Ernst-Georg Schmid: Deriving the elemental composition from a molformula in pgchem::tigress

pgchem::tigress can generate molecular formulae like C3H6NO2- from chemical structures.

But what if we need access to the elemental composition as a relation, e.g:

element	count
C	3
N	1
O	2

Fortunately, PostgreSQL is awesome:

CREATE OR REPLACE FUNCTION elemental_composition(molformula TEXT)
  RETURNS TABLE(element TEXT, count INTEGER) AS
$BODY$
DECLARE token TEXT[];
DECLARE elements TEXT[];
BEGIN
elements := ARRAY['C','N','O','P','S','Cl'];
molformula := REPLACE(REPLACE(molformula,'-',''),'+','');

FOREACH element IN ARRAY elements LOOP
count := 1;
token := REGEXP_MATCHES(molformula, element || '[\d?]*');

IF (token[1] IS NOT NULL) THEN
    token := REGEXP_MATCHES(token[1],'[0-9]+');
        IF (token[1] iS NOT NULL) THEN
            count := token[1]::INTEGER;
        END IF;
RETURN NEXT;
END IF;
END LOOP;
RETURN;
END;
$BODY$
  LANGUAGE plpgsql IMMUTABLE STRICT
  COST 1000;

SELECT * FROM elemental_composition('C3H6NO2-');

And that's it. Did I already mention that PostgreSQL is awesome? :-)