# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/<DisplayContents>[\s\S]*?<Contents>(.*?)<\\/Contents>[\s\S]*?<\\/DisplayContents>/$1/gm;t' <<< "<DisplayContents>
<ID>8</ID>
<Type>102</Type>
<Contents>A whole bunch of stuff in this tag</Contents>
</DisplayContents>
<DisplayContents>
<ID>8</ID>
<Type>102</Type>
<Contents>A whole bunch of stuff in this tag</Contents>
</DisplayContents>"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html