Using Regular Expression to find position instead of strpos or stripos in PHP

I really can't believe that there isn't any article explaining using regular expression to find a position of a given string. It is very command to use strpos or stripos to find the first occurrences of a given string in PHP. However, problem comes if strpos or stripos gives you the wrong result. Assuming you are looking for the symbol "RM" (Ringgit) on a given text. However, on the given text there exist a word called "RMX9182 is the code for this item selling at RM2000". It is obvious that you want your program to retrieve the symbol on "RM2000" instead of "RMX9182". Using the following strpos or stripos will definitely give you a wrong result.

$text = "RMX9182 is the code for this item selling at RM2000";
$position = stripos($text, "RM"); // return 0