Or this:

DECLARE @pString VARCHAR(8000) = '-[ab]+[cd]-[ef]';

WITH E1(N) AS (

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1

), --10E+1 or 10 rows

E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows

E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max

cteTally(N) AS (--==== This provides the "zero base" and limits the number of rows right up front

-- for both a performance gain and prevention of accidental "overruns"

SELECT 0 UNION ALL

SELECT TOP (DATALENGTH(ISNULL(@pString,1))) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4

),

cteStart(N1, pString) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)

SELECT t.N+1, pString

FROM cteTally t

CROSS APPLY (SELECT pString = CASE WHEN LEFT(@pString,1) IN ('+','-') THEN STUFF(@pString,1,1,'') ELSE @pString END) x

WHERE SUBSTRING(x.pString,t.N,1) IN ('+','-') OR t.N = 0

)

--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.

SELECT

ItemNumber = ROW_NUMBER() OVER(ORDER BY s.N1),

Items = REPLACE(REPLACE(

SUBSTRING(pString,s.N1,ISNULL(NULLIF((LEAD(s.N1,1,1) OVER (ORDER BY s.N1) - 1),0)-s.N1,8000))

,'[',''),']',''),

Signs = CASE

WHEN s.N1-1 = 0 THEN CASE WHEN LEFT(@pString,1) ='-' THEN '-' ELSE '+' END

ELSE SUBSTRING(pString,s.N1-1,1) END

FROM cteStart s;