$re = '/(?<!function )(?<!\/\/)print/';
$str = '<?php
// Description of file
...
print("Foobar");
// print("Foo");
//print("bar");
// Open and print file
function printtemplate($file) {
...
}
...
printtemplate($file);
...
?>';
$subst = "//$0";
$result = preg_replace($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