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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 to remove all spaces, periods and other non-word characters from a string

I have this [^\w\.#-] regex expression that removes any character that is not a word character from the given string it works fine. Except for the two cases that I want it to cater also that is to also remove any spaces or full-stops . if one exists in the string.
Can you please help me in editing this regex for it, I tried getting a hold of regex over internet but it doesn't seems that easy.
Regex.Replace(title, #"[^\w\.#-]", "",RegexOptions.None, TimeSpan.FromSeconds(1.5));

Remove dot from your negative character class. You only need to place those character in your negative character class that you want to keep in the replaced string.
You can use:
string repl = Regex.Replace(title, #"[^\w#-]", "", TimeSpan.FromSeconds(1.5));
Space is already being removed since space is not considered a word character.

Your regex is fine. It appears that the problem is in the way that you are trying to use it.
The replacement does not happen in place, you need to capture the result in order to get the new string:
var newTitle = Regex.Replace(title, #"[^\w\.#-]", "", RegexOptions.None, TimeSpan.FromSeconds(1.5));
This expression works as expected (demo) - it keeps only word characters, dots, dashes, and at # signs.

Regex-like construction to match %([text]) where [text] can contain escaped parens

I'm trying to resolve tokens in a string.
What I would like is given input like this:
string input = "asdf %(text) %(123) %(a\)a) asdf";
That I could run that through regex.Replace() and have it replace on "%(text)", "%(123)" and "%(a\)a)".
That is, that it would match everything between a starting "%(" and a closing ")" unless the closing ")" was escaped. (But of course, then you could escape the slash with another slash, which would prevent it from escaping the end paren...)
I'm pretty sure standard regular expressions can't do this, but I'm wondering if any of the various fancy expanded capabilities of the C# regular expression library could, rather than just iterating across the string totally manually? Or some other method that could do this? I feel like it's a common enough program that there has to be some way to solve it without implementing the solution from scratch, given the immensity of the .net framework? If I do have to implement iterating through the string and replacing with string.Replace(), I will, but it just seems so inelegant.

How about
var regex = new Regex(#"%\(.*?(?<!\\)(?:\\\\)*\)");
var result = regex.Replace(source,"");
%\( match literal %(
.*? match anything non-greedy
(?<!\\) preceding character to next match must not be \
(?:\\\\)* match zero or more literal \\ (i.e. match escaped \
\) match literal )

This is working for me :
String something = "\"asdf %(text) %(123) %(a\\)a) asdf\";";
String change = something.replaceAll("%\\(.*\\)", "");
System.out.println(change);
The output
"asdf asdf";

Retrive a Digit from a String using Regex

What I am trying to do is fairly simple, although I am running into difficulty. I have a string that is a url, it will have the format http://www.somedomain.com?id=someid what I want to retrive is the someid part. I figure I can use a regular expression but I'm not very good with them, this is what I tried:
Match match = Regex.Match(theString, #"*.?id=(/d.)");
I get a regex exception saying there was an error parsing the regex. The way I am reading this is "any number of characters" then the literal "?id=" followed "by any number of digits". I put the digits in a group so I could pull them out. I'm not sure what is wrong with this. If anyone could tell me what I'm doing wrong I would appreciated it, thanks!

No need for Regex. Just use built-in utilities.
string query = new Uri("http://www.somedomain.com?id=someid").Query;
var dict = HttpUtility.ParseQueryString(query);
var value = dict["id"]

You've got a couple of errors in your regex. Try this:
Match match = Regex.Match(theString, #".*\?id=(\d+)");
Specifically, I:
changed *. to .* (dot matches all non-newline chars and * means zero or more of the preceding)
added a an escape sequence before the ? because the question mark is a special charcter in regular expressions. It means zero or one of the preceding.
changed /d. to \d* (you had the slash going the wrong way and you used dot, which was explained above, instead of * which was also explained above)

Try
var match = RegEx.Match(theString, #".*\?id=(\d+)");

The error is probably due to preceding *. The * character in regex matches zero or more occurrences of previous character; so it cannot be the first character.
Probably a typo, but shortcut for digit is \d, not /d
. matches any character, you need to match one or more digits - so use a +
? is a special character, so it needs to be escaped.
So it becomes:
Match match = Regex.Match(theString, #".*\?id=(\d+)");
That being said, regex is not the best tool for this; use a proper query string parser or things will eventually become difficult to manage.

C# regex replace unexpected behavior

Given $displayHeight = "800";, replace whatever number is at 800 with int value y_res.
resultString = Regex.Replace(
im_cfg_contents,
#"\$displayHeight[\s]*=[\s]*""(.*)"";",
Convert.ToString(y_res));
In Python I'd use re.sub and it would work. In .NET it replaces the whole line, not the matched group.
What is a quick fix?

Building on a couple of the answers already posted. The Zero-width assertion allows you to do a regular expression match without placing those characters in the match. By placing the first part of the string in a group we've separated it from the digits that you want to be replaced. Then by using a zero-width lookbehind assertion in that group we allow the regular expression to proceed as normal but omit the characters in that group in the match. Similarly, we've placed the last part of the string in a group, and used a zero-width lookahead assertion. Grouping Constructs on MSDN shows the groups as well as the assertions.
resultString = Regex.Replace(
im_cfg_contents,
#"(?<=\$displayHeight[\s]*=[\s]*"")(.*)(?="";)",
Convert.ToString(y_res));
Another approach would be to use the following code. The modification to the regular expression is just placing the first part in a group and the last part in a group. Then in the replace string, we add back in the first and third groups. Not quite as nice as the first approach, but not quite as bad as writing out the $displayHeight part. Substitutions on MSDN shows how the $ characters work.
resultString = Regex.Replace(
im_cfg_contents,
#"(\$displayHeight[\s]*=[\s]*"")(.*)("";)",
"${1}" + Convert.ToString(y_res) + "${3}");

Try this:
resultString = Regex.Replace(
im_cfg_contents,
#"\$displayHeight[\s]*=[\s]*""(.*)"";",
#"\$displayHeight = """ + Convert.ToString(y_res) + #""";");

It replaces the whole string because you've matched the whole string - nothing about this statement tells C# to replace just the matched group, it will find and store that matched group sure, but it's still matching the whole string overall.
You can either change your replacer to:
#"\$displayHeight = """ + Convert.ToString(y_res) + #""";"
..or you can change your pattern to just match the digits, i.e.:
#"[0-9]+"
..or you could see if C# regex supports lookarounds (I'm not sure if it does offhand) and change your match accordingly.

You could also try this, though I think it is a little slower than my other method:
resultString = Regex.Replace(
im_cfg_contents,
"(?<=\\$displayHeight[\\s]*=[\\s]*\").*(?=\";)",
Convert.ToString(y_res));

Check this pattern out
(?<=(\$displayHeight\s*=\s*"))\d+(?=";)
A word about "lookaround".