sed replace character in string code example

Example 1: replace using sed

#on my mac
sed -i -e 's/old-text/new-text/g' text.txt

Example 2: 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'