Replace first 5 characters - awk and sed

Input file:

$ cat file.txt
sl909678342-slp10
sl919678362-slp11
sl929678322-slp12
sl929678382-slp12

Required output:

Replace first 5 characters in each line of the above file with 'XXXXX'. i.e. the required output:

XXXXX678342-slp10
XXXXX678362-slp11
XXXXX678322-slp12
XXXXX678382-slp12

Awk solution:

$ awk '{
    first=substr($0,1,5)
    gsub(/./,"X",first)
    end=substr($0,6)
    print first end
}' file.txt

Read about awk substr function here

Sed solution:

Using 'extended regular expressions' with sed (-r option)

$ sed -r "s/^(.{0})(.{5})/\1XXXXX/" file.txt

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