About search in builtin tools like Replace, Index, etc.

Can the search text be an regular expression? For example, I can remove a string, say ".xls",  from a text t by the code Replace(t, ".xls", "").  But what if I want to remove ".xls" or ".xlsx"?  I would like to supply an regexp for the search part like ".xlsx*" like Replace(t, ".xlsx*", ""). But I tried it and it didn't work.  Or shoule it work?

The built-in Text extension has Actions that support regular expressions for find, replace, etc.