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How to perform a RegEx replace only if another a separate filter is matched using .NET?

Given a string (a path) that matches /dir1/, I need to replace all spaces with dashes.
Ex: /dir1/path with spaces should become /dir1/path-with-spaces.
This could easily be done in 2 steps...
var rgx = new Regex(#"^\/dir1\/");
var path = "/dir1/path with spaces";
if (rgx.isMatch(path))
{
path = (new Regex(#" |\%20")).Replace(path, "-");
}
Unfortunately for me, the application is already built with a simple RegEx replace and cannot be modified, so I need to have the RegEx do the work. I thought I had found the answer here:
regex: how to replace all occurrences of a string within another string, if the original string matches some filter
And was able create and test (?:\G(?!^)|^(?=\/dir1\/.*$)).*?\K( |\%20), but then I learned it does not work in this app because the \K is an unrecognized escape sequence (not supported in .NET).
I also tried a positive lookbehind, but I wasn't able to get it to replace all the spaces (only the last if the match was greedy or the first if not greedy). I could put in enough checks to handle the max number of spaces, but as soon as I check for 10 spaces, someone will pass in a path with 11 spaces.
Is there a RegEx only solution for this problem that will work in the .NET engine?

You can leverage the unlimited width lookbehind pattern in .NET:
Regex.Replace(path, #"(?<=^/dir1/.*?)(?: |%20)", "-")
See the regex demo
Regex details
(?<=^/dir1/.*?) - a positive lookbehind that matches a location that is immediately preceded with /dir1/ and then any zero or more chars other than a newline char, as few as possible
(?: |%20) - either a space or %20 substring.

C# Regex Pattern Conundrum

I have a regex that I've verified in 3 separate sources as successfully matching the desired text.
http://regexlib.com/RETester.aspx
http://derekslager.com/blog/posts/2007/09/a-better-dotnet-regular-expression-tester.ashx,
http://sourceforge.net/projects/regextester/
But, when I use the regex in my code. It does not produce a match. I have used other regex with this code and they have resulted in the desired matches. I'm at a loss...
string SampleText = "starttexthere\r\nothertexthereendtexthere";
string RegexPattern = "(?<=starttexthere)(.*?)(?=endtexthere)";
Regex FindRegex = new Regex(#RegexPattern);
Match m = FindRegex.Match(SampleText);
I don't know if the problem is my regex, or my code.

The problem is that your text contains a \r\n which means it is split across two lines. If you want to match the whole string you have to set the option to match across multiple lines, and to change the behavior of the . to include the \n (new-line character) in matched
Regex FindRegex = new Regex(#RegexPattern, RegexOptions.Multiline | RegexOptions.Singleline);

You don't need RegexOptions.Multiline.
The problem in your case is that the dot matches any character except line break characters (\r\ and \n).
So, you'll need to define your regex pattern like so: (?<=starttexthere)[\w\r\n]+(?=endtexthere) in order to specifically match text across line breaks.
Here's an online running sample: http://ideone.com/ZXgKar

Regular Expression question

RegexBuddy shows the matches are OK, but in C# when I try use replace, a semicolon and a curly bracket are not replaced.
The expression I am using is the following:
#"({\\)(.+?)(}+)|(\s?\\)(.+?)(\b)|}$"
and the input text (rtf) is included in the screenshot.
This the code:
Regex reg2 = new Regex(#"\\b([\s\S]+?)\\b0");
MatchCollection matches = reg2.Matches(text);
foreach (Match match in matches)
{
string output = reg.Replace(match.Value, "");
MessageBox.Show(output);
}

You are trying to match nested structures with regular expressions. Look at your screenshot: in the first line there are three opening braces and one closing brace, in your third line you have one opening and two closing braces etc.
While .NET does provide ways to do nested pattern matching with regexes, your regex is not using them (and it's extremely mystifying to me what exactly you're hoping to achieve).
You most certainly need to use a different way to parse RTF files; unfortunately I don't know whether the .NET libraries provide an RTF parser.

Regex for string enclosed in <*>, C#

I am trying to get all strings enclosed in <*> by using following Regex:
Regex regex = new Regex(#"\<(?<name>\S+)\>", RegexOptions.IgnoreCase);
string name = e.Match.Groups["name"].Value;
But in some cases where I have text like :
<Vendors><Vtitle/> <VSurname/></Vendors>
It's returning two strings instead of four, i.e. above Regex outputs
<Vendors><Vtitle/> //as one string and
<VSurname/></Vendors> //as second string
Where as I am expecting four strings:
<Vendors>
<Vtitle/>
<VSurname/>
</Vendors>
Could you please guide me what change I need to make to my Regex.
I tried adding '\b' to specify word boundry
new Regex(#"\b\<(?<name>\S+)\>\b", RegexOptions.IgnoreCase);
, but that didn't help.

You'll get most of what what you want by using the regex /<([^>]*)>/. (No need to escape the angle brackets' as angle brackets aren't special characters in most regex engines, including the .NET engine.) The regex I provided will also capture trailing whitespace and any attributes on the tag--parsing those things reliably is way, way beyond the scope of a reasonable regex.
However, be aware that if you're trying to parse XML/HTML with a regex, that way lies madness

Regexes are the wrong tool for parsing XML. Try using the System.Xml.Linq (XElement) API.

Your regex is using \S+ as the wildcard. In english, this is "a series of one or more characters, none of which is non-whitespace". In other words, when the regex <(?<name>\S+)> is applied to this string: '`, the regex will match the entire string. angle brackets are non-whitespace.
I think what you want is "a series of one or more characters, none of which is an angle bracket".
The regex for that is <(?<name>[^>]+)> .
Ahhh, regular expressions. The language designed to look like cartoon swearing.

Simple regex pattern

i'm using C# and i'm trying to allow only alphabetical letters and spaces. my expression at the moment is:
string regex = "^[A-Za-z\s]{1,40}$";
my IDE says that \s is an "Unrecognized escape sequence"
what am i missing?

"\" is a c# escape character as well as a regex escape character. Try:
string regex = #"^[A-Za-z\s]{1,40}$";

You need to put an # in front of your string to turn it into a verbatim string literal:
string regex = #"^[A-Za-z\s]{1,40}$";
Right now, the \ in your regex is being interpreted as trying to escape the following s, which the compiler doesn't understand.
Alternatively, you can just escape the backslash with another one:
string regex = "^[A-Za-z\\s]{1,40}$";
but in general, prefer the first approach to the second.

An additional note, your regex doesn't do what you describe. You say a max of 1 space in between words. In order to do that, you need to move the "\s" out of the character list. The pattern you're currently using allows "any alphanumeric or space from 1 to 40 times" which allows for multiple successive spaces. You'll need something more like the following:
string regex = #"^(?:[A-Za-z]+\s?)+$";
This means "any alphanumeric 1 or more times followed by an optional space, this whole thing one or more times". I don't know how to limit the whole string to 40 characters when you don't know the size of the first expression in advance. Maybe this can be achieved with a "look behind" expression, but I'm not sure. You might have to do it in two steps.