Insert a character at a specific location using sed

How to insert a dash into a string af开发者_运维百科ter 4 characters using sed (e.g., 201107 to 2011-07)?

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

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

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

This will insert a - after every four characters.

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

The following will match perform the replacement only on strings of four digits, followed by strings of two digits.

$ echo '201107' | sed 's/\([0-9]\{4\}\)\([0-9]\{2\}\)/\1-\2/'
2011-07

Sed regex is a little awkward, because of the lack of \d, and the need to escape parenthesis and braces (but not brakets). My solution ought to be 's/(\d{4})(\d{2})/\1-\2/' if not for these peculiarities.

I know this is diggin up a massive dinosaur, but if you wanted to eliminate the 6 tailing zeros

echo '20110715000000' | sed 's/\([0-9]\{4\}\)\([0-9]\{2\}\)\([0-9]\{2\}\)\([0-9]\{6\}\)/\1-\2-\3/'