Magic in nested branches

why.sql

CREATE TABLE test_data (
    housenumber text
);

INSERT INTO test_data VALUES
('123;456'),
('789;'),
(';'),
('abc;def'),
('');

CREATE OR REPLACE FUNCTION display_housenumber_nonnumeric(raw_housenumber text)
RETURNS text AS $$
  SELECT substring(raw_housenumber from 1 for position(';' in raw_housenumber) - 1)
         || '–'
         || substring(raw_housenumber from position(';' in raw_housenumber) + 1);
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION display_housenumber_numeric(raw_housenumber text)
RETURNS text AS $$
  SELECT min(value)::text || '–' || max(value)::text
     FROM unnest(string_to_array(raw_housenumber, ';')::int[]) AS value;
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION display_housenumber(raw_housenumber text)
RETURNS text AS $$
  SELECT CASE
    WHEN raw_housenumber !~ ';' THEN raw_housenumber
    WHEN raw_housenumber ~ '[^0-9;]' THEN display_housenumber_nonnumeric(raw_housenumber)
    else display_housenumber_numeric(raw_housenumber)
  END
$$ LANGUAGE SQL IMMUTABLE;

SELECT display_housenumber('') FROM test_data;
SELECT display_housenumber(housenumber) FROM test_data;