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set own string format using c#

I'm currently developing an application for our companys stock. In this application I have to create a format check button. So if the text in the textbox is not in the right format, it should throw an error.
I thought using String.Format() method but I think I'm doing it wrong and I don't know if it's possible using this method. I tried the following:
string format = String.Format("XXX.XX.XX.XX.X", txtStockFormat.Text);
if (format != txtStockFormat.Text)
{
MessageBox.Show("if");
}
else
{
MessageBox.Show("else");
}
I don't know if String.Format() is the right way to achieve what I want and I think I don't quite understand it right. I already searched on this site and I saw that there are loads of given formats but I couldn't found a way to set my own format.
Can someone tell me how I can achieve this?
Suggestion appreciated :)

As the comments on your question already said, you have to use Regex. I had a similar problem about 1 month ago and it took me a while to understand how Regex works.
I think you are looking for something like this:
string pattern = #"[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9]";
Match match = Regex.Match(txtStockFormat.Text, pattern);
if (match.Success)
{
// Do your stuff
}
else
{
// Do your stuff
}
In your question you wanted the format XXX.XX.XX.XX.X. So let's have a look at the first three letters. In my code example the first three letters could be "a-z", "A-Z" and numbers from "0-9". If you want only uppercase letters for the first three chars you have to change the pattern to [A-Z].
string pattern = #"[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9][a-zA-Z0-9]\.[a-zA-Z0-9]";
^ ^ ^ ^
| | | |
1st char 2nd char 3rd char Point (separator) etc.
You can just replace the pattern I wrote above with the pattern you want. I defined the pattern for your example you posted in your question.
UPDATE:
As #Rawling already said in the comments, to make it clearer and simpler, this will have the same effect:
string pattern = #"[a-zA-Z0-9]{3}\.[a-zA-Z0-9]{2}\.[a-zA-Z0-9]{2}\.[a-zA-Z0-9]{2}\.[a-zA-Z0-9]";

Your question is not clear, as you asked to set a string format but in code you are trying to compare a text with that format.
Anyway if you want to change a string to your own desired format, you can simply use ToString method like _yourString.ToString(_yourFormat).

Correction in this simple regular expression

I am new to regular expressions and the one that i have written might be a very simple one but donot know where I am wrong.
#"^([a-zA-Z._]+)#([\d]+)"
This RE is for the following string:
somename#somenumber
Now i am trying to retrieve the somename and somenumber. This is what i did:
ac.name = m.Groups[0].Value;
ac.number = m.Groups[1].Value;
Here ac.name reads the complete string, and ac.number reads somenumber. Where am I wrong in ac.name?

i guess the regex is correct, the problem is, you get the ac.name not from group 1 but group(0), which is the whole string. try this:
ac.name = m.Groups[1].Value;
ac.number = m.Groups[2].Value;

This regex is correct. I think your mistake is in somewhere else. You seem to use C#. So, you should think about the regex usage in the language.
Looking to the code sample in MSDN, you need to use 1-based indexes while accessing Groups instead of zero-based (as also Kent suggested). So, use this:
String name = m.Groups[1].Value;
String number = m.Groups[2].Value;

use this regex (\w+)#(\d+([.,]\d+)?)
Groups[1] will be contain name
Groups[2] will be contain number

I think you should move the + into the capture group:
#"^([a-zA-Z._]+)#([\d]+)"

If this is C#, try without the ^
([a-zA-Z\._]+)#([\d]+)
I just tried it out and it groups properly
Update: escaped the .
If you want only one match (and hence the ^ in original expression), use .Match instead of .Matches method. See MSDN documentation on Regular Expression Classes.

Regex for a string

It would be great if someone could provide me the Regular expression for the following string.
Sample 1: <div>abc</div><br>
Sample 2: <div>abc</div></div></div></div></div><br>
As you can see in the samples provided above, I need to match the string no matter how many number of </div> occurs. If there occurs any other string between </div> and <br>, say like this <div>abc</div></div></div>DEF</div></div><br> OR <div>abc</div></div></div></div></div>DEF<br>, then the Regex should not match.
Thanks in advance.

Try this:
<div>([^<]+)(?:<\/div>)*<br>
As seen on rubular
Notes:
This only works if there are not tags in the abc part (or anything that has a < symbol).
You might want to use start and end of string anchors (^<div>([^<]+)(?:<\/div>)*<br>$ if you want your string to match the pattern exactly.
If you want to allow the abc part to be empty, use * instead of +
That being said, you should be wary of using regex to parse HTML.
In this example, you can use regex because you are parsing a (hopefully) known, regular subset of HTML. But a more robust solution (ie: an [X]HTML parser like HtmlAgilityPack) is preferred when it comes to parsing HTML.

You need to use a real parser. Things like infinitely nested tags can't be handled via regex.

You could also include a named group in the the expression, e.g.:
<div>(?<text>[^<]*)(?:<\/div>)*<br>
Implemented in C#:
var regex = new Regex(#"<div>(?<text>[^<]*)(?:<\/div>)*<br>");
Func<Match, string> getGroupText = m => (m.Success && m.Groups["text"] != null) ? m.Groups["text"].Value : null;
Func<string, string> getText = s => getGroupText(regex.Match(s));
Console.WriteLine(getText("<div>abc</div><br>"));
Console.WriteLine(getText("<div>123</div></div></div></div></div><br>"));

NullUserException's answer is good. Here are a couple of questions, and variations, depending on what you want.
Do you want to prevent anything from occurring before the open div tag? If so, keep the ^ at the beginning of the regex. If not, drop it.
The rest of this post refers to the following section of the regex:
([^<]+?)
Do you want to capture the contents of the div, or just know that it matches your form? To capture, leave it as is. If you don't need to capture, drop the parentheses from the above.
Do you want to match if there is nothing inside the div? If so change the + in the above to *
Finally, although it will work fine, you don't need the ? in the above.

I think, this regex is more flexible:
<div\b[^><]*+>(?>.*?</div>)(?:\s*+</div>)*+\s*+<br(?:\s*+/)?>
I don't include the ^ and $ in the beginning and the end of my regex because we cannot assure that your sample will always in a single line.

How to Extract the Word Following a Symbol?

I have a string that could have any sentence in it but somewhere in that string will be the # symbol, followed by an attached word, sort of like #username you see on some sites.
so maybe the string is "hey how are you" or it's "#john hey how are you".
IF there's an "#" in the string i want to pull what comes immediately after it into its own new string.
in this instance how can i pull "john" into a different string so i could theoretically notify this person of his new message? i'm trying to play with string.contains or .replace but i'm pretty new and having a hard time.
this btw is in c# asp.net

You can use the Substring and IndexOf methods together to achieve this.
I hope this helps.
Thanks,
Damian

Here's how you do it without regex:
string s = "hi there #john how are you";
string getTag(string s)
{
int atSign = s.IndexOf("#");
if (atSign == -1) return "";
// start at #, stop at sentence or phrase end
// I'm assuming this is English, of course
// so we leave in ' and -
int wordEnd = s.IndexOfAny(" .,;:!?", atSign);
if (wordEnd > -1)
return s.Substring(atSign, wordEnd - atSign);
else
return s.Substring(atSign);
}

You should really learn regular expressions. This will work for you:
using System.Text.RegularExpressions;
var res = Regex.Match("hey #john how are you", #"#(\S+)");
if (res.Success)
{
//john
var name = res.Groups[1].Value;
}
Finds the first occurrence. If you want to find all you can use Regex.Matches. \S means anything else than a whitespace. This means it also make hey #john, how are you => john, and #john123 => john123 which may be wrong. Maybe [a-zA-Z] or similar would suit you better (depends on which characters the usernames is made of). If you would give more examples, I could tune it :)
I can recommend this page:
http://www.regular-expressions.info/
and this tool where you can test your statements:
http://regexlib.com/RESilverlight.aspx

The best way to solve this is using Regular Expressions. You can find a great resource here.
Using RegEx, you can search for the pattern you are after. I always have to refer to some documentation to write one...
Here is a pattern to start with - "#(\w+)" - the # will get matched, and then the parentheses will indicate that you want what comes after. The "\w" means you want only word characters to match (a-z or A-Z), and the "+" indicates that there should be one or more word characters in a row.

You can try Regex...
I think will be something like this
string userName = Regex.Match(yourString, "#(.+)\\s").Groups[1].Value;

RegularExpressions. Dont know C#, but the RegEx would be
/(#[\w]+) / - Everything in the parans is captured in a special variable, or attached to RegEx object.

Use this:
var r = new Regex(#"#\w+");
foreach (Match m in r.Matches(stringToSearch))
DoSomething(m.Value);
DoSomething(string foundName) is a function that handles name (found after #).
This will find all #names in stringToSearch

Regex Question

In my C# Console App I'm trying to use Regex to search a string to determine if there is a match or not. Below is my code but it is not quite working right so I will explain further. sSearchString is set to "_One-Call_Pipeline_Locations" and pDS.Name is a filename it is searching against. Using the code below it is set to true for Nevada_One-Call_Pipeline_Locations and Nevada_One-Call_Pipeline_LocationsMAXIMUM. There should be a match for Nevada_One-Call_Pipeline_Locations But Not for Nevada_One-Call_Pipeline_LocationsMAXIMUM. How can I change my code to do this properly?
Thanks in advance
if (Regex.IsMatch(pDS.Name, sSearchString))

Change the sSearchString to ".*_One-Call_Pipeline_Locations$"

You need to specify that a matching name must end with the text you have entered using the dollar token.
sSearchString = "_One-Call_Pipeline_Locations$";

Since you provided no details as to what else should match, we can only assume that if the string ends with "Nevada_One-Call_Pipeline_Locations", then it matches? Is this correct?
If so, you don't need Regex:
if (pDS.Name.EndsWith("Nevada_One-Call_Pipeline_Locations"))
{ //...