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how to change one line in a file?

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# 1  
Old 02-18-2009
how to change one line in a file?
Suppose I have a file. The content of the file is:
Fwf33
#enable_a STH.
fwqfw
egvega

What I want to do is: if "STH" = "YES", do not change it, otherwise, change it to "YES". e.g.

if
“enable_a ERR”
change it to be
“enable_a YES”

if
“enable_a YES”
do not change the file.

How can I implement this using perl, sed, or other unix command?
Thanks,
# 2  
Old 02-18-2009
Code:
sed 's/enable_a 	ERR/enable_a 	YES/' FILE > NEWFILE

# 3  
Old 02-18-2009
Code:
awk '/enable_a/ && $NF=="ERR"{$NF="YES"}1'  FILE > NEWFILE

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