<?php
$input='foo/bar#baz';
$array = array("#" => "_", "/" => "-");
$output = preg_replace('@([a-z][#/])@e', '"<span>".strtr("$0", $array)."</span>"', $input);
var_dump($output);
?>

string(37) "fo<span>o-</span>ba<span>r_</span>baz"