A regex, therefor, is possible, but not too pretty (source):

(?:[0369]|
[147](?:[0369]*[147][0369]*[258])*(?:[0369]*[258]|[0369]*[147][0369]*[147])|
[258](?:[0369]*[258][0369]*[147])*(?:[0369]*[147]|[0369]*[258][0369]*[258])
)*

Example: http://www.rubular.com/r/ZcRDblHg8M

Here’s another approach, using .Net’s stacks as a simple counter:

\b
(?>             # No regrets - don't backtrack on if/else decisions.
    [0369]      # = 0 (mod 3)
    |
    [147]       # = 1 (mod 3)
    (?:         # if possible pop 2, else push 1
        (?<-Sum>){2}|(?<Sum>)
    )
    |
    [258]       # = 2 (mod 3)
    (?:         # if possible pop 1, else push 2
        (?<-Sum>)|(?<Sum>){2}
    )
)+
\b
(?(Sum)(?!)) # Assert nothing's left in the stack

Why? Well, I was bored while I shaved. Luckily, this regex is simple enough for Mono. Working example: http://ideone.com/Yp6Ti (ok, maybe not, mono is missing 111222)

(?>
    (?=[0-9])   # optimization - don't multiply when we don't have a digit.
    # multiply the content of the stack by 10
    # for each item on Stack, push 10 items to a Temp stack.
    (?(Decimal)
        (?<-Decimal>
            (?<Temp>){10}
        )
    ){100000}
    (?(Decimal)(?!))
    # Push all items from Temp back to Stack
    (?(Temp)
        (?<-Temp>
            (?<Decimal>)
        )
    ){100000}
    (?(Temp)(?!))
    # match a digit, and push its value to the stack
    (?:
        0                 |
        1 (?<Decimal>)    |
        2 (?<Decimal>){2} |
        3 (?<Decimal>){3} |
        4 (?<Decimal>){4} |
        5 (?<Decimal>){5} |
        6 (?<Decimal>){6} |
        7 (?<Decimal>){7} |
        8 (?<Decimal>){8} |
        9 (?<Decimal>){9}
    )
)+

The idea is very simple: when we see a new digit, we multiply the depth of the stack by 10, and add the number represented by the new digit. The value of the number can be verified using:

match.Groups["Decimal"].Captures.Count

A curious bit here is the use of the loop to copy stacks:

(?(Temp)
    (?<-Temp>
        (?<Decimal>)
    )
){100000}

I’d expect this to be enough:

(?<-Temp> (?<Decimal>) )*
(?(Temp)(?!))

It turns out the above loop is only executed once, and the condition always fails. It is probably a documented optimization, I’ll look more into that later. As a proof of concept, the workaround should do.

It is even possible to perform basic arithmetic operations on these stacks such as adding, subtracting, multiplying and such from within the regex engine, but that may be a few extra steps too many.
It should go without saying, of course, that regex isn’t a good option here – this is for recreational use. The run time and complexity are far from ideal.

See also:

• https://gist.github.com/940593 – Sample program that captures a decimal number followed by the same number in binary base.
• How to determine if a number is a prime with regex?

<Column Name="Editor" Member="CreatedBy" />
<Column Name="Author" Member="ModifiedBy" />
<Column Name="Created" />
<Column Name="Modified" />

Also, here’s a template of the SPMetal command:

"%programfiles%\Common Files\Microsoft Shared\Web Server Extensions\14\BIN\spmetal.exe" "/web:http://myserver/someweb/foo" /language:csharp /code:SPFoo.cs /serialization:unidirectional /parameters:FooSPMetalParameters.xml

See also:

• Overriding SPMetal Defaults by Using a Parameters XML File
• SPMetal Parameters XML Schema

Well, my first try was this:

\b((?<Acronym>\w)\w*\W+)+
\((?<-Acronym>\k<Acronym>)+\)
(?(Acronym)(?!))

What I had to do is to reverse the stack. I came up with this regex:

\b((?<Acronym>\w)\w*\W+)+
(?<=(?<-Acronym>.(?=.*?(?<Reverse>\k<Acronym>)))+)(?(Acronym)(?!))
\((?<-Reverse>\k<Reverse>)+\)
(?(Reverse)(?!))

Here’s a sample program: https://gist.github.com/765417
I tried creating an ideone version, but it fails (probably a mono bug)

A quick search brought up Linguistic Forms’ Regex Balancing Group in Depth, which has a clever approach to the problem in the best use I’ve seen yet to balancing groups, and taught me a few new tricks. I’d recommend reading it. The tl;dr version is this: when I see a closed bracket I look-behind my current position using whatever was matched since the last open bracket, revisit it, and check it again. So, if I see I had a { (by matching it again), I know I should now match a }.

While reading the post I had an interesting idea – how says I must push to the stack what I matched? What if wanted to push an arbitrary string to the stack? When I see an open bracket I really want to push a closing bracket – but how can I?
The trick is to use a look-ahead: find the next occurrence of the character I want to push, and push it:

{(?=.*?(<Stack>}))

Next, when I want to match a closing brace, I already have the right one in stack.
Using this approach, here’s a regex that matches tokens with matching balanced parentheses of 3 different typed:

(
    [^(){}\[\]]+
    | \( (?=[^)]*  (?<Stack> \) ) )
    | \[ (?=[^\]]* (?<Stack> \] ) )
    | \{ (?=[^}]*  (?<Stack> \} ) )
    | \k<Stack> (?<-Stack>)
)+?
(?(Stack) (?!))

Of course, this approach does have limitations – you may not find the character you’d like to push (which might be a good thing – it allows you to fail early). It also gets much trickier if you want to balance something more complicated constant strings, but that’s another subject.

Here’s a sample program one ideone: http://ideone.com/WaXfZ

Happy matching.

public void ProcessRequest(HttpContext context)
{
	string docId = context.Request.QueryString["id"];
	string versionLabel = context.Request.QueryString["v"];
	SPSite currentSite = SPContext.Current.Site;
	string url = null;
	try
	{
		url = GetUrlFromID(currentSite, docId, versionLabel);
	}
	catch (Exception ex)
	{
		string message = String.Format("Error finding document by id {0} and version [{1}], {2}"
			, docId, versionLabel, ex.Message);
		SPUtility.TransferToErrorPage(message);
	}

	if (url == null)
	{
		string message = String.Format("Could not find document with id {0} and version [{1}]"
			, docId, versionLabel);
		SPUtility.TransferToErrorPage(message);
	}
	else
	{
		context.Response.Write(url);
	}
}

private string GetUrlFromID(SPSite site, string docId, string version)
{
	//FindUrlById throws an exception if version is empty or null,
	// so I check it here to make sure it works.
	if (String.IsNullOrEmpty(version))
	{
		string[] urls = DocumentId.FindUrlsById(site, docId);
		return urls.Single(); // think about what you do here
	}
	else
	{
		return DocumentId.FindUrlById(site, docId, version);
	}
}

After creating and deploying your package, which is effortless in Visual Studio 2010, instead of using the default permalinks:

/_layouts/DocIdRedir.aspx?ID=GROOVY-10-2

you can use your own handler (you may want a shorter solution name and handler):

/_layouts/mySolution/VersionedDocumentId.ashx?id=GROOVY-10-2&v=2.5

See also:

• Source code of FindUrlById
• Source code of the default handler, DocIdRedir.aspx – found in
  C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\CONFIG\BIN\Microsoft.Office.DocumentManagement.Pages.dll

The first example captures three words. Besides spaces and word boundaries, the interesting bits read:

• (?<A>\w)+ – Capture the first and second word. Push each letter to the A stack. A note here is the it might have been more correct to write (?:\w(?<A>))+, so I don’t push a value I don’t use to the stack, but I think the (?<A>\w) is clearer. I could have also compressed it to ((?<A>\w)+\s+){2}, but then it would have been even less readable.
• (?<-A>\w)+ – On the third word, this will only match as many letters as matched before. An alternative syntax here would be (\w(?<-A>))+.
• (?(A)(?!)) – Fail the match if there are still letters in the A stack. This one is a must, in case the third word is shorter than the first two.
• \b – Last but the least, I need to make sure I match at word boundaries.

Note that In the above regex I don’t care if the second word is longer or of equal length to the first. If this is a problem for you, it can be checked using the following pattern. I won’t explain it too much, but the idea here is to push two letters to the B stack for each letter you remove from the A stack:

\b
(?<A>\w)+   # First word
\s+
(?<B-A>(?<B>)\w)+  # Second word - consume all A's, push an extra B
(?(A)(?!))   # Make sure A is finished
(?<B>\w)*    # Add more B's (or none)
\s+
(?<-B>\w)+ # Third word
(?(B)(?!))
\b

Finally, here’s a regex to capture a sequence of words, where the length of each word is equal to the sum of the lengths of the two previous words:

\b
(?:
    (?<A>\w)+   # First word - push each letter
    \s+
    (?=
        (?<A>\w)+  # Second word - push each letter
        \s+
        (?<-A>\w)+ # Third word - pop letters
        (?(A)(?!))
        \b
    )    #Look Ahead
)+
\w+\s+\w+\b #Capture last two words (already looked at them)

This might look a bit strange: I match a single word at a time, and use the look-ahead to check the following two words. The result is a sequence of words whose lengths make a Fibonacci series.

See Also:
Regex Balancing Group in Depth – An excellent introduction to balancing groups and the basic concepts and tools used in .Net regular expressions.

Looking for a solution, I’ve found this thread, suggesting it’s an IE6 bug that caused it to load the same background images dozens of times. The suggested solution, however, didn’t work for me. I came up with a slightly better solution. Simply add this CSS rule to prevent the background images from loading in IE6:

* html .ui-datepicker tbody a {background-image:none !important;}

The next step is to disable animation. The date picker has many elements – animating them is too much work for IE6, resulting in the animation not showing at all – but still delaying the calendar. Wonderful. The solution is to disable the animation for IE6 by setting its duration to 0. You may also want to set the animation to null, because some animations ignore the zero duration. This is done for IE6 only, as there is no need to penalize modern browsers.

$('input.Calendar').datepicker({
           showAnim: jQuery.support.boxModel ? 'drop' : null, // optional
           duration: jQuery.support.boxModel ? 'normal' : ''
        });

1. Access Denied – At first I received SharePoint’s Access Denied page. I soon remembered I had to elevate the permissions (lambda style).
2. Another Access Denied – Even after using RunWithElevatedPrivileges, we have to reopen our SPSite and SPWeb, we can’t use SPContext (it has the old permissions). Which is a shame, because we also have to close them later (here this is done with using, but in the real code I manage it on another class).
3. Security Validation Exception – This one took me a few hours. When searching for the answer I saw someone suggesting to use AllowUnsafeUpdates (which a colleague said I should use on a different project). That means setting “web.AllowUnsafeUpdates = true;” before I make any updates, and setting it back to false later.
   Microsoft.SharePoint.SPException:
The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again. --->
System.Runtime.InteropServices.COMException (0x8102006D): The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.

4. ValidateFormDigest – While commenting the code I realized I don’t fully understand what AllowUnsafeUpdates does and why I use it. I came across a very detailed article, that explained I could use ValidateFormDigest instead of this little hack.

Sample code for users adding or updating an item:

//A user cannot add items, needs elevated permissions.
SPSecurity.RunWithElevatedPrivileges(() =>
{
  //when using RunWithElevatedPrivileges we cannot use
  // SPContext.Current - These SP objects have the old
  // privileges. We need to open a new SPSite and SPWeb.
  using (SPSite site = new SPSite(SPContext.Current.Site.ID))
  {
    using (SPWeb web = site.OpenWeb("/training"))
    {
      //Validate form POST data.
      SPUtility.ValidateFormDigest();

      //demo code for adding or editing an item
      SPList registrations = web.Lists["Registrations"];
      SPItem newItem = registrations.Items.Add();
      newItem["Title"] = "Added new Item";
      newItem.Update();
      //end of demo code.
    }
  }
});

//A user cannot add items, needs elevated permissions.
SPSecurity.RunWithElevatedPrivileges(() =>
{
  //when using RunWithElevatedPrivileges we cannot use
  // SPContext.Current - These SP objects have the old
  // privileges. We need to open a new SPSite and SPWeb.
  using (SPSite site = new SPSite(SPContext.Current.Site.ID))
  {
    using (SPWeb web = site.OpenWeb("/training"))
    {
      web.AllowUnsafeUpdates = true;

      //code for adding or editing an item...

      web.AllowUnsafeUpdates = false;
    }
  }
});

Meeting Request

To make these ICS files dynamically, I use the .Net library DDay.iCal. It’s aewsome.
It took some trial and error, but at the end I was able to create events that work well for all versions of Outlook.
Unlike the solutions I’ve found on the web, this one doesn’t use the office interop thing, so I don’t need outlook installed on the server. I’m using a regular SMTP mail message and add the ICS as attachment.

Anyway, assuming you know how to send mails and how to generate the right ICS for your event (we’ll get to that later), this is the way to send it to Outlook:
First the variables:

//init the message with your defaults (from, to, subject, etc)
MailMessage message = initMailMessage();
string iCal = initICal(parameters ...);

//Add the attachment, specify it is a calendar file.
System.Net.Mail.Attachment attachment =
System.Net.Mail.Attachment.CreateAttachmentFromString(
iCal, new ContentType("text/calendar"));
attachment.TransferEncoding = TransferEncoding.Base64;
attachment.Name = "EventDetails.ics"; //not visible in outlook

message.Attachments.Add(attachment);

sendMailMessage(message);

Ok. That’s probably disappointing, since you can’t just copy-paste this code and make it work. I’m including more code you may want to use, but first some rules for the ICS files you’re about to create:

Rules of Thumb

• Make sure to specify the Method property – should be Publish or Request. Outlook won’t accept the file without it.
• Add an Organizer – or Outlook 2007 won’t let people save the event.
• Add the meeting’s subject and description to the ICS file, but also as the mail’s subject and body. Outlook may display either one.

Now for that extra code.
This one is pretty simple – sends an SMTP message:

private static void sendMailMessage(MailMessage mailMessage)
{
string mailHost = "Ask.Someone.com";
SmtpClient smtpClient = new SmtpClient(mailHost, 25);
smtpClient.DeliveryMethod =
   SmtpDeliveryMethod.PickupDirectoryFromIis;
smtpClient.Send(mailMessage);
}

This fine function creates the ICS:

[WebMethod(Description =
   "Send an appointment with much details.")]
public void SendAppointment(string from, string to,
   string title, string body, DateTime startDate,
   double duration, string location, string organizer,
   bool updatePreviousEvent, string eventId,
   bool allDayEvent,
   int recurrenceDaysInterval, int recurrenceCount)
{
  iCalendar iCal = new iCalendar();

  // outlook 2003 needs this property,
  //  or we'll get an error (a Lunar error!)
  iCal.Method = "PUBLISH";

  // Create the event
  Event evt = iCal.Create();

  evt.Summary = title;

  evt.Start = new iCalDateTime(startDate.Year,
    startDate.Month, startDate.Day, startDate.Hour,
    startDate.Minute, startDate.Second);
  evt.Duration = TimeSpan.FromHours(duration);
  evt.Description = body;
  evt.Location = location;

  if (recurrenceDaysInterval > 0)
  {
    RecurrencePattern rp = new RecurrencePattern();
    rp.Frequency = FrequencyType.Daily;
    rp.Interval = recurrenceDaysInterval; // interval of days

    rp.Count = recurrenceCount;
    evt.AddRecurrencePattern(rp);
  }
  evt.IsAllDay = allDayEvent;

  //organizer is mandatory for outlook 2007 - think about
  // trowing an exception here.
  if (!String.IsNullOrEmpty(organizer))
    evt.Organizer = organizer;


  if (!String.IsNullOrEmpty(eventId))
    evt.UID = eventId;

  //"REQUEST" will update an existing event with the same
  // UID (Unique ID) and a newer time stamp.
  if (updatePreviousEvent)
    iCal.Method = "REQUEST";

  // Save into calendar file.
  iCalendarSerializer serializer =
    new iCalendarSerializer(iCal);
  //serializer.Serialize(@"iCalendar.ics");

  string icalData = serializer.SerializeToString();

  //send the iCal data. Also sends the subject and body
  //on the mail.
  SendAppointmentFromICalWithMailTitle(from, to,
    icalData, title, body);
}

See Also:

• DDay.iCal – The iCal C# library.
• http://www.mavetju.org/programming/outlook-ics.php – This helped at the beginning, but written in Perl. Which is weird.
• http://chuckdotnet.blogspot.com/2007/10/send-outlook-meeting-requests-with.html – A VB.Net version, creating his own iCal and using Alternate Views.
• The iTip RFC – RFC 2446 – Had to read parts of it.