$excerpts = array(
    "Hello world: This.is, your first post... Edit or delete it, then start blogging!",
    "Hello world: This.is, your first post… Edit or delete it, then start blogging!",
);

foreach ( $excerpts as $excerpt ) {
    preg_match( '#(.*?)([.!?]|…)(\s|$)#', $excerpt, $matches );
    if ( $matches )
        $excerpt = $matches[0];
    echo $excerpt . '<br />';
}