I strongly propose a simple method of inputting the variables simply by tap and hold which would auto inject them into wherever the cursor is on the line.

My reasons are:

1.) Android system keyboard's often feature characters such as percent % and more in an keyboard extended menu, just like numeric text is in an extended keyboard menu. This makes the job of adding the Batch Rename variables extra longer and tedious

2.) There are Batch Rename variables with the same letter but with different case size (upper case and lower case). This can be confusing to a first time user but also cause some delay with the Android system keyboard especially when the keyboard would auto-capitalize the letters. One such example of the variable with different size is:
%n and %N

3.) The speed productivity of Tap+Hold for variables entering would be much benefiting.

4.) Perhaps a vibration haptic feedback could occur on the phone when tap+hold a variable, especially for users with larger fingers. Vibration would form part of the accuracy of input.

Related topic: http://neatbytes.uservoice.com/forums/274528-solid-explorer-2-0/suggestions/9511950-remove-first-or-last-characters-batch-rename-imp