PL/PGSQL function to convert text Grid Reference into OSGB Geometry

function_get_geom_from_gridref.sql

-- convert a grid-reference E.g. SH123456 into a northing easting geometry for postgis with SRID=27700 (OSGB36)
-- standing on the shoulders of...
-- http://www.movable-type.co.uk/scripts/latlong-os-gridref.html
-- https://github.com/chrisveness/geodesy/blob/master/osgridref.js [MIT]
-- consider this MIT licensed also.
CREATE OR REPLACE FUNCTION get_geom_from_grid_ref(IN grid_ref character varying)
    RETURNS public.geometry
    LANGUAGE 'plpgsql'
    
AS $BODY$
DECLARE
    parts text[];
    l1 integer; l2 integer;
    e100km integer; n100km integer;
    easting varchar; northing varchar;
BEGIN
	parts := regexp_matches(grid_ref,  '^([S,N,H,J,O,T])([A,B,C,D,E,F,G,H,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z]) ?([0-9]{5}) ?([0-9]{5})$');
    if parts is null or array_length(parts, 1) = 0 then
		parts := regexp_matches(grid_ref,  '^([S,N,H,J,O,T])([A,B,C,D,E,F,G,H,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z]) ?([0-9]{4}) ?([0-9]{4})$');
    end if;
	 
    if parts is null or array_length(parts, 1) = 0 then
        parts := regexp_matches(grid_ref,  '^([S,N,H,J,O,T])([A,B,C,D,E,F,G,H,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z]) ?([0-9]{3}) ?([0-9]{3})$');
    end if;
	 
     -- abandon.. not a grid ref
     if parts is null or array_length(parts, 1) = 0 then
        RAISE NOTICE 'Invalid grid reference: %', grid_ref;
        return null; 
     end if;

--    // get numeric values of letter references, mapping A->0, B->1, C->2, etc:
--    var l1 = gridref.toUpperCase().charCodeAt(0) - 'A'.charCodeAt(0);
--    var l2 = gridref.toUpperCase().charCodeAt(1) - 'A'.charCodeAt(0);
--    // shuffle down letters after 'I' since 'I' is not used in grid:
--    if (l1 > 7) l1--;
--    if (l2 > 7) l2--;
	  l1 := ascii(parts[1]) - ascii('A');
      l2 := ascii(parts[2]) - ascii('A');
      IF l1 > 7 THEN l1 := l1 - 1; END IF;
      IF l2 > 7 THEN l2 := l2 - 1; END IF;
   
--    // convert grid letters into 100km-square indexes from false origin (grid square SV):
--    var e100km = ((l1-2)%5)*5 + (l2%5);
--    var n100km = (19-Math.floor(l1/5)*5) - Math.floor(l2/5);

	  e100km := ((l1-2)%5)*5 + (l2%5);
      n100km := (19-floor(l1/5)*5) - floor(l2/5);
      
      IF (e100km<0 or e100km>6 or n100km<0 or n100km>12) THEN
      	RAISE EXCEPTION 'Invalid grid reference: %', grid_ref;
      END IF;
      
      easting  := e100km::varchar || rpad(parts[3], 5, '0');
      northing := n100km::varchar || rpad(parts[4], 5, '0');
      
      
      
return public.ST_GeomFROMEWKT('SRID=27700;POINT(' || easting || ' ' || northing ||  ')');
END
$BODY$;

function_get_gridref_from_geom.sql

-- convert SRID:27700-based OSGB36 coordinate point geometry to a OS Grid Reference  E.g. SH123456
-- standing on the shoulders of...
-- http://www.movable-type.co.uk/scripts/latlong-os-gridref.html
-- https://github.com/chrisveness/geodesy/blob/master/osgridref.js [MIT]
-- consider this MIT licensed also.
CREATE OR REPLACE FUNCTION get_grid_ref_from_geom(geom public.geometry)
 RETURNS character varying
 LANGUAGE plpgsql
AS $function$
        DECLARE
            e integer;     n integer;
            e100k integer; n100k integer;
            l1 integer;    l2 integer;
            letterPair varchar;
        begin

                e := ST_X(geom)::integer;
                n := ST_Y(geom)::integer;

        -- // get the 100km-grid indices
                e100k := floor(e/100000);
                n100k := floor(n/100000);

        --      // translate those into numeric equivalents of the grid letters
        --    var l1 = (19-n100k) - (19-n100k)%5 + Math.floor((e100k+10)/5);
        --    var l2 = (19-n100k)*5%25 + e100k%5;

                l1 := (19-n100k) - (19-n100k)%5 + floor((e100k+10)/5);
                l2 := (19-n100k)*5%25 + e100k%5;

        --    // compensate for skipped 'I' and build grid letter-pairs
        --    if (l1 > 7) l1++;
        --    if (l2 > 7) l2++;

                IF l1 > 7 THEN l1 := l1 + 1; END IF;
                IF l2 > 7 THEN l2 := l2 + 1; END IF;

        --  var letterPair = String.fromCharCode(l1+'A'.charCodeAt(0), l2+'A'.charCodeAt(0));
                letterPair := chr( ascii('A') + l1  ) || chr( ascii('A') + l2  );

        --    // strip 100km-grid indices from easting & northing, and reduce precision
        --    e = Math.floor((e%100000)/Math.pow(10, 5-digits/2));
        --    n = Math.floor((n%100000)/Math.pow(10, 5-digits/2));
                e := floor(e%100000);
                n := floor(n%100000);


        --        // pad eastings & northings with leading zeros (just in case, allow up to 16-digit (mm) refs)
        --    e = ('00000000'+e).slice(-digits/2);
        --    n = ('00000000'+n).slice(-digits/2);
        --    return letterPair + ' ' + e + ' ' + n;

                return letterPair || e || n;

        END
        $function$