# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/^(([^\n\r.]*).*)(?:(?:\r?\n|\r)\2.*)*/\1/gmi;t' <<< "This is test string for my app. d
This is test string for my app.
This is test string for my app. abcd
This is test string for my app.
This is test string for my app.
This is test string for my app.
This is test string for my app. d
This is test string for my app.
This is test string for my app.
This is another string."
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