diff --git a/hsmw-highlight b/hsmw-highlight
index 0494278..76175e5 100644
--- a/hsmw-highlight
+++ b/hsmw-highlight
@@ -394,7 +394,7 @@ typeset -gA HSMW_HIGHLIGHT_STYLES
         # parameters that refer to commands, functions, and builtins.
         local MATCH; integer MBEGIN MEND
         if [[ $REPLY == none ]] && (( ${+parameters} )) &&
-           [[ ${arg[1]} == \$ ]] && [[ ${arg:1} = (#m)([[:alpha:]_][[:alnum:]_]#|[[:digit:]]##) ]] &&
+           [[ ${arg[1]} == \$ ]] && [[ ${arg:1} = (#m)([a-zA-Z_][a-zA-Z0-9_]#|[0-9]##) ]] &&
            (( ${+parameters[${MATCH}]} ))
            then
           -hsmw-highlight-main-type ${(P)MATCH}