Here's a non-recursive way which I think fits your criteria:
use strict;
use warnings;
use Test::More;
my $wiki =
'_/one *two*/ three_ null _/four *five*/ six_ null _/seven *eight
+*/ nine_';
my $expected =
'<u><i>one <b>two</b></i> three</u> null <u><i>four <b>five</b></i
+> six</u> null <u><i>seven <b>eight</b></i> nine</u>';
my %h = (
'*' => 'b' ,
'/' => 'i' ,
'_' => 'u' ,
);
my $DBG = 1;
sub flip {
my $s = shift;
my $z = $h{$s};
$h{$s} = $z =~ /\// ? substr ($z, 1, 1) : "/$z";
return "<$z>";
}
sub tf {
diag "Pre: '$_'\n\n" if $DBG;
s{([_*/])}{flip($1)}eg
};
$_ = $wiki;
diag "IN <= '$wiki'\n\n" if $DBG;
tf();
is ($_, $expected, " repeated replace works");
done_testing;