Why order matters in this RegEx with alternation?

by

The order matters since that is the order which the Regex engine will try to match.

Case 1: Number of rooms|[0-9]*

In this case the regex engine will first try to match the text “Number of room”. If this fails will then try to match numbers or nothing.

Case 2: [0-9]*|Number of rooms:

In this case the engine will first try to match number or nothing. But nothing will always match. In this case it never needs to try “Number of rooms”

This is kind of like the || operator in C#. Once the left side matches the right side is ignored.

Update:
To answer your second question. It behaves differently with the RegularExpressionValidator because that is doing more than just checking for a match. 

// .....
Match m = Regex.Match(controlValue, ValidationExpression);
return(m.Success && m.Index == 0 && m.Length == controlValue.Length); 
// .....

It is checking for a match as well as making sure the length of the match is the whole string. This rules out partial or empty matches.

More Related Contents:

  1. Regex Expression for asp.net [duplicate]
  2. What is the best way to read/parse the XML files [closed]
  3. Japenese character in a filename changes to garbage if I download file from IE 11
  4. ASP.NET Web Site or ASP.NET Web Application?
  5. MVC3 and Entity Framework
  6. What is the App_Data folder used for in Visual Studio?
  7. Return XML from a controller’s action in as an ActionResult?
  8. How serious is this new ASP.NET security vulnerability and how can I workaround it?
  9. Unable to start debugging on the web server. Could not start ASP.NET debugging VS 2010, II7, Win 7 x64
  10. What perfmon counters are useful for identifying ASP.NET bottlenecks?
  11. Global ASAX – get the server name
  12. Regular expression for asp:RegularExpressionValidator with format MMddyy (leap year issue)
  13. I’m lost. What happened to ASP.NET MVC 5?
  14. You must add a reference to assembly ‘netstandard, Version=2.0.0.0
  15. Difference between Html.RenderAction and Html.Action
  16. In ASP.NET, when should I use Session.Clear() rather than Session.Abandon()?
  17. Store String Array In appSettings?
  18. How to deploy ASP.NET MVC 4 application using localDB to local IIS on Windows 7?
  19. Cannot use a leading ../ to exit above the top directory
  20. What’s the difference between ASP.NET 5, .NET Core, and ASP.NET Core 5? [closed]
  21. Web API Queryable – how to apply AutoMapper?
  22. ‘System.Net.Http.HttpContent’ does not contain a definition for ‘ReadAsAsync’ and no extension method
  23. Validating file types by regular expression
  24. What does the web.config compilation/assemblies element do?
  25. The configuration section ‘system.web.extensions’ cannot be read because it is missing a section declaration
  26. ASP.NET Web Site or ASP.NET Web Application?
  27. Regex for Money
  28. How do I get msdeploy to create App_Data if it doesn’t exist, but not delete any contents of the remote directory?
  29. .NET “code nugget blocks”?
  30. How do I make a “generic error” page in my ASP.NET application so that it handles errors triggered when serving that page itself?

Leave a Comment