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Regex to match only numbers , no apostrophes

I want to match only numbers in the following string
String : "40’000"
Match : "40000"
basically tring to ignore apostrophe.
I am using C#, in case it matters.
Cant use any C# methods, need to only use Regex.

Replace like this it replace all char excpet numbers
string input = "40’000";
string result = Regex.Replace(input, #"[^\d]", "");

Since you said; I just want to pick up numbers only, how about without regex?
var s = "40’000";
var result = new string(s.Where(char.IsDigit).ToArray());
Console.WriteLine(result); // 40000

I suggest use regex to find the special characters not the digits, and then replace by ''.
So a simple (?=\S)\D should be enough, the (?=\S) is to ignore the whitespace at the end of number.
DEMO

Replace like this it replace all char excpet numbers and points
string input = "40’000";
string result = Regex.Replace(input, #"[^\d^.]", "");

Don't complicate your life, use Regex.Replace
string s = "40'000";
string replaced = Regex.Replace(s, #"\D", "");

How do I replace all strings found with a regular expression with itself concatenated with another string?

For example, my regular expression found the string: some\file\path.xml and I want it to be changed to new_root\some\file\path.xml. Is there a way to do this using the regex replace method? If not, what is the preferred way to do this?

It appears that you can do what you are asking using Regex.Replace.
Check out Substitutions in Regular Expressions article on MSDN.
Example:
var path = #"C:\some\file\path.xml";
var result = Regex.Replace(path, #"(C:\\)(.*)", "$1new_root\\$2");
Result is C:\new_root\some\file\path.xml.

You don't need regex for that, just find the string you want with a buid-in function and concatenate with what you want.
For a more general search/replace you can do this:
string pattern = #"(?>\w+\\)+\w+.xml";
string replacement = "new_root\\$0";
Regex rgx = new Regex(pattern);
string result = rgx.Replace(input, replacement);

get text inside quotes using regex

I want to extract the data inside the quotes using regex
My Text is : boundary="s323sd2342423---"
Now i need to extract the value inside the double quotes without using substring.
I tried the following but didnt help.
String pattern = #"boundary=""(?<value>[^""]*";
Match m = Regex.Match(rawMessage, pattern);
while (m.Success)
{
boundaryString = m.Groups["value"].Value;
m = m.NextMatch();
}

You need to close opening bracket of a group
String pattern = #"boundary=""(?<value>[^""]*)";
now if you go with
Console.WriteLine(m.Groups["value"].Value);
will print:
s323sd2342423---

You can use this pattern and it will work.
String pattern = #"boundary=\""(?<value>.+?)\""";

With the following Regex you'll get what you want without any grouping
(?<=boundary=")[^"]+(?=")
Code to get the the quoted text:
string txt = "boundary=\"s323sd2342423---\"";
string quotedTxt = Regex.Match(txt, #"(?<=boundary="")[^""]+(?="")").Value;

As per you details:
I want to extract the data inside the quotes using regex
Why not you just use this pattern:
"(?<value>[^"]+)"

.NET regex replace using backreference

I have a fairly long string that contains sub strings with the following format:
project[1]/someword[1]
project[1]/someotherword[1]
There will be about 10 or so instances of this pattern in the string.
What I want to do is to be able to replace the second integer in square brackets with a different one. So the string would look like this for instance:
project[1]/someword[2]
project[1]/someotherword[2]
I''m thinking that regular expressions are what I need here. I came up with the regex:
project\[1\]/.*\[([0-9])\]
Which should capture the group [0-9] so I can replace it with something else. I'm looking at MSDN Regex.Replace() but I'm not seeing how to replace part of a string that is captured with a value of your choosing. Any advice on how to accomplish this would be appreciated. Thanks much.
*Edit: * After working with #Tharwen some I have changed my approach a bit. Here is the new code I am working with:
String yourString = String yourString = #"<element w:xpath=""/project[1]/someword[1]""/> <anothernode></anothernode> <another element w:xpath=""/project[1]/someotherword[1]""/>";
int yourNumber = 2;
string anotherString = string.Empty;
anotherString = Regex.Replace(yourString, #"(?<=project\[1\]/.*\[)\d(?=\]"")", yourNumber.ToString());

Matched groups are replaced using the $1, $2 syntax as follows :-
csharp> Regex.Replace("Meaning of life is 42", #"([^\d]*)(\d+)", "$1($2)");
"Meaning of life is (42)"
If you are new to regular expressions in .NET I recommend http://www.ultrapico.com/Expresso.htm
Also http://www.regular-expressions.info/dotnet.html has some good stuff for quick reference.

I've adapted yours to use a lookbehind and lookahead to only match a digit which is preceded by 'project[1]/xxxxx[' and followed by ']':
(?<=project\[1\]/.*\[)\d(?=\]")
Then, you can use:
String yourString = "project[1]/someword[1]";
int yourNumber = 2;
yourString = Regex.Replace(yourString, #"(?<=project\[1\]/.*\[)\d(?=\]"")", yourNumber.ToString());
I think maybe you were confused because Regex.Replace has lots of overloads which do slightly different things. I've used this one.

If you want to process the value of a captured group before replacing it, you'll have to separate the different parts of the string, make your modifications and put them back together.
string test = "project[1]/someword[1]\nproject[1]/someotherword[1]\n";
string result = string.Empty;
foreach (Match match in Regex.Matches(test, #"(project\[1\]/.*\[)([0-9])(\]\n)"))
{
result += match.Groups[1].Value;
result += (int.Parse(match.Groups[2].Value) + 1).ToString();
result += match.Groups[3].Value;
}
If you just want to replace text verbatim, it's easier: Regex.Replace(test, #"abc(.*)cba", #"cba$1abc").

you can use String.Replace (String, String)
for example
String.Replace ("someword[1]", "someword[2]")

How can I use RegEx (Or Should I) to extract a string between the starting string '__' and ending with '__' or 'nothing'

RegEx has always confused me.
I have a string like this:
IDE\DiskDJ205GA20_____________________________A3VS____\5&1003ca0&0&0.0.0
Or Sometimes stored like this:
IDE\DiskSJ305GA23_____________________________PG33S\6&2003Sa0&0&0.0.0
I want to get the 'A3VS' or 'PG33S' string. It's my firmware and is varied in length and type. I used to use:
string[] split = PNP.Split('\\'); //where PHP is my string name
var start = split[1].LastIndexOf('_');
string mystring = split[1].Substring(start + 1);
But that only works for strings that don't end with __ after the firmware string. I noticed that some have an additional random '_' after it.
Is RegEx the way to solve this? Or is there another way better

just without RegEx it can be expressed like this:
var firmware = PNP.Split(new[] {'_'}, StringSplitOptions.RemoveEmptyEntries)[1].Split('\\')[0];

string s = split[1].TrimEnd('_');
string mystring = s.Substring(s.LastIndexOf('_') + 1);

If you want the RegEX way to do it here it is:
Regex regex = new Regex(#"\\.*_+(?<firmware>[A-Za-z0-9]+)_*\\");
var m1 = regex.Match("IDE\DiskSJ305GA23_____________________________PG33S\6&2003Sa0&0&0.0.0");
var g1 = m1.Groups["firmware"].Value;
//g1 == "PG33S"
Keep in mind you have to use [A-Za-z0-9] instead of \w in the capture subexpression since \w also matches an underscore (_).