And I appreciate the effort expended in minimizing the scope of
> the warning.  But I think it's a readability loss overall, and when
> 5.8 is passé, we'll still be stuck with it.
>   1) The Perl Best Practices book recommends putting /m on all
> regexes for consistency and readability.  In that paradigm, '^' and
> '$' always mean start/end-of-line and never start/end-of-string,
> which lightens the reader's mental load.
>   2) Perl::Critic has been recommending /m on all regexes since 0.13,
> so lots of people have adopted this habit.
>   3) People who adopted that habit were not bitten by this 5.8 ->
> 5.10 behavior change.
>   4) The new warning would mean that the above practice is now
> officially discouraged, at least in terms of the absoluteness of the
> practice.
>   5) Authors (like me) with working 5.8 code will have to remove
> harmless /m modifiers from various places just to get their code to
> run quietly.
>   6) While the modifier is indeed useless in 5.10, it's not at all
> harmful or wasteful, unlike say sort in void context which wasted CPU.
> If this patch does go in, we'll probably adapt Perl::Critic to know
> about this special case, and I'll probably go through all of my CPAN
> code to remove the extraneous modifiers, and I'll forget about this
> whole issue in a few months.  But what a pain for so little gain...

I dont see the problem. If you use the same modifiers everywhere there
will be no warning.

The only place it will warn is when there is a mismatch between the
modifiers on a qr// and on the match processing the qr//.


if (/$qr/m) {}

If you never do stuff like this, for instance if you always write:

if (/$qr/m) {}

there will be no warning.

perl -Mre=debug -e "/just|another|perl|hacker/"

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