Dear GNU, Not sure if this is bug. But I think first one is legal command that is logically similar to second which works Thank you, Raj. #1-- Error [raj@rajvista ~]$ echo "Raj" | tr "[[:alpha:]]" "[[:lower:]]" tr: misaligned [:upper:] and/or [:lower:] construct #2--Ok [raj@rajvista ~]$ echo "Raj" | tr "[[:upper:]]" "[[:lower:]]" raj [raj@rajvista ~]$ [raj@rajvista ~]$ tr --version tr (GNU coreutils) 6.12 Copyright (C) 2008 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Written by Jim Meyering.