Regular Expressions in T-SQL

Given that a recent blog of mine lamented the dangers of using xprocs, I thought it would be a good time to show some of the wonderful stuff you can do thanks to SQL Server extensibility features such as xprocs and COM objects. It’s not that xprocs or in-process COM objects are inherently evil -- it’s just that they can be misused and that they can be challenging for the average developer to code properly.

In today’s blog, I’ll show you how to filter T-SQL queries using Regular Expressions. Regular Expressions, you’ll recall, allow sophisticated string searching and matching that goes beyond simple wildcards. T-SQL’s LIKE operator (and PATINDEX() function, which has similar functionality) supports basic wildcards and some simple pattern matching, but has never had anything approaching Regular Expression support. People who’ve written much T-SQL have no doubt encountered situations where they needed string searches that exceeded the meager capabilities of LIKE.

Lurking on any machine that has Windows Scripting Host installed (virtually all machines these days, although scripting can be disabled), is a powerful Regular Expressions facility, the VBScript.RegExp scripting object. You can get to it from any COM client that supports the IDispatch interface. IDispatch, you’ll recall, is COM’s popular late-binding interface – it allows applications to use COM components without knowing anything about them at compile-time. In T-SQL, we get to IDispatch via the sp_OA stored procedures. Via a simple UDF, we can access the RegExp object as though it were part of T-SQL:

use pubs

GO

DROP function dbo.fn_regex

GO

CREATE FUNCTION

dbo.fn_regex(@pattern varchar(255), @matchstring varchar(8000))

RETURNS int

AS

BEGIN

       declare @obj int

       declare @res int

       declare @match bit

       set @match=0

 

       exec @res=sp_OACreate 'VBScript.RegExp',@obj OUT

       IF (@res <> 0) BEGIN

              RETURN NULL

       END

 

       exec @res=sp_OASetProperty @obj, 'Pattern', @pattern

       IF (@res <> 0) BEGIN

              RETURN NULL

       END

 

       exec @res=sp_OASetProperty @obj, 'IgnoreCase', 1

       IF (@res <> 0) BEGIN

              RETURN NULL

       END

 

       exec @res=sp_OAMethod @obj, 'Test',@match OUT, @matchstring

       IF (@res <> 0) BEGIN

              RETURN NULL

       END

 

       exec @res=sp_OADestroy @obj

       return @match

END

GO

The UDF above does several interesting things. Note the fact that we call the sp_OA procs directly from our function. If you’ve done much UDF coding, you’re probably aware of the fact that you can’t call regular stored procedures from a UDF. Fortunately for us, although the sp_OA procs are prefixed with "sp_", they’re actually extended procedures, which you can call from a UDF. Equally fortunate is the fact that they aren’t “spec procs”—extended procedures implemented internally by the server. Their entry points are in ODSOLE70.DLL, so they’re callable from a UDF just like any other regular xproc.

The algorithm we use here is embarrassingly simple: We create the object, set some properties, then call the Test method to see whether we have a match. As the code below illustrates, once we’ve wrapped our Regular Expression functionality in a UDF, we can use it to filter a query.

 

SELECT au_lname

FROM authors

WHERE dbo.fn_regex('G.*',au_lname)<>0

 

au_lname

Green

Greene

Gringlesby

Ringer

Ringer

Straight

Stringer

As you can see, you don’t need SQLCLR or anything beyond SQL Server 2000 to add this powerful functionality to T-SQL.  And, contrary to what I would have guessed, this technique is surprisingly fast – I think most users would find the performance quite acceptable, especially for smaller tables.  You’ll have to try it yourself to see how it works in your environment, but it was not nearly as slow as I expected a technique built around constructing and tearing down a COM object with each search iteration to be.

So, even though xprocs and COM objects can be abused and can certainly cause problems when not coded properly, on balance, SQL Server, coupled with the objects and facilities lying around on most users’ machines, offers some great extensibility and power with minimal effort.

 

This technique first appeared in my last book, The Guru’s Guide to SQL Server Architecture and Internals . That book has lots of additional details about Regular Expression use from T-SQL, including how to do so using the .NET Framework’s Regex object rather than VBScript.RegExp. See it for additional Regular Expression search techniques as well as for more info on SQL Server's SQLOLE facility, the component that makes all of this possible.

Comments

• Anonymous
  May 10, 2005
  RePost : http://www.yeyan.cn/Database/RegularExpressionsTSQL.aspx

• Anonymous
  June 29, 2005
  Can you use regular expressions in a WHERE clause?
  Ken Henderson shows us how, in this recent blog...

• Anonymous
  July 15, 2005
  
  This code will not run on Yukon.
  
  Period.
  
  1) 'allow updates' is a NOP since direct catalog updates are no longer allowed.
  2) There is no more system_function_schema. It was never documented anyway.
  
  It is very unwise to use undoc'ed internal APIs. Don't expect code like this to port to Yukon.
  
  cdibble@microsoft.com
  SQL Engine PM
  

• Anonymous
  August 09, 2005
  The comment has been removed

• Anonymous
  August 21, 2005
  Great article - this sort of functionality is very useful; a lot of the time, LIKE's selection of pattern matching just doesn't do what you need. I've already used it to identify records with garbage data in them.
  
  However, this approach doesn't scale too happily, as a new COM object is created per row. I found another article that builds on this idea, but reduces the number of object references that need to be created: http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=27205
  
  Working against my development database with 250K, the original query took almost 7 minutes; the revised query took less than 2 minutes, so it's a worthwhile performance improvement.

• Anonymous
  September 17, 2005
  In SQL 2005 this can be done easily with the SQL-CLR. I use a regular expression UDF as an example in a newsletter. http://www.novicksoftware.com/coding-in-sql/Vol3/cis-v3-N13-dot-net-clr-in-sql-server.htm
  I'm pretty sure the regular expression version is going to be faster than creating a com object.

• Anonymous
  July 11, 2006
  The comment has been removed

• Anonymous
  July 24, 2006
  we've written similar code.  especially important is the bi David Mumladze posted about factoring out the com create/destroy code if performance on medium to large tables is expected.
  
  something else worth noting is that with this approach indexes are useless.  since SQL doesn't know about the implementation of the function, it's hands are pretty tied and it has to hand over all values resulting in a table scan every time (unless some other criteria helps knock down the record count 1st.)
  
  we generally recommend to our customers that they use features like like as much as possible and only use regex when nothing else will do.
  
  great blog, btw.  love the bits about xp procs.  it explains a great deal about some behavior we've seen.  we've got a bunch of xp procs that work really well, but i'd still love to get the chance to convert them to clr flavor.

• Anonymous
  August 23, 2006
  funny ringtones

• Anonymous
  September 30, 2006
  THANKS KEN!! finally regex on sql server that I was actually able to make work!!!

• Anonymous
  February 06, 2007
  Fantastic !! Thank you so much !

• Anonymous
  July 31, 2007
  I need help in other sentence. I want know how many if five or more letters be repetead in a word. You can help me in this??

• Anonymous
  January 21, 2009
  PingBack from http://www.keyongtech.com/2092955-how-to-optimize-search-on