$name $=$ preg$\_$replace$(\text{'}/<(.)$s$(.)$c$(.)$r$(.)$i$(.)$p$(.)$t$/$i$\text{'},\text{'}\text{'},$ $$\_$GET$[$ 'name' $])$;

In this code, the developer used a regular expression to replace any form of the tag, no matter what case of the characters is used, with a null value.

Which character in the script was omitted from the regular expression? How do you know?