# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/\/upload\/[^\/]+\//\/upload\//gm;t' <<< "<div class=\"slide\"><a class=\"slide__card fancybox\" rel=\"gallery1\" href=\"/upload/f2d/f2d520b5d31920e1227c4414cd4f214a.jpg\"><img src=\"/upload/f2d/700_525_1/f2d520b5d31920e1227c4414cd4f214a.jpg\" /><span class=\"zoom-cell\"></span></a></div><div class=\"slide\"><a class=\"slide__card fancybox\" rel=\"gallery1\" href=\"/upload/3ff/3ff30ba3ec6e5d5b1b753b3749ff601c.jpg\"><img src=\"/upload/3ff/700_525_1/3ff30ba3ec6e5d5b1b753b3749ff601c.jpg\" /><span class=\"zoom-cell\"></span></a></div><div class=\"slide\"><a class=\"slide__card fancybox\" rel=\"gallery1\" href=\"/upload/0aa/0aa859109670c4aac06eaee7eec33d6f.jpg\"><img src=\"/upload/0aa/700_525_1/0aa859109670c4aac06eaee7eec33d6f.jpg\" /><span class=\"zoom-cell\"></span></a></div><div class=\"slide\"><a class=\"slide__card fancybox\" rel=\"gallery1\" href=\"/upload/b56/b56537718cabbaa75e0b56056ae8a1db.jpg\"><img src=\"/upload/b56/700_525_1/b56537718cabbaa75e0b56056ae8a1db.jpg\" /><span class=\"zoom-cell\"></span></a></div><div class=\"slide\"><a class=\"slide__card fancybox\" rel=\"gallery1\" href=\"/upload/e99/e99800f9ece27c61fb0c395aed4423d1.jpg\"><img src=\"/upload/e99/700_525_1/e99800f9ece27c61fb0c395aed4423d1.jpg\" /><span class=\"zoom-cell\"></span></a></div>"
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