sed match { code example

Example 1: sed from match to other match

Just use following structure to get result between two different patterns:
sed -n '/^pattern1/,/^pattern2/p;/^pattern2/q' file.txt

Example 2: 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'