bash split strings

Given: foo=/tmp/my.dir/filename.tar.gz
We can use these expressions:
path = ${foo%/*}
To get: /tmp/my.dir (like dirname)
file = ${foo##*/}
To get: filename.tar.gz (like basename)
base = ${file%%.*}
To get: filename
ext = ${file#*.}
To get: tar.gz

${variable%pattern}
Trim the shortest match from the end
${variable##pattern}
Trim the longest match from the beginning
${variable%%pattern}
Trim the longest match from the end
${variable#pattern}
Trim the shortest match from the beginning

If we are interested split a string by a delimiter, we can use "tr"
for i in $filename;do
echo $i|tr "_" "\n"
done