$re = '/(<ApplicationPath>.*\\\\West\\\\.*<\/ApplicationPath>\n)<CommandLine \/>/m';
$str = '<ApplicationPath>F:\\Objects\\Trees\\Conifers\\North America\\West\\comporgel.zip</ApplicationPath>
<CommandLine />
<ApplicationPath>F:\\Objects\\Trees\\Conifers\\North America\\West\\Christmas.zip</ApplicationPath>
<Photo />
<ApplicationPath>F:\\Objects\\Trees\\Conifers\\North America\\West\\spruce1.zip</ApplicationPath>
<CommandLine />
<ApplicationPath>F:\\Objects\\Trees\\Conifers\\North America\\East\\spruce1.zip</ApplicationPath>
<CommandLine />';
$subst = "$1<CommandLine>trees -nowindow</CommandLine>";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php