<?php

$oldString = '\t this is my \n odd \\ \\\ \\\\ string \0';
echo $oldString . "\n";
$newString = preg_replace('/(\\\\\S)/', '<span class="b">$1</span>', $oldString);
echo $newString;

?>

\t this is my \n odd \ \\ \\ string \0
<span class="b">\t</span> this is my <span class="b">\n</span> odd \ <span class="b">\\</span> <span class="b">\\</span> string <span class="b">\0</span>