#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11112988
use warnings;

my $wiki =
    '_/one *two*/ th/ree_ null _/four *five*/ six_  null _/se_ven *eig
+ht*/ nine_';

my $expected =
    '<u><i>one <b>two</b></i> th/ree</u> null <u><i>four <b>five</b></
+i> six</u>  null <u><i>se_ven <b>eight</b></i> nine</u>'; 

my %h = (
    '*' => 'b' ,
    '/' => 'i' ,
    '_' => 'u' ,
);

my $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]

Outputs;

passed

_/one *two*/ th/ree_ null _/four *five*/ six_  null _/se_ven *eight*/ 
+nine_

<u><i>one <b>two</b></i> th/ree</u> null <u><i>four <b>five</b></i> si
+x</u>  null <u><i>se_ven <b>eight</b></i> nine</u>

<u><i>one <b>two</b></i> th/ree</u> null <u><i>four <b>five</b></i> si
+x</u>  null <u><i>se_ven <b>eight</b></i> nine</u>
[download]