Issue (only) when user does not add characters in an orderly manner

[Ifycode]

Case 1: (When using any of the allowed characters - aAnNsS)
E.g. say user types in 'nnnnaA' and generates '7854cZ', then use arrow key or taps in somewhere in the middle to add e.g. letter 's' like so:
'nnsnnaA'
'7854cZF'

's' is not recognised, but assumes you just added another 'A' which now generates the letter 'F'.

Case 2: (When using not-allowed characters)
E.g. say user types in 'nnnnaA' and generates '2679eW', then use arrow key or taps in somewhere in the middle to add e.g. letter 'h' like so: 'nnhnnaA'

Result: '2679eWZ'

'h' (& all other not-allowed characters are) programmed to yield empty string ''. It still assumes you just added another 'A' which now generates the letter 'Z'. Causing the first to have 6 characters and the generated string to have 7 characters (instead of 6) as shown below.

'nnnnaA'
'2679eWZ'