That's simple for me, but very difficult for most other users because they did not read the manual and so don't know about the superb Find Select feature.

If you hold the SHIFT key while pressing the Find Next button in the find dialog everything from current cursor position to end of the found search string is selected. And it's not really surprising, that this Find feature is also available via macro command Find. What is not described in the help is that you have to release the SHIFT key shortly if you want to modify the selection further with the cursors after the find (for example unselect the founded search string).

With this knowledge you now should be able to simple record the macro. But for me it's more difficult to write and post the macro code because I don't know some of your settings. So I have to think about all possible settings which could have influence on the macro execution and have to add extra code to make sure the macro works under all possible configurations.

The following macro is independet of macro property Continue if a Find with Replace not found. You could discard the IfNotFound part and the Else part of the IfSel code, if you do not enable this property.

Because of the special Key HOME handling in the macro code this macro is also indepedent of the current setting of option Home Key Always Goto Column 1 at Configuration - Editor - Miscellaneous. I don't know if you have enabled it or not and if you have preceding white-spaces in the line AAAA and/or FFFF.

InsertMode
ColumnModeOff
HexOff
Top
Loop
Find "AAAA"
IfNotFound
ExitLoop
EndIf
Key HOME
IfColNumGt 1
Key HOME
EndIf
Key DOWN ARROW
StartSelect
Find Select "FFFF"
IfSel
Key HOME
Key HOME
EndSelect
Delete
Else
EndSelect
ExitLoop
EndIf
EndLoop