$re = '~<span class="sense"><span class="bld">A<\/span>(?s)(?!.*<span class="bld">)~m';
$str = '<span class="sense"><span class="bld">A</span> [[lay bare at the side]], [[expose]], τι τῆς πλευρᾶς <span class="bibl">Arr. <span class="title">Tact.</span>40.5</span>, cf. <span class="bibl">D.C.49.6</span> (Pass.). </span><span class="sense"><span class="bld">2</span> metaph., [[lay bare]], [[disclose]], τὸν πάντα λόγον <span class="bibl">Hdt.1.126</span>, cf. <span class="bibl">8.19</span>, <span class="bibl">9.44</span>; τὸ βούλευμα <span class="bibl">Conon 50</span>:—Pass., <b class="b3">παρεγυμνώθη διότι</b>… <span class="bibl">Plb.1.80.9</span>.</span>
<span class="sense"><span class="bld">A</span> [[lay bare at the side]], [[expose]], τι τῆς πλευρᾶς <span class="bibl">Arr. <span class="title">Tact.</span>40.5</span>, cf. <span class="bibl">D.C.49.6</span> (Pass.).';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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