Postgres 11

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CREATE OR REPLACE FUNCTION escape_html(p_input text)
  RETURNS text AS 
$$
DECLARE   
  l_clean_value text;
  l_to_replace text[];
  l_tag text;
BEGIN

-- ersetze alle "einfach" < und > zuerst 
  l_clean_value := regexp_replace(p_input, '(\s+)<(\s+)', '\1&lt;\2', 'g');
  l_clean_value := regexp_replace(l_clean_value, '(\s+)>(\s+)', '\1&gt;\2', 'g');

-- finde alle tags die nicht geschlossen werden
  select coalesce(array_agg(tag), '{}')
     into l_to_replace
  from (
    select translate(x2.open_tag, '<>', '') as tag
    from regexp_matches(l_clean_value, '<\w+>', 'g') as x1(open_tags),
         unnest(x1.open_tags) as x2(open_tag)
    except 
    select translate(x3.close_tag, '</>', '') as close_tag
    from regexp_matches(l_clean_value, '</\w+>', 'g') as x2(close_tags),
         unnest(x2.close_tags) as x3(close_tag)
  ) x;
  
  -- in dem array l_to_replace stehen jetzt alle tags die kein schliessendes Tag haben
  -- diese können direkt ersetzt werden
  foreach l_tag in array l_to_replace loop
    l_clean_value := replace(l_clean_value, concat('<', l_tag, '>'), concat('&lt;', l_tag, '&gt;'));
  end loop;
  return l_clean_value;
END;
$$
LANGUAGE plpgsql;

escape_html
<table><tr><td><nobr>Daten1</nobr></td><td> <nobr>Daten2</nobr> </td></tr> <tr><td>Daten3</td></tr> <tr><td>Daten4</td></tr> <tr><td>Daten5</td></tr> <tr><td></td></tr> <tr><td>DatenText</td></tr> </table>