how do i do a php preg_replace for bbcode with out skipping any?

[Christopher]

ok i have this
"function BBCode($str) {
$find = array(
"(\[[Bb]\](.+)\[/[Bb]\])",
"(\[[Ii]\](.+)\[/[Ii]\])",
"(\[[Ss]\](.+)\[/[Ss]\])"
);
$replace = array(
"<span class=\"bold\">$1</span>",
"<span class=\"italics\">$1</span>",
"<span class=\"strikethrough\">$1</span>"
);
$str = preg_replace($find,$replace,$str);
return $str;
}"
and when i pass it this
"Bold italicsstrikethrough onlyBold"
i get
"<span class="bold"><span class="italics">Bold italics</span>[/b]<span class="strikethrough">strikethrough only</span>Bold</span>"
how do i make it give me
"<span class="bold"><span class="italics">Bold italics</span></span><span class="strikethrough">strikethrough only</span><span class="bold">Bold</span>"