I can't think of any way to do this with simple regexps, . . .
Then don't use a regex. Why does everyone think that all string manipulation should be handled with a regex?!? Use a simple character-by-character parser, noting state as you move through the string.
# UNTESTED !!!
my $in_quote = 0;
my $in_bold = 0;
my $final_string = '';
foreach my $char ( split //, $string ) {
if ( $char eq "'" ) {
$in_quote = 1 - $in_quote;
}
else if ( $char eq '*' && !$in_quote ) {
$final_string .= '<';
$final_string .= $in_bold ? '/' : '';
$final_string .= 'b>';
$in_bold = 1 - $in_bold;
}
else {
$final_string .= $char;
}
}