Using the replace operator to convert lowercase to uppercase

Hello, I am trying to use the replace operator to replace all lowercase letters in the values belonging to an attribute, with their uppercase counterparts. My "replace what" regex expression is working (it is [a-z] ), but my "replace by" expression ( [A-Z] ) simply replaces every lowercase character with "[A-Z]", not the uppercase version of the character it is supposed to replace. What is the correct regex expression for the replace by parameter to replace the characters with their uppercase versions?

Thanks!