# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/^\/(media\/catalog\/product\/cache)\/\d+\/thumbnail\/(\d+x\d+)\/[a-z0-9]+\/(\S+)/\/pub\/$1\/$2\/$3/gm;t' <<< "/media/catalog/product/cache/1/thumbnail/543x403/db978388cfd007780066eaab38556cef/l/e/legal_slide01.jpg
# check if match with above:
/pub/media/catalog/product/cache/543x403/l/e/legal_slide01.jpg
/media/catalog/product/cache/1/thumbnail/543x403/db978388cfd007780066eaab38556cef/n/u/number1.png
# check if match with above:
/pub/media/catalog/product/cache/543x403/n/u/number1.png"
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