$re = '/span
:[^)]*[)];?/';
$str = '<span style="background-image: url ("http://mantis.we.intern/custom/userfiles/image/6y0eC4vzptnIxsikHs0AJA.png"); bgcolor:\'red\'">11111<br />
<span style="background-image: url(\'https:// asd asdmantis.we.intern/custom/userfiles/image/6y0eC4vzptnIxsikHs0AJA.png\')">22222
<span style="background-image:url(\'https://fmantis.we.intern/custom/userfiles/image/6y0eC4vzptnIxsikHs0AJA.png\')">3333
<span style="background-image:url( \'https://fmantis.we.intern/custom/userfiles/image/6y0eC4vzptnIxsikHs0AJA.png\')">444
<span style="background-image: url ( "http://mantis.we.intern/custom/userfiles/image/6y0eC4vzptnIxsikHs0AJA.png");">555<br />
<span style="background-image: url(xx https://trk.workexpert.net/web/include/ckeditor_432/plugins/icons.png?t=E0LB& xx); >666';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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 PHP, please visit: http://php.net/manual/en/ref.pcre.php