Debugging rtf to text function - sql-server

Question

I have a function we use to convert RTF formatted text to plain text. It has worked pretty well in the past, and seems to work pretty well so far on the text in question.
However, somewhere in my dataset of 230,000 records, it makes a bad SUBSTRING call and aborts the entire thing (without telling me the offending record).
Is there any way I can get some feedback into what is going on?
I know that SQLServer functions do not allow PRINT statements, or INSERT statements.
And the dataset of 230,000 records is not mine, but a clients. I really don't want to have to try to go record by record and see which one is causing the error.
SQL Function below:
CREATE FUNCTION [dbo].[RTF2Text]
(
#rtf nvarchar(max)
)
RETURNS nvarchar(max)
AS
BEGIN
DECLARE #Pos1 int;
DECLARE #Pos2 int;
DECLARE #hex varchar(316);
DECLARE #Stage table
(
[Char] char(1),
[Pos] int
);
INSERT #Stage
(
[Char]
, [Pos]
)
SELECT SUBSTRING(#rtf, [Number], 1)
, [Number]
FROM [master]..[spt_values]
WHERE ([Type] = 'p')
AND (SUBSTRING(#rtf, Number, 1) IN ('{', '}'));
SELECT #Pos1 = MIN([Pos])
, #Pos2 = MAX([Pos])
FROM #Stage;
DELETE
FROM #Stage
WHERE ([Pos] IN (#Pos1, #Pos2));
WHILE (1 = 1)
BEGIN
SELECT TOP 1 #Pos1 = s1.[Pos]
, #Pos2 = s2.[Pos]
FROM #Stage s1
INNER JOIN #Stage s2 ON s2.[Pos] > s1.[Pos]
WHERE (s1.[Char] = '{')
AND (s2.[Char] = '}')
ORDER BY s2.[Pos] - s1.[Pos];
IF ##ROWCOUNT = 0
BREAK
DELETE
FROM #Stage
WHERE ([Pos] IN (#Pos1, #Pos2));
UPDATE #Stage
SET [Pos] = [Pos] - #Pos2 + #Pos1 - 1
WHERE ([Pos] > #Pos2);
SET #rtf = STUFF(#rtf, #Pos1, #Pos2 - #Pos1 + 1, '');
END
SET #rtf = REPLACE(#rtf, '\pard', '^*^');
SET #rtf = REPLACE(#rtf, '\par', '^*^');
SET #rtf = REPLACE(#rtf, '\t', '^~^');
SET #rtf = STUFF(#rtf, 1, CHARINDEX(' ', #rtf), '');
IF len(#rtf) > 0
WHILE (Right(#rtf, 1) IN (' ', CHAR(13), CHAR(10), '}'))
BEGIN
SELECT #rtf = SUBSTRING(#rtf, 1, (LEN(#rtf + 'x') - 2));
IF LEN(#rtf) = 0 BREAK
END
SET #Pos1 = CHARINDEX('\''', #rtf);
WHILE #Pos1 IS NOT NULL AND #Pos1 > 0
BEGIN
IF #Pos1 IS NOT NULL AND #Pos1 > 0
BEGIN
SET #hex = '0x' + SUBSTRING(#rtf, #Pos1 + 2, 2);
SET #rtf = REPLACE(#rtf, SUBSTRING(#rtf, #Pos1, 4), CHAR(CONVERT(int, CONVERT (binary(1), #hex,1))));
SET #Pos1 = CHARINDEX('\''', #rtf);
END
END
SET #rtf = COALESCE(#rtf, '') + ' ';
SET #Pos1 = PATINDEX('%\%[0123456789][\ ]%', #rtf);
WHILE #Pos1 IS NOT NULL AND #Pos1 > 0 AND #rtf != ''
BEGIN
SET #Pos2 = CHARINDEX(' ', #rtf, #Pos1 + 1);
IF #Pos2 < #Pos1
SET #Pos2 = CHARINDEX('\', #rtf, #Pos1 + 1);
IF #Pos2 < #Pos1
BEGIN
SET #rtf = SUBSTRING(#rtf, 1, #Pos1 - 1);
SET #Pos1 = 0;
END
ELSE
BEGIN
SET #rtf = STUFF(#rtf, #Pos1, #Pos2 - #Pos1 + 1, '');
SET #Pos1 = PATINDEX('%\%[0123456789][\ ]%', #rtf);
END
END
IF RIGHT(#rtf, 1) = ' '
SET #rtf = SUBSTRING(#rtf, 1, LEN(#rtf) -1);
RETURN #rtf;
END

Answer 1

Not to be too rude, but have you actually tested on your function?
Have you run any unit tests to try and break your function, e.g. invalid values, boundary conditions etc.?
Have you checked documentation to see under what conditions SUBSTRING can throw an exception?
I have run these cases and am getting exceptions:
SELECT dbo.[RTF2Text]( NULL )
SELECT dbo.[RTF2Text]( '' )
SELECT dbo.[RTF2Text]( '1' )
SELECT dbo.[RTF2Text]( 'blah' )
If you know under what conditions/input values your function will fail, then it is a simple matter of checking for these in your table.

Answer 2

I had a similar situation and have very little knowledge of SQL functions however needed to strip RTF and tried this code. Debugging suggested that this function was failing was here as I was getting Invalid length parameter.
IF RIGHT(#rtf, 1) = ' '
SET #rtf = SUBSTRING(#rtf, 1, LEN(#rtf) -1);
As I have minimal knowledge and lack of time I just added a second if to make sure it wasn't doing a subtraction i.e. -1 from 0 which worked for my dataset.
IF RIGHT(#rtf, 1) = ' '
IF LEN(#rtf) > 0
SET #rtf = SUBSTRING(#rtf, 1, LEN(#rtf) -1);