If you would like to remove the specific character or replacing string in the particular file by the command.

My string is "TTthelinuxXX" and change to be "**thelinux**"

Use the below command to view the output, if you want to update permanently in file execute with "-i" option 



$ sed -r 's/[TX]+/*/g' filename.txt



$ sed -r -i 's/[TX]+/*/g' filename.txt


Another one is found and remove all a-z lower case character from particular file,

$ sed -r 's/[a-z]//g' filename.txt


Another one is found and remove all A-Z uppercase character from particular file,



$ sed -r 's/[A-Z]//g' filename.txt.


have any blank lines can remove that part

sed '/^\s*$/d'



$ sed '/^$/d' filename_in.txt 



$ grep -v '^$' filename_in.txt