Question

Language = C#.NET

Anything that is between [STX] and [ETX] must be accepted rest of the things must be rejected.

string startparam = "[STX]";
string endparam = "[ETX]";

String str1 = "[STX]some string 1[ETX]"; //Option 1
String str2 = "sajksajsk [STX]some string 2 [ETX] saksla"; //Option 2
String str3 = "[ETX] dksldkls [STX]some string 3 [ETX]ds ds"; //Option 3
String str4 = "dksldkls [STX]some string 4.1[ETX]ds ds [STX] some string 4.2[ETX] jdskjd"; //Option 4

/* the various strings can be appended and converted to a single 
   string using string builder or treat them as different strings*/

ProcessString (string str , string startparam , string endparam)
{
   //What To Write here using RegEX or String Functions in c#

}

/* The output after passing these to a ProcessString () */     
/* Append Output To a TextBox or Append it to a String using For Loop.*/

/* Output Required */

some string 1 
some string 2
some string 3
some string 4.1 
some string 4.2

=============================================================================

EDIT 2

Language = C#

string str = "
[STX]some string 1[ETX]
sajksajsk [STX]some string 2 [ETX] saksla
[ETX] dksldkls [STX]some string 3 [ETX]ds ds
dksldk[STX]ls [STX]some st[ETX]ring 4.1[ETX]ds ds [STX]some string 4.2[ETX] jdskjd";

How can i get the same output if the string array is one single string

/* output */
some string 1 
some string 2
some string 3
some string 4.1 
some string 4.2


/*case 1*/ 
the above string can be "[STX] djkdsj [STX]dskd1[ETX] dsnds[ETX]" 
the output should be just "dskd1"

/*case 2*/ 
the above string can be "[STX] djkdsj [STX]dskd1[ETX] ddd" 
the output should be just "dskd1"

/*case 3*/ 
the above string can be " kdsj [STX]dskd1[ETX] dsnds[ETX]" 
the output should be just "dskd1"

/*case 4*/ 
the above string can be "[STX] djk[STX]dsj [STX]dskd2[ETX] ddd" 
the output should be just "dskd2"

The real problem comes when [STX] followed by [STX] i want to consider the newer [STX] and start string processing from the newer [STX] occurance. Eg. Case 2 above

=============================================================================

EDIT 3 : New Request

Language = C#

If i want the data between [STX] and [STX] also can that also be done.

New RegEx which will extract data between
1. [STX] some Data [STX]
2. [STX] some Data [ETX]

Eg.

/* the above string can be */
"[STX] djk[STX]dsj [STX]dskd2[ETX] ddd" 
/* the output should be just */
djk
dsj
dskd2

As [STX] means a transmission has been started so i want to extract data between STX as well.

Answer 1

(?<=\[STX\])(?:(?!\[STX\]).)*?(?=\[ETX\])

matches any text (except newlines) between [STX] and [ETX]:

(?<=\[STX\])  # Are we right after [STX]? If so,...
(?:           # match 0 or more of the following:
 (?!\[STX\])  # (as long as it's not possible to match [STX] here)
 .            # exactly one character
 )*?          # repeat as needed until...
(?=\[ETX\])   # there is a [ETX] ahead.

This will always match somestring in each of the following:

blah blah [STX]somestring[ETX] blah blah
[STX]somestring[ETX] blah [STX]somestring[ETX] (hey, two matches here!)
[STX] not this! [STX]somestring[ETX] not this either! [ETX]
blah [ETX] [STX]somestring[ETX] [STX] bla bla

A full reference on positive/negative lookbehind and lookahead assertions (three of which are used in this regex) can be found in Jan Goyvaerts’ excellent regular expression tutorial at http://www.regular-expressions.info/lookaround.html.