sed - extract a group of Letters/numbers

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# 1  
Old 10-01-2013
sed - extract a group of Letters/numbers

I have a file with hundreds of lines in it. I wanted to extract anything that matches the following:

KR followed by 4 digits:
example KR1201

cat list | sed "s/[KR[0-9$]//g"

Is the closest I've come, and obviously it is not what I want. This would remove all of the items that I want and leave me with the rest of the text. I've tried sed -n and /p' but this doesn't work and I'm confused about the syntax. I know other ways to do this with egrep and grep -w but I want to learn to do it in sed.

The one thing I don't want to do is print the rest of the line; I want only to print the KR1201 or KR followed by 4 digits. There is something missing in my syntax; not sure how to encapsulate the pattern.
# 2  
Old 10-01-2013
As with sort, sed takes pathname operands. Using:
cat file | sed [options] [OtherOperands]

instead of:
sed [options] [OtherOperands] file

wastes system resources, slows down your system for other users who may be sharing your system, and makes your command line run slower.

If you have an input file named list containing:
nothing from this line

Previous line was empty; neither of these should appear in output

and you run the command:
sed -n 's/\(.*\)\(KR[0-9][0-9][0-9][0-9]\).*/\2/p' list

using a standards conforming version of sed, you get the output:

The Linux sed man page says:
POSIX.2 BREs should be supported, but they aren't completely because of performance problems.
and in the description of the sed s(ubstitute) command says:
The replacement may contain the special character & to refer to that portion of the pattern space which matched, and the special escapes \1 through \9 to refer to the corresponding matching sub-expressions in the regexp.
But, there is no other mention of sub-expressions on that man page.

Note that this sed command makes no effort to find more than one occurrence of your pattern if it appears more than once on a single line
This User Gave Thanks to Don Cragun For This Post:
# 3  
Old 10-02-2013
Really appreciate it!

I've learned a lot from your sed/sort tutorials and today was able to solve a sort problem on my own for once after reading all your posts. I thank you for taking the time out of your day to really create detailed posts with examples! This is much more helpful than any textbook!
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