Extracting blocks from XML file

I have a large XML file and I want to extract blocks from it.

For example, there is a large block:

<tag>
....
</tag>

I want to check if there is a specific string within the block - if yes, then extract the entire block from <tag> to </tag>.

I tried using nawk but I didnt know how to make the "if" part. It's a Sun machine, thus Sun OS.

sed -n "/\<${1}\>/,/\<\/${1}\>/p" INFILE > OUTFILE

would probably do you, passing the tag as the parameter $1

not really, Nuth
"/\<${1}\>/,/\<\/${1}\>/p"

a) ${1} is not wrong but not needed and in this context, confusing, use $1 instead, so
"/\<$1\>/,/\<\/$1\>/p"

b) \<abc\> in sed is used to isolate word-boundary 'abc'
it casually match
'<abc>' but also ',abc,' or 'abc' and ' abc ' ...
you need to match '<abc>' and '</abc>' also use:
"/<$1>/,/<\/$1>/p"

c) assumed 'tag' is NOT expected in the output AND it is on
it own line, a correct sed cmd would be:

sed -n "/<tag>/,/<\/tag>/{;/<tag>/d;/<\/tag>/d;p;}"

repalce tag by $1 if you want. if you like precision:

"/^<tag>$/,/^<\/tag>$/{;/^<tag>$/d;/^<\/tag>$/d;p;}"

on a production system, i also would check for blancs