Keeping only A-Z . , in a text file

Hi, im wanting to no, what is needed to remove all characters like £ $ from a text file, to leave only normal letters remaining? Thanks.

read file (for example usinf fgetc)
check char - using isupper, or isalpha or other suitable function
write char to the new file if it is ok
delete the old file and rename the new file

if file is not too big:
read file (for example usinf fgetc)
check char - using isupper, or isalpha or other suitable function
add it to the buffer if condition passed

when the whole file is processed - overwrite it with the buffer prepared