#!/usr/bin/perl -w

use strict;

my $txt = "foo *text* bar *bat* bounce";

$txt =~ s/\*(.*?)\*/<em>$1<\/em>/g;

print $txt."\n";

##</code><code>##

s/ substitute
\* a star
(.*?) then a group of arbitrary characters (but only match once, see below)
\* then another star
/ replace it with
<em>$1<\/em> what we just got, surrounded by <em>'s
/g; do this across the entire string;

##</code><code>##

foo <em>text* bar *bat</em> bounce
- instead of -
foo <em>text</em> bar <em>bat</em> bounce