sprintf function for PostgreSQL with indexed arguments

sprintf.psql

-- Inspired by https://wiki.postgresql.org/wiki/Sprintf,
-- but really, pretty different.
CREATE FUNCTION sprintf(_format TEXT, VARIADIC _args ANYARRAY) RETURNS TEXT
LANGUAGE PLPGSQL
STRICT IMMUTABLE
AS $$
DECLARE _f INT DEFAULT 0; -- Index of current character in format string
DECLARE _c TEXT; -- Current character
DECLARE _match TEXT;
DECLARE _matches TEXT[];
DECLARE _result TEXT DEFAULT '';
BEGIN
  LOOP
    _f := _f + 1;
    IF _f > CHAR_LENGTH(_format) THEN
      EXIT;
    END IF;
    _c = SUBSTRING(_format, _f, 1);
    IF _c = '%' THEN
      _matches := REGEXP_MATCHES(SUBSTRING(_format FROM _f), '(^%(\d*)%)');
      IF _matches IS NOT NULL THEN
        _match := _matches[2]; 
        IF _match = '' THEN
          _result := _result || _c;
          _f := _f + 1;
        ELSE
          _result := _result || _args[_match::INT]::TEXT; 
          _f := _f + CHAR_LENGTH(_matches[1]) - 1;
        END IF;
      ELSE
        _result := _result || _c;
      END IF;
    ELSE
      _result := _result || _c;
    END IF;
  END LOOP;
  RETURN _result;
END
$$;

Here's how you use this bugger:

sprintf('The odds are %1%%%, got it? %1%!', 50);

Which becomes:

'The odds are 50%, got it? 50!'