regexp_replace:вмъкнете интервал в низ, ако вече не присъства

Ето едно възможно решение с помощта на REGEXP_REPLACE функция:

-- Match the string "st." followed by zero or more spaces and a word character,
-- replace it with "st." followed by exactly one space and the captured character
select city,
       regexp_replace(city, 'st\.\s*(\w)', 'st. \1' ) as city_formatted
  from t
 order by city;

Изход:

CITY               CITY_FORMATTED      
------------------ --------------------
st.   triple space st. triple space    
st.  double space  st. double space    
st. ulrich         st. ulrich          
st.paul            st. paul