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Regex pattern in c# start with # and end with 9;

Need regex pattern that text start with"#" and end with " ";
I tried the below pattern
string pattern = "^[#].*?[ ]$";
but not working

Since is an hex code of tab character, why not just using StartsWith and EndsWith methods instead?
if(yourString.StartsWith("#") && yourString.EndsWith("\\t"))
{
// Pass
}

This patterns works fine. I have tested it.
string pattern = "#(.*?)9";
See below link to test it online.
https://regex101.com/r/iR6nP6/1
C#
const string str = "dadasd#beetween9ddasdasd";
var match = Regex.Match(str, "#(.*?)9");
Console.WriteLine(match.Groups[1].Value);

In regex syntaxt, the [] denotes a group of characters of which the engine will attempt to match one of. Thus, [&#x9] means, match one of an &, #, x or 9 in no particular order.
If you are after order, which seems you are, you will need to remove the []. Something like so should work: string pattern = "^#.*?&#x9$";

you mean something like:
string pattern = "^#.*?[ ]$"
There are also many fine regex expression helpers on the web. for example https://regex101.com/ It gives a nice explanation of how your text will be handled.

You should use \t to match tab character
You can use special character sequences to put non-printable characters in your regular expression. Use \t to match a tab character (ASCII 0x09)
Try following Regex
^\#.*\t\;$

Using an escape character with a beginning wildcard in regex in c#

Below is a sample of an email I am using from a database:
2.2|[johnnyappleseed#example.com]
Every line is different, and it may or may not be an email, but it will always. I am trying to use regular expressions to get the information inside the brackets. Below is what I have been trying to use:
^\[\]$
Unfortunately, every time I try to use it, the expression isn't matching. I think the problem is using the escape characters, but I am not sure. If this is not how I use the escape characters with this, or if I am wrong completely, please let me know what the actual regex should be.

Close to yours is ^.*\[(.*)\]$:
^ start of the line
.* anything
\[ a bracket, indicating the start of the email
(.*) anything (the email), as a capturing group
\] a square bracked, indicating the end of the email
$ end of the line
Note that your Regex is missing the .* parts to match the things between the key characters [ and ].

Your regex - ^\[\]$ - matches a single string/line that only contains [], and you need to obtain a substring inbetween the square brackets somewhere further inside a larger string.
You can use
var rx = new Regex(#"(?<=\[)[^]]+");
Console.WriteLine(rx.Match(s).Value);
See regex demo
With (?<=\[) we find the position after [ and then we match every character that is not ] with [^]]+.
Another, non-regex way:
var s = "2.2|[johnnyappleseed#example.com]";
var ss = s.Split('|');
if (ss.GetLength(0) > 1)
{
var last = ss[ss.GetLength(0)-1];
if (last.Contains("[") && last.Contains("#")) // We assume there is an email
Console.WriteLine(last.Trim(new[] {'[', ']'}));
}
See IDEONE demo of both approaches

Regex for checking numbers in a string

I am looking for help with a regex for checking a string that could contain 10 digits separated by other characters or alphabets. For example
call1234567890
1234567890call
12.34_567.890_call
I have tried \D*(\d\D*){10}$ as suggested in other posts , but this matches with any string that has numbers even if 1 and characters after 1. So
Silly_1_me is also being caught

You must need to include starting anchor ^ so that it would do an exact line match or otherwise, it would do a partial string match.
#"^\D*(\d\D*){10}$"
DEMO
For multiline input , its better to use the below regex.
#"^[^\n\d]*(\d[^\n\d]*){10}$"

^(?!(?:.*\d){11,})(?:.*\d){10}[^\d]*$
Try this.See demo.
http://regex101.com/r/hQ9xT1/21

C# Regex for validating words with no space, no special character

I have written the following Regex for matching only those words with no space and no special character. But it is matching with words containing space too. What is wrong in it?
Regex rgx = new Regex("[a-zA-Z0-9]+");
if (!rgx.IsMatch(TextBox_EntityType.Text))
{
}

You can change the logic of your check so it does the opposite, and you take the appropriate action:
Regex rgx = new Regex("[^a-zA-Z0-9]");
# Match if there is something that is not alphanumeric
if (rgx.IsMatch(TextBox_EntityType.Text))
{
# Do what should be done if the text contains non-alphanumeric
}
This one works just as well because .IsMatch() looks for a match anywhere in a string (it tries its best to find a match), so either you make it match the whole string with anchors like Nikhil suggested, or invert the logic like I did (and which I believe should be slightly more efficient, but not benchmarked).

It should be ^[a-zA-Z0-9]+$
Added ^ and $.
The ^ matches the start of the string and $ matches the end.

Matching an (easy??) regular expression using C#'s regex

Ok sorry this might seem like a dumb question but I cannot figure this thing out :
I am trying to parse a string and simply want to check whether it only contains the following characters : '0123456789dD+ '
I have tried many things but just can't get to figure out the right regex to use!
Regex oReg = new Regex(#"[\d dD+]+");
oReg.IsMatch("e4");
will return true even though e is not allowed...
I've tried many strings, including Regex("[1234567890 dD+]+")...
It always works on Regex Pal but not in C#...
Please advise and again i apologize this seems like a very silly question

Try this:
#"^[0-9dD+ ]+$"
The ^ and $ at the beginning and end signify the beginning and end of the input string respectively. Thus between the beginning and then end only the stated characters are allowed. In your example, the regex matches if the string contains one of the characters even if it contains other characters as well.
#comments: Thanks, I fixed the missing + and space.

Oops, you forgot the boundaries, try:
Regex oReg = new Regex(#"^[0-9dD +]+$");
oReg.IsMatch("e4");
^ matches the begining of the text stream, $ matches the end.

It is matching the 4; you need ^ and $ to terminate the regex if you want a full match for the entire string - i.e.
Regex re = new Regex(#"^[\d dD+]+$");
Console.WriteLine(re.IsMatch("e4"));
Console.WriteLine(re.IsMatch("4"));

This is because regular expressions can also match parts of the input, in this case it just matches the "4" of "e4". If you want to match a whole line, you have to surround the regex with "^" (matches line start) and "$" (matches line end).
So to make your example work, you have to write is as follows:
Regex oReg = new Regex(#"^[\d dD+]+$");
oReg.IsMatch("e4");

I believe it's returning True because it's finding the 4. Nothing in the regex excludes the letter e from the results.

Another option is to invert everything, so it matches on characters you don't want to allow:
Regex oReg = new Regex(#"[^0-9dD+]");
!oReg.IsMatch("e4");