Suppose we have the following list

for k in newfile{1..4}.txt; do echo $k; done

newfile1.txt
newfile2.txt
newfile3.txt
newfile4.txt

remove from end

for k in newfile{1..4}.txt; do echo ${k%.txt}; done

newfile1
newfile2
newfile3
newfile4

remove from beginning

for k in newfile{1..4}.txt; do echo ${k#new}; done

file1.txt
file2.txt
file3.txt
file4.txt

offset

for k in newfile{1..4}.txt; do echo ${k:7} done

1.txt
2.txt
3.txt
4.txt

offset:length

for k in newfile{1..4}.txt; do echo ${k:7:1}; done

1
2
3
4

replace

for k in newfile{1..4}.txt; do echo ${k/new/old}; done

oldfile1.txt
oldfile2.txt
oldfile3.txt
oldfile4.txt

length

for k in newfile{1..4}.txt; do echo ${#k}; done

12
12
12
12