# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(?s)<body>\K\s*<tag(?:(?!<inner_block=25>).)+<\/tag>//;t' <<< "<body>
<tag>
<number=1>
<inner_block=5>
</tag>
<tag>
<number=12>
<inner_block=15>
</tag>
<tag>
<number=21>
<inner_block=25>
</tag>
<tag>
<number=29>
<inner_block=35>
</tag>
</body>"
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