Thanks for the suggestions Stamimail,
1. Make a synchronized scroll - to "Old Filenames:" and "New Filenames:" boxes.
I'll consider making the vertical position of the two edits locked together.
2. Add a serial number that counts the file names, to the left of the two boxes.
I will consider an option to show the current 'line' number to the left of the filename.
3. There are some cases the "New Format:" is good for all but one or two files.
A status box letting the user know if all old filenames matched the old format would be useful.
I'll consider showing this status and showing checkboxes to override the new format.
4. Consider to make "?" button instead of or in addition to ">" button
A Help button at the bottom left would be useful. F1 to open help for the renamer would also be helpful. ...added to my TODO list.
For now, help for regex and preprocessor syntax can be found under the > button.
5. Change the text to:
#uppercase:<x> Convert x to UPPERCASE
#lowercase:<x> Convert x to lowercase
#titlecase:<x> Convert x to Title Case
I'll consider showing the description text as examples.
6. Replace "Regular expressions" checkbox with new "Regular expressions Syntax" tab.
I will consider two tabs, one for the basic %1 syntax and another for Regex.