Learning Sed

sed 's/day/night/'

s Substitute command
/../../ Delimiter
day Regular Expression Pattern Search Pattern
night Replacement string

Using & as the matched string

% echo "123 abc" | sed 's/[0-9]*/& &/'
123 123 abc

Using \1 to keep part of the pattern

sed 's/\([a-z]*\) \([a-z]*\)/\2 \1/'

/g – Global replacement

sed 's/[^ ][^ ]*/(&)/g' <old >new

Multiple commands with -e command

sed -e 's/a/A/' -e 's/b/B/' <old >new

Passing arguments into a sed script

#!/bin/sh
sed -n 's/'"$1"'/&/p'

notice :
{} () has to be escaped as \{\} \(\)