And I thought that $1 and $2 are read-only ... ah I see you use the /r flag.

anyway, markup should be paired.°

my $wiki =
    '_one*';

my $expected = $wiki;


$html = $wiki =~ s{ (?:^|\s) \K ([*_/]+) | ([*_/]*) (?=$|\s) }
  { $1 ? $1 =~ s|.|<$h{$&}>|gr : $2 =~ s|.|</$h{$&}>|gr }gexr;

print $html eq $expected ? "passed" : "FAILED", "\n\n";
print $wiki, "\n\n", $expected, "\n\n", $html, "\n";
[download]

FAILED

_one*

_one*

<u>one</b>
[download]

I've updated the tests in Re: wiki regex reprocessing replacement (UPDATED^2) with markup to ignore

Funny enough, the monastery fails too :)

FAILED

_one*

_one*

one  

°) yes I know, wasn't explicitely tested