sed replace with regex code example

Example 1: sed replace in file

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

Example 2: sed replace from match

Just use a similar example with using ".*" regex with sed as following:
sed 's/match_pattern.*/replacement_of_match_and_rest_of_line/' file.txt

Example 3: sed use match in replacement

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'