Regular Expressions
the following codes in this article throws light to some of the commonly used regular expression..all are coded with c#,asp.net

How to Find a text and Replace with other text 
string FindAndReplace(string text)
{
 string str = text;
 Regex rgx = new Regex( class="str">@"\bbuddha\b");
 if (rgx.IsMatch(text))
   {
     str = rgx.Replace(str, "mybuddha");
   }
   return str;
}


Testing complexity of password using regular expression validator
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="TextBox1"
            ErrorMessage="RegularExpressionValidator" ValidationExpression="(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9]).{7,15}"></asp:RegularExpressionValidator></div>

Splitting comma seperated files with a br tag
string SplitLine(string text)
    {
        string str = text;
        Regex rgx = new Regex(@",\s*");
        if (rgx.IsMatch(text))
        {
            str = rgx.Replace(str, ",<br>" );
        }
        return str;
    }

SplitLine("Microsoft,Linux,Oracle");
will return 
Microsoft,
Linux,
Oracle



Extracting Query Strings from URLs 
Regex rgx= new Regex( "\\?(?<query>[^<>#\"]+)" );



Extracting Filenames from Paths
Regex rgx = new Regex( ?
"(?<path>(\\\\(?<file>[^\\\\/:*?\"<>|.][^\\\\/:*?\"<>|]*))+)" );



Extracting Extensions from Filenames
Regex rgx = new Regex( ?
"\\\\[^\\\\/:*?\"<>|]+\\.(?<ext>[^.\\\\/:*?\"<>|]+)" );



Formatting U.S. Dates
Regex rgx = new Regex( @"^(\d{1,2})[-\/.]?(\d{1,2})[-\/.]?((?:\d{2}|\d{4}))$" );



Formating Dates
  string FormatDates(string text)
    {
        string str = text;
        Regex rgx = new Regex(@"(?:(?<=^)|(?<=[^\d]))(\d)(?=[^\d])");

        if (rgx.IsMatch(text))
        {
            str = rgx.Replace(str, @"0$1");
        }

        return str;

    }
1/1/2007 will become 01/01/2007



Validating Credit Card Numbers
<asp:RegularExpressionValidator Id="revInput" RunAt="server"
ControlToValidate="txtInput"
ErrorMessage="Please enter a valid value"
ValidationExpression="^(\d{4}-){3}|(\d{4} ){3}\d{4}|\d{15,16}|?
\d{4} \d{2} \d{4} \d{5}|\d{4}-\d{2}-\d{4}-\d{5}$">
</asp:RegularExpressionValidator>



Validating Dates in MM/DD/YYYY Format
<asp:RegularExpressionValidator Id="revInput" RunAt="server"
ControlToValidate="txtInput"
ErrorMessage="Please enter a valid value"
ValidationExpression="^(0?2/(0?[1-9]|[12][0-9])|
(0?[469]|11)/(0?[1-9]|[12][0-9]|30)|
(0?[13578]|1[02])/(0?[1-9]|[12][0-9]|3[01]))/\d{4}$">
</asp:RegularExpressionValidator>