SHELL

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'

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