Sed - An Introduction and Tutorial by Bruce Barnett

With no flags,the first matched substitution is changed. With the "g" option,all matches are changed. If you want to modify a particular pattern that is not the first one on the line,you could use "(" and ")" to mark each pattern,and use "1" to put the first pattern back unchanged. This next example keeps the first word on the line but deletes the second:

sed 's/([a-zA-Z]*) ([a-zA-Z]*) /1 /' new

Yuck. There is an easier way to do this. You can add a number after the substitution command to indicate you only want to match that particular pattern. Example:

sed 's/[a-zA-Z]* //2' new

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