how to make a 'input' text contain only one letter maj and three Numbers ( example code for Garrach_hazem )

how to make a <input> text contain only one letter maj and three Numbers ( example code for Garrach_hazem )

Each time the user modify the input value, it will be verified:

if the value is 'part-valid' ( in this case: 0 or 1 uppercase letter), we store it
else, we replace it with last stored valid value

So, this is a simple way to be sure that each change is valid compatible.

Simple improvments can be to:

[debug version] <input> real-time validator [ code example for @Garrach_hazem ]