SHELL

replace using sed

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

replace tab sed

sed -i.bak $'s/	/  /' file.txt	#replace tab with sed command

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'