# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(<.*Page #.*$)/$1\nfoo\nbar\n/gm;t' <<< " <text:p text:style-name=\"P27\"><text:span text:style-name=\"T20\">Page #</text:span><text:span text:style-name=\"T19\"><text:tab/></text:span><text:span text:style-name=\"T20\">Content</text:span></text:p>
<text:p text:style-name=\"P27\"><text:span text:style-name=\"T20\">Page #</text:span><text:span text:style-name=\"T19\"><text:tab/></text:span><text:span text:style-name=\"T20\">Content</text:span></text:p>
<text:p>some other text</text:p>
<text:p text:style-name=\"P27\"><text:span text:style-name=\"T20\">Page #</text:span><text:span text:style-name=\"T19\"><text:tab/></text:span><text:span text:style-name=\"T20\">Content</text:span></text:p>"
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