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From: C.DeRykus on 3 Nov 2009 23:00
On Nov 3, 11:18 am, Ben Morrow <b...(a)morrow.me.uk> wrote:
> Quoth "C.DeRykus" <dery...(a)gmail.com>:
>
>
>
> > On Nov 2, 11:07 am, Paul Lalli <mri...(a)gmail.com> wrote:
>
> > > for (qw/n t f r v/) {
> > >    my $meta = eval("\\$_");
> > >    $string =~ s/$meta/\\$_/;
>
> > > }
>
> > Did that work? I don't understand why the eval is needed
> > at all:
>
> > my $string = "1\n 2\t 3\f 4\r 5\cK";
> > for (qw/n t f r cK/) {
> >     my $meta = "\\$_";
> >     $string =~ s/$meta/\\$_/;
> > }
> > print $string;   #  1\n 2\t 3\f  4\r  5\cK
>
> That's... evil. It relies on the fact that regexes undergo two separate
> expansion phases, and requires that variable expansion happens in the
> first phase but other qqish escapes are expanded in the second. I'm not
> entirely convinced that's documented behaviour: anyone care to dig out
> perlre and prove it one way or the other?
>
> For extra added evil:
>
>     my $bs = "\\";
>     $string =~ s/$bs$_/$bs$_/g for qw/n r t f/;
>

Perl magic is evil? Say it ain't so :)

I didn't spot a full explanation in perlre but I see perlop
steps through the compilation in "gory details of parsing
quoted constructs" and ends with what happens at runtime
in "parsing regular expressions".

This closely mirrors Chapter 7's section - Perl Regular
Expressions in J.Friedl's "Mastering Regular Expressions"
1st ed.


--
Charles DeRykus
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