in reply to how to use string RE (against $_ and capture)

In your first code sample, you have:

my $re_str=qr{([0-6BS])};
$_="B";
my $ans;
if (eval m{$re_str})

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Why are you using eval? It should not be needed.

In your second code sample:

my $re_str="qr{([0-6BS])}";
$_="B";
my $ans;
if (eval m{$re_str})

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The m{$re_str} will be interpreted as m/qr{([0-6BS])}/ which is looking to match something that starts with "qr". (And I'm not sure how the "{([0-6BS])}" will get interpreted as. "{" and "}" in a regex are normally used to quantify the number of matches.)

To store a regex as a hash key, you could prefix the regex with a character that won't otherwise appear as the first character of any of your hash keys. So, something like this:

#not tested, YMMV
for $key (keys %options)
{
    if ($key =~ s/^#//)
    {
        if (/$key/)
        {
            print "Match: $1\n";
        }
    }
    else
    }
        if ($_ eq $key)
        {
            print "Equals: $1\n";
        }
    }
}

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Re^2: how to use string RE (against $_ and capture)
by AnomalousMonk (Archbishop) on Nov 12, 2016 at 00:15 UTC
    ... I'm not sure how the "{([0-6BS])}" will get interpreted as. "{" and "}" in a regex are normally used to quantify the number of matches.

    Apparently, if the RE compiler can't see anything in the regex that looks like a count or count range, it just gives up on the whole counted quantifier thing and takes the curlies as literal characters. 

    c:\@Work\Perl>perl -wMstrict -le
"$_ = 'xxqr{S}xx';
 print qq{matched '$&', \$1 is '$1'} if m/qr{([0-6BS])}/;
"
matched 'qr{S}', $1 is 'S'

    [download]
    (Tested under ActiveState 5.8.9, Strawberry 5.14.4.1.)

    To store a regex as a hash key, you could prefix the regex with a character that won't otherwise appear as the first character of any of your has keys.

    I still prefer some sort of separate tag to distinguish pattern from exact matching: 

    c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my %options = (
   qr{ x (Y) z }xms => { type => 'pattern',  name => 'foo', },
   '(?x) x (Y) z '  => { type => 'pattern',  name => 'fum', },
   'xYzzy'          => { type => 'exact',    name => 'bar', },
   'zzzzz'          => { type => '?????',    name => 'zot', },
   );
 dd \%options;
 ;;
 $_ = 'xYzzy';
 for my $p (sort keys %options) {
   my ($p_type, $p_name) = @{ $options{$p} }{ qw(type name) };
   if ($p_type eq 'pattern') {
     if ($_ =~ $p) { print qq{'$p_name' pattern match of $p, \$1 is '$
+1'}; }
     }
   elsif ($p_type eq 'exact') {
     if ($_ eq $p) { print qq{'$p_name' exact match of '$p'}; }
     }
   else {
     die qq{unknown: $p; type '$p_type'; name '$p_name'};
     }
   }
"
{
  "(?^msx: x (Y) z )" => { name => "foo", type => "pattern" },
  "(?x) x (Y) z " => { name => "fum", type => "pattern" },
  "xYzzy" => { name => "bar", type => "exact" },
  "zzzzz" => { name => "zot", type => "?????" },
}

'foo' pattern match of (?^msx: x (Y) z ), $1 is 'Y'
'fum' pattern match of (?x) x (Y) z , $1 is 'Y'
'bar' exact match of 'xYzzy'
unknown: zzzzz; type '?????'; name 'zot' at -e line 1.

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    (Test under same Perl versions as above.)

    Give a man a fish:  <%-{-{-{-<

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