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C# - Removing single word in string after certain character

I have string that I would like to remove any word following a "\", whether in the middle or at the end, such as:
testing a\determiner checking test one\pronoun
desired result:
testing a checking test one
I have tried a simple regex that removes anything between the backslash and whitespace, but it gives the following result:
string input = "testing a\determiner checking test one\pronoun";
Regex regex = new Regex(#"\\.*\s");
string output = regex.Replace(input, " ");
Result:
testing a one\pronoun
It looks like this regex matches from the backslash until the last whitespace in the string. I cannot seem to figure out how to match from the backlash to the next whitespace. Also, I am not guaranteed a whitespace at the end, so I would need to handle that. I could continue processing the string and remove any text after the backslash, but I was hoping I could handle both cases with one step.
Any advice would be appreciated.

Change .* which match any characters, to \w*, which only match word characters.
Regex regex = new Regex(#"\\\w*");
string output = regex.Replace(input, "");

".*" matches zero or more characters of any kind. Consider using "\w+" instead, which matches one or more "word" characters (not including whitespace).
Using "+" instead of "*" would allow a backslash followed by a non-"word" character to remain unmatched. For example, no matches would be found in the sentence "Sometimes I experience \ an uncontrollable compulsion \ to intersperse backslash \ characters throughout my sentences!"

With your current pattern, .* tells the parser to be "greedy," that is, to take as much of the string as possible until it hits a space. Adding a ? right after that * tells it instead to make the capture as small as possible--to stop as soon as it hits the first space.
Next, you want to end at not just a space, but at either a space or the end of the string. The $ symbol captures the end of the string, and | means or. Group those together using parentheses and your group collectively tells the parser to stop at either a space or the end of the string. Your code will look like this:
string input = #"testing a\determiner checking test one\pronoun";
Regex regex = new Regex(#"\\.*?(\s|$)");
string output = regex.Replace(input, " ");

Try this regex (\\[^\s]*)
(\\[^\s]*)
1st Capturing group (\\[^\s]*)
\\ matches the character \ literally
[^\s]* match a single character not present in the list below
Quantifier: * Between zero and unlimited times, as many times as possible, giving back as needed [greedy]
\s match any white space character [\r\n\t\f ].

Regex goes wrong - which characters need escaping?

I want to extract an RTMP link from a website and has so far managed to find the line where it's located:
string line = GetLine(innerHTML, "turbo:");
// The string line now contains something like this:
// turbo: 'rtmp://fcs21-1.somewebsite.com/reflect/2996910732;0',
Match match = Regex.Match(line, #"turbo: '(rtmp://[*]+);0',$",
RegexOptions.IgnoreCase);
string key;
if (match.Success)
key = match.Groups[1].Value;
There aren't any matches. What I would like to extract from this line:
turbo: 'rtmp://fcs21-1.somewebsite.com/reflect/2996910732;0',
is this piece:
rtmp://fcs21-1.somewebsite.com/reflect/2996910732
What am I missing in the Regex?

Your character class - [*] matches just a *, with quantifier +, it matches 1 or more *, nothing else. Clearly it won't match your string.
I guess you meant to use .* instead, which matches 0 or more occurrences of any character but \n.
Try changing your regex to:
"turbo: '(rtmp://.*);0',$"
or even better, given your text, and what you want to extract, you can simply use:
"turbo: '([^;]*);0',$

[*] matches only *. To match any character, prefer .. (Re
(actually, . fails to match a newline. If a newline may appear, prefer something to the effect of (.|\n) -- note that the backslash will need to be escaped.)

Try this:
Match match = Regex.Match(line, #"^turbo: '(rtmp://[^;]+);0',$", RegexOptions.IgnoreCase);
This will take into account the start of the string with the ^ symbol, and the matching selection will match anything that isn't a ; all the way up to an actual ;.

Trying to understand this regex

I have this regex
^(\\w|#|\\-| |\\[|\\]|\\.)+$
I'm trying to understand what it does exactly but I can't seem to get any result...
I just can't understand the double backslashes everywhere... Isn't double backslash supposed to be used to get a single backslash?
This regex is to validate that a username doesn't use weird characters and stuff.
If someone could explain me the double backslashes thing please. #_#
Additional info: I got this regex in C# using Regex.IsMatch to check if my username string match the regex. It's for an asp website.

My guess is that it's simply escaping the \ since backslash is the escape character in c#.
string pattern = "^(\\w|#|\\-| |\\[|\\]|\\.)+$";
Can be rewritten using a verbatim string as
string pattern = #"^(\w|#|\-| |\[|\]|\.)+$";
Now it's a bit easier to understand what's going on. It will match any word character, at-sign, hyphen, space, square bracket or period, repeated one or more times. The ^ and $ match the begging and end of the string, respectively, so only those characters are allowed.
Therefore this pattern is equivalent to:
string pattern = #"^([\w# \[\].-])+$";

Double slash are supposed to be single slash. Double slash are used to escape the slash itself, as slashes are used for other escape characters in C# String context e.g. \n stands for new line
With double slashes sorted out, it becomes ^(\w|#|\-| |\[|\]|\.)+$
Break down this regex, as | means OR, and \w|#|\-| |\[|\]|\. would mean \w or # or \- or space or \[ or \] or \.. That is, any alphanumeric character, #, -, space, [, ] and . characters. Note that this slash is regex escape, to escape -, [, ] and . characters as they all have special meanings in regex context
And, + means the previous token (i.e. \w|#|\-| |\[|\]|\.) repeated one or more times
So, the entire thing means one or more of any combination of alphanumeric character, #, -, space, [, ] and . characters.

There are online tools to analyze regexes. Once such is at http://www.myezapp.com/apps/dev/regexp/show.ws
where it reports
Sequence: match all of the followings in order
BeginOfLine
Repeat
CapturingGroup
GroupNumber:1
OR: match either of the followings
WordCharacter
#
-
[
]
.
one or more times
EndOfLine
As others have noted, the double backslashes just escape a backslash so you can embed the regex in a string. For example, "\\w" will be interpreted as "\w" by the parser.

^ means beginning of the line.
the parentheses is use for grouping
\w is a word character
| means OR
# match the # character
\- match the hyphen character
[ and ] matches the squares brackets
\. match a period
+ means one or more
$ the end of line.
So the regex is use to match a string which contains only word characters or an # or an hyphen or a space or squares brackets or a dot.

Here's what it means:
^(\\w|#|\\-| |\\[|\\]|\\.)+$
^ - Means the regex starts at the beginning of the string. The match shouldn't start in the middle of the string.
Here's the individual things in the parentheses:
\\w - Indicates a "word" character. Normally, this is shown as \w, but this is being escaped.
# - Indicates an # symbol is allowed
\\- - Indicates a - is allowed. This is escaped since the dash can have other meanings in regex. Since it's not in a character class, I don't believe this is technically needed.
- A space is allowed
\\[ and \\] - [ and ] are allowed.
\\. - A period is a valid character. Escaped because periods have special meanings in regex.
Now all of those characters have | as delimiters in the parentheses - this means OR. So any of those characters are valid.
The + at the end means one or more characters as described in parentheses are valid. The $ means the end of the regex must match the end of the string.
Note that the double slashes aren't necessary if you just prefix the string like this:
#"\w" is the same as "\\w"

Replace with wildcards

I need some advice. Suppose I have the following string: Read Variable
I want to find all pieces of text like this in a string and make all of them like the following:Variable = MessageBox.Show. So as aditional examples:
"Read Dog" --> "Dog = MessageBox.Show"
"Read Cat" --> "Cat = MessageBox.Show"
Can you help me? I need a fast advice using RegEx in C#. I think it is a job involving wildcards, but I do not know how to use them very well... Also, I need this for a school project tomorrow... Thanks!
Edit: This is what I have done so far and it does not work: Regex.Replace(String, "Read ", " = Messagebox.Show").

You can do this
string ns= Regex.Replace(yourString,"Read\s+(.*?)(?:\s|$)","$1 = MessageBox.Show");
\s+ matches 1 to many space characters
(.*?)(?:\s|$) matches 0 to many characters till the first space (i.e \s) or till the end of the string is reached(i.e $)
$1 represents the first captured group i.e (.*?)

You might want to clarify your question... but here goes:
If you want to match the next word after "Read " in regex, use Read (\w*) where \w is the word character class and * is the greedy match operator.
If you want to match everything after "Read " in regex, use Read (.*)$ where . will match all characters and $ means end of line.
With either regex, you can use a replace of $1 = MessageBox.Show as $1 will reference the first matched group (which was denoted by the parenthesis).
Complete code:
replacedString = Regex.Replace(inStr, #"Read (.*)$", "$1 = MessageBox.Show");

The problem with your attempt is, that it cannot know that the replacement string should be inserted after your variable. Let's assume that valid variable names contain letters, digits and underscores (which can be conveniently matched with \w). That means, any other character ends the variable name. Then you could match the variable name, capture it (using parentheses) and put it in the replacement string with $1:
output = Regex.Replace(input, #"Read\s+(\w+)", "$1 = MessageBox.Show");
Note that \s+ matches one or more arbitrary whitespace characters. \w+ matches one or more letters, digits and underscores. If you want to restrict variable names to letters only, this is the place to change it:
output = Regex.Replace(input, #"Read\s+([a-zA-Z]+)", "$1 = MessageBox.Show");
Here is a good tutorial.
Finally note, that in C# it is advisable to write regular expressions as verbatim strings (#"..."). Otherwise, you will have to double escape everything, so that the backslashes get through to the regex engine, and that really lessens the readability of the regex.

Regex Expressions for all non alphanumeric symbols

I am trying to make a regular expression for a string that has at least 1 non alphanumeric symbol in it
The code I am trying to use is
Regex symbolPattern = new Regex("?[!##$%^&*()_-+=[{]};:<>|./?.]");
I'm trying to match only one of !##$%^&*()_-+=[{]};:<>|./?. but it doesn't seem to be working.

If you want to match non-alphanumeric symbols then just use \W|_.
Regex pattern = new Regex(#"\W|_");
This will match anything except 0-9 and a-z. Information on the \W character class and others available here (c# Regex Cheet Sheet).
https://www.mikesdotnetting.com/article/46/c-regular-expressions-cheat-sheet

You could also avoid regular expressions if you want:
return s.Any(c => !char.IsLetterOrDigit(c))

Can you check for the opposite condition?
Match match = Regex.Match(#"^([a-zA-Z0-9]+)$");
if (!match.Success) {
// it's alphanumeric
} else {
// it has one of those characters in it.
}

I didn't get your entire question, but this regex will match those strings that contains at least one non alphanumeric character. That includes whitespace (couldn't see that in your list though)
[^\w]+

Your regex just needs little tweaking. The hyphen is used to form ranges like A-Z, so if you want to match a literal hyphen, you either have to escape it with a backslash or move it to the end of the list. You also need to escape the square brackets because they're the delimiters for character class. Then get rid of that question mark at the beginning and you're in business.
Regex symbolPattern = new Regex(#"[!##$%^&*()_+=\[{\]};:<>|./?,-]");
If you only want to match ASCII punctuation characters, this is probably the simplest way. \W matches whitespace and control characters in addition to punctuation, and it matches them from the entire Unicode range, not just ASCII.
You seem to be missing a few characters, though: the backslash, apostrophe and quotation mark. Adding those gives you:
#"[!##$%^&*()_+=\[{\]};:<>|./?,\\'""-]"
Finally, it's a good idea to always use C#'s verbatim string literals (#"...") for regexes; it saves you a lot of hassle with backslashes. Quotation marks are escaped by doubling them.