# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/\[img]([^'\"&?]*?\/([^'\"&?\/]*\.(?:gif|jpe?g|png|bmp)))\[\\/img]/<img src="$1" alt="$2"\/>/g;t' <<< "style_Forever 21 [img]http://cdn9.lbstatic.nu/files/looks/medium/2013/08/15/3225332_IMG_20130810_2.jpg[/img][img]http://cdn11.lbstatic.nu/files/looks/medium/2013/08/15/3225294_1000340_10201946229836897_118758683_n.jpg[/img]"
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