Something
public text v1 · immutable'/\bOPEN\b/' => ' <span class="sv_keyword_def" title="<span class="sv_keyword_def" title="Prix d\'ouverturede la journée">OPEN</span> '
$this -> strHTML = preg_replace( $this -> words_defined, $this -> definitions, $this -> strHTML );
gives me : <span class="sv_keyword_def" title="<span class="sv_keyword_def" title="Prix d'ouverturede la journée">OPEN</span>