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Introduction to Linux - A Hands on Guide
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This example will remove comments everywhere except the lines between the two keywords:
sed -e '1,/start/ s/#.*//' -e '/stop/,$ s/#.*//'
What this is doing is using the -e parameter to specify that the pieces between single quotes are to be used as a SED expression. The first expression means:
Starting with the first line (specified by the 1) and continuing until "the pattern 'start' is found" (specified by the /start/) perform the substitution of replacing "#" followed by "any number of" (specifed by the *) "any character" (specified by the ".") with nothing (specified by the emptyness between the second / and the last / in the substitution command).
I'll leave decoding the second expression as an exercise for you.
For your homework, you may also need to know that '^' matches the beginning of a line.
forrestt has given you a link to the addressing feature of SED. The general construct is to first specify an "address range" and then operate within that range.
sed '/brown/,/car/s/dog/cat/' filename
Start with the first line containing "brown", and end with the first line containing "car". In this range, replace the first occurrence on each line of "dog" with "cat"