Can the Select list in a SQL Statement use Regular Expressions - sql

Question

I have a SQL statement,
select ColumnName from Table
And I get this result,
Error 192.168.1.67 UserName 0bce6c62-1efb-416d-bce5-71c3c8247b75 An existing ....
So anyway the field has a lot of stuff in it, I just want to get out the 'UserName'.
Can I use a regex for that?
I mean it would be kind of like this,
select SUBSTRING(ColumnName, 0, 5) from Table
Except the SUBSTRING would be replaced with a regex of some kind. I am comfortable with regex, but I am not sure how to apply it in this case, or even if you can.
If I could get this working it would be great because I plan to pull the data into a temporary table, and do some quite complicated things matching it with other tables etc. If I can get this all working it would save me writing a C# app to do it with.
Thanks.

Answer 1

No, out of the box, SQL Server doesn't support regexs.
You could retrofit those by means of a SQL-CLR assembly that you deploy into SQL Server.

Answer 2

I think going you should use SUBSTRING anyway. Using regular expression is more flexible but also lead to a large processing overhead. This becomes even worse if your have to process a large recordsets.
You have to justify if there's the need for flexibility in first place.
If so you should read about it here:
http://msdn.microsoft.com/en-us/magazine/cc163473.aspx
Using T-SQL only can look like that:
SELECT 'Error 192.168.1.67 XUserNameX 0bce6c62-1efb-416d-bce5-71c3c8247b75 An existing' expr
INTO log_table
GO
WITH
split1 (expr, cstart, cend)
AS (
SELECT
expr, 1, 0
FROM
log_table a
), split2 (expr, cstart, cend, div)
AS (
SELECT
a.expr, a.cend + 1, CHARINDEX(' ', a.expr, a.cend + 1), 1
FROM
split1 a
UNION ALL
SELECT
a.expr, a.cend + 1, CHARINDEX(' ', a.expr, a.cend + 1), div+1
FROM
split2 a
WHERE
a.cend > 1
), substrings(expr, div)
AS (
SELECT
SUBSTRING(expr, cstart, cend - cstart), div
FROM
split2
)
SELECT expr from
substrings a
where
a.div = 3

Answer 3

UPDATE
we cannot tell where the start of the
username is. Unless we can say 'find
me the start character after the
second space'
That is fairly straightforward:
Filter out strings that have fewer than
two spaces (alternatively, have three
or more words);
Find the position after the first
space (alternatively, the beginning
of the second word);
Find the position after the the first
space after the first space
(alternatively, the beginning of the
third word);
Determine the length of the third
word using the position of the next
space (or the end of the string is
there are only three words);
Use the above values with the
SUBSTRING() function to return the
third word.
Example:
WITH MyTable (ColumnName)
AS
(
SELECT NULL
UNION ALL
SELECT ''
UNION ALL
SELECT 'One.'
UNION ALL
SELECT 'Two words.'
UNION ALL
SELECT 'Three word sentence.'
UNION ALL
SELECT 'Sentence containing four words.'
UNION ALL
SELECT 'Five words in this sentence.'
UNION ALL
SELECT 'Sentence containing more than five words.'
),
AtLeastThreeWords (ColumnName, pos_word_2_start)
AS
(
SELECT M1.ColumnName, CHARINDEX(' ', M1.ColumnName) + LEN(' ') + 1
FROM MyTable AS M1
WHERE LEN(M1.ColumnName) - LEN(REPLACE(M1.ColumnName, ' ', '')) >= 2
),
MyTable2 (ColumnName, pos_word_3_start)
AS
(
SELECT M1.ColumnName,
CHARINDEX(' ', M1.ColumnName, pos_word_2_start) + LEN(' ') + 1
FROM AtLeastThreeWords AS M1
),
MyTable3 (ColumnName, pos_word_3_start, pos_word_3_end)
AS
(
SELECT M1.ColumnName, M1.pos_word_3_start,
CHARINDEX(' ', M1.ColumnName, pos_word_3_start) + LEN(' ')
FROM MyTable2 AS M1
),
MyTable4 (ColumnName, pos_word_3_start, word_3_length)
AS
(
SELECT M1.ColumnName, M1.pos_word_3_start,
CASE
WHEN pos_word_3_start < pos_word_3_end
THEN pos_word_3_end - pos_word_3_start
ELSE LEN(M1.ColumnName) - pos_word_3_start + 1
END
FROM MyTable3 AS M1
)
SELECT M1.ColumnName,
SUBSTRING(M1.ColumnName, pos_word_3_start, word_3_length)
AS word_3
FROM MyTable4 AS M1;
ORIGINAL ANSWER:
Is the problem that the position and/or length of the username value may not be constant in the data but always follows the string 'username '? If so, you can use CHARINDEX with SUBSTRING e.g.
WITH MyTable (ColumnName)
AS
(
SELECT 'Error 192.168.1.67 UserName 0bce6c62-1efb-416d-bce5-71c3c8247b75 An existing ....'
UNION ALL
SELECT 'Username onedaywhen is invalid'
),
MyTable1 (ColumnName, pos1)
AS
(
SELECT M1.ColumnName, CHARINDEX('UserName ', M1.ColumnName) + LEN('UserName ') + 1
FROM MyTable AS M1
),
MyTable2 (ColumnName, pos1, pos2)
AS
(
SELECT M1.ColumnName, M1.pos1,
CHARINDEX(' ', M1.ColumnName, pos1) - M1.pos1
FROM MyTable1 AS M1
)
SELECT SUBSTRING(M1.ColumnName, M1.pos1, M1.pos2)
FROM MyTable2 AS M1;
...though you'd need to make it more robust e.g. when there is no trailing space after the username value etc.