# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/.+?<span class="remaining">([0-9]+)?<\\/span>(.+?)<\\/span>/$1$2/gm;t' <<< "<span class=\"grey black-text lighten-2 credit-counter\"><span class=\"remaining\">2</span> credits</span>"
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