Insert a character at a specific location using sed

Question

How to insert a dash into a string after 4 characters using sed (e.g., 201107 to 2011-07)?

Answer 1

echo "201107" | sed 's/201107/2011-07/'

Should work. But just kidding, here's the more general solution:

echo "201107" | sed 's/^\(.\{4\}\)/\1-/' 

This will insert a - after the first four characters.

HTH

Answer 2

For uncertain sed version (at least my GNU sed 4.2.2), you could just do:

echo "201107" | sed 's/./&-/4'

/4 -> replace for the 4th occurrence (of search pattern .)

& -> back reference to the whole match

I learned this from another post: https://unix.stackexchange.com/questions/259718/sed-insert-character-at-specific-positions