Soundex

CREATE OR REPLACE FUNCTION soundex(input text) RETURNS text
IMMUTABLE STRICT COST 500 LANGUAGE plpgsql
AS $$
DECLARE
  soundex text = '';
  char text;
  symbol text;
  last_symbol text = '';
  pos int = 1;
BEGIN
  WHILE length(soundex) < 4 LOOP
    char = upper(substr(input, pos, 1));
    pos = pos + 1;
    CASE char
    WHEN '' THEN
      -- End of input string
      IF soundex = '' THEN
        RETURN '';
      ELSE
        RETURN rpad(soundex, 4, '0');
      END IF;
    WHEN 'B', 'F', 'P', 'V' THEN
      symbol = '1';
    WHEN 'C', 'G', 'J', 'K', 'Q', 'S', 'X', 'Z' THEN
      symbol = '2';
    WHEN 'D', 'T' THEN
      symbol = '3';
    WHEN 'L' THEN
      symbol = '4';
    WHEN 'M', 'N' THEN
      symbol = '5';
    WHEN 'R' THEN
      symbol = '6';
    ELSE
      -- Not a consonant; no output, but next similar consonant will be re-recorded
      symbol = '';
    END CASE;
 
    IF soundex = '' THEN
      -- First character; only accept strictly English ASCII characters
      IF char ~>=~ 'A' AND char ~<=~ 'Z' THEN
        soundex = char;
        last_symbol = symbol;
      END IF;
    ELSIF last_symbol != symbol THEN
      soundex = soundex || symbol;
      last_symbol = symbol;
    END IF;
  END LOOP;
 
  RETURN soundex;
END;
$$;