$re = '/(?<=\s)(?<start>%%)?%dMODIFIER_PROPERTY_(\w+)%(?(start)|%%)(?=[^%]|$)/m';
$str = '"DOTA_Tooltip_modifier_viper_poison_attack_slow_Description" "Velocidad de movimiento reducida un %dMODIFIER_PROPERTY_MOVESPEED_BONUS_PERCENTAGE%%%, resistencia mágica reducida un %dMODIFIER_PROPERTY_MAGICAL_RESISTANCE_BONUS%%%. Recibiendo daño a lo largo del tiempo."
%%%dMODIFIER_PROPERTY_SOMETHING%
%%dMODIFIER_PROPERTY_SOMETHING%
%%%%dMODIFIER_PROPERTY_SOMETHING%
%%%dMODIFIER_PROPERTY_SOMETHING%%
%%%dMODIFIER_PROPERTY_SOMETHING%%%
%dMODIFIER_PROPERTY_SOMETHING%
%dMODIFIER_PROPERTY_SOMETHING%%
%dMODIFIER_PROPERTY_SOMETHING%%%
%dMODIFIER_PROPERTY_SOMETHING%%%%';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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