> perl -we'use strict; use P;
my $re_str=qr{([0-6BS])};
$_="B";
my $ans;
if (eval m{$re_str}) {
  $ans=$1;
}
P "ans=%s, re_s=%s", $ans, $re_str;
'
ans=B, re_s=(?^:([0-6BS]))