MODX Output Modifier checks if $input contains a string defined in the $options value.

snippet.containing.php

<?php
/**
* Returns a value if input contains string
* [[*link_attributes:containing=`blank=New Window&self=This window`]]
*/
parse_str($options, $cases);

$output = 'no-match';
foreach ($cases as $needle => $value) {
    if (strpos($input, $needle) !== false) {
	    $output = $value;
	}
}

return $output;