# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(?<=auth\s-\s).*(?=(?:Firefox|Chrome)\\/[\d\.]+)//gm;t' <<< "::1 - OPTIONS - /signin - 204 - 0 - 0.163 ms http://localhost:8080/auth - Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36 -
::1 - POST - /signin - 200 - 545 - 106.758 ms http://localhost:8080/auth - Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36 -
::1 - OPTIONS - /signin - 204 - 0 - 0.126 ms http://localhost:8080/auth - Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:76.0) Gecko/20100101 Firefox/76.0 -"
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