# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(<img .*?imgur\.com(.*?))(?=\\/?\s?>)/$1 class="newClass"/gm;t' <<< "I have been searching <img src=\"https://i.imgur.com/NknpNdu.jpg\" alt=\"\"> for... I have been searching <img src=\"https://i.imgur.com/NknpNdu.jpg\" alt=\"\"> for... I have been searching <img src=\"https://i.imgur.com/NknpNdu.jpg\" alt=\"\"> for... I have been searching <img src=\"https://i.imgur.com/NknpNdu.jpg\" alt=\"\" /> for... "
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