sed replace in files in directory code example

Example 1: find and replace in all files in directory centos

grep -rl 'apples' /dir_to_search_under | xargs sed -i 's/apples/oranges/g'

Example 2: sed replace in file

sed -i 's/foo/bar/g' hello.txt

Example 3: sed replace

For using match in sed replacement, just border it with '\(' and '\)':
echo Before123 | sed 's/Before\([0-9]*\)/\1After/g'
123After	# number is matched withtin '\( \)' and replaced in '\1'
Example with 2 match replacements
echo a_b | sed 's/\(^.*\)_\(.*$\)/first is \1 and \2 is after/g'