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How do I replace all instances of any special characters between each occurrence of a set of delimiters in a string?

I'm attempting to replace all instances of any special characters between each occurrence of a set of delimiters in a string. I believe the solution will include some combination of a regular expression match to retrieve the text between each set of delimiters and a regular expression replace to replace each offending character within the match with a space. Here’s what I have so far:
string input = "***XX*123456789~N3*123 E. Fake St. Apt# 456~N4*Beverly Hills*CA*902122405~REF*EI*902122405~HL*1*1*50*0~SBR*P*18*******MA~NM1*IL*1*Tom*Thompson*T***MI*123456789A~N3*456 W. False Ave.*Apt. #6B~N4*Beverly Hills*CA*90210~DMG*";
string matchPattern = "(~N3\\*)(.*?)(~N4\\*)";
string replacePattern = "[^0-9a-zA-Z ]?";
var matches = Regex.Matches(input, matchPattern);
foreach (Match match in matches)
{
match.Value = "~N3*" + Regex.Replace(match.Value, replacePattern, " ") + "~N4*";
}
MessageBox.Show(input);
I would expect the message box to show the following:
"***XX*123456789~N3*123 E Fake St Apt 456~N4*Beverly Hills*CA*902122405~REF*EI*902122405~HL*1*1*50*0~SBR*P*18*******MA~NM1*IL*1*Tom*Thompson*T***MI*123456789A~N3*456 W False Ave *Apt 6B~N4*Beverly Hills*CA*90210~DMG*"
Obviously this isn’t working because I can’t assign to the matched value inside the loop, but I hope you can follow my thought process. It is important that any characters which are not between the delimiters remain unchanged. Any direction or advice would be helpful. Thank you so much!

Use a Regex.Replace with a match evaluator where you may call the second Regex.Replace:
string input = "***XX*123456789~N3*123 E. Fake St. Apt# 456~N4*Beverly Hills*CA*902122405~REF*EI*902122405~HL*1*1*50*0~SBR*P*18*******MA~NM1*IL*1*Tom*Thompson*T***MI*123456789A~N3*456 W. False Ave.*Apt. #6B~N4*Beverly Hills*CA*90210~DMG*";
string matchPattern = #"(~N3\*)(.*?)(~N4\*)";
string replacePattern = "[^0-9a-zA-Z ]";
string res = Regex.Replace(input, matchPattern, m =>
string.Format("{0}{1}{2}",
m.Groups[1].Value,
Regex.Replace(m.Groups[2].Value, replacePattern, " "), // Here, you modify just inside the 1st regex matches
m.Groups[3].Value));
Console.Write(res); // Just to print the demo result
// => ***XX*123456789~N3*123 E Fake St Apt 456~N4*Beverly Hills*CA*902122405~REF*EI*902122405~HL*1*1*50*0~SBR*P*18*******MA~NM1*IL*1*Tom*Thompson*T***MI*123456789A~N3*456 W False Ave Apt 6B~N4*Beverly Hills*CA*90210~DMG*
See the C# demo
Actually, since ~N3* and ~N4* are literal strings, you may use a single capturing group in the pattern and then add those delimiters as hard-coded in the match evaluator, but it is up to you to decide what suits you best.

C# RegEx - get only first match in string

I've got an input string that looks like this:
level=<device[195].level>&name=<device[195].name>
I want to create a RegEx that will parse out each of the <device> tags, for example, I'd expect two items to be matched from my input string: <device[195].level> and <device[195].name>.
So far I've had some luck with this pattern and code, but it always finds both of the device tags as a single match:
var pattern = "<device\\[[0-9]*\\]\\.\\S*>";
Regex rgx = new Regex(pattern);
var matches = rgx.Matches(httpData);
The result is that matches will contain a single result with the value <device[195].level>&name=<device[195].name>
I'm guessing there must be a way to 'terminate' the pattern, but I'm not sure what it is.

Use non-greedy quantifiers:
<device\[\d+\]\.\S+?>
Also, use verbatim strings for escaping regexes, it makes them much more readable:
var pattern = #"<device\[\d+\]\.\S+?>";
As a side note, I guess in your case using \w instead of \S would be more in line with what you intended, but I left the \S because I can't know that.

depends how much of the structure of the angle blocks you need to match, but you can do
"\\<device.+?\\>"

I want to create a RegEx that will parse out each of the <device> tags
I'd expect two items to be matched from my input string:
1. <device[195].level>
2. <device[195].name>
This should work. Get the matched group from index 1
(<device[^>]*>)
Live demo
String literals for use in programs:
#"(<device[^>]*>)"

Change your repetition operator and use \w instead of \S
var pattern = #"<device\[[0-9]+\]\.\w+>";
String s = #"level=<device[195].level>&name=<device[195].name>";
foreach (Match m in Regex.Matches(s, #"<device\[[0-9]+\]\.\w+>"))
Console.WriteLine(m.Value);
Output
<device[195].level>
<device[195].name>

Use named match groups and create a linq entity projection. There will be two matches, thus separating the individual items:
string data = "level=<device[195].level>&name=<device[195].name>";
string pattern = #"
(?<variable>[^=]+) # get the variable name
(?:=<device\[) # static '=<device'
(?<index>[^\]]+) # device number index
(?:]\.) # static ].
(?<sub>[^>]+) # Get the sub command
(?:>&?) # Match but don't capture the > and possible &
";
// Ignore pattern whitespace is to document the pattern, does not affect processing.
var items = Regex.Matches(data, pattern, RegexOptions.IgnorePatternWhitespace)
.OfType<Match>()
.Select (mt => new
{
Variable = mt.Groups["variable"].Value,
Index = mt.Groups["index"].Value,
Sub = mt.Groups["sub"].Value
})
.ToList();
items.ForEach(itm => Console.WriteLine ("{0}:{1}:{2}", itm.Variable, itm.Index, itm.Sub));
/* Output
level:195:level
name:195:name
*/

C# Regex Split - How do I split string into 2 words

I have the following string:
String myNarrative = "ID: 4393433 This is the best narration";
I want to split this into 2 strings;
myId = "ID: 4393433";
myDesc = "This is the best narration";
How do I do this in Regex.Split()?
Thanks for your help.

If it is a fixed format as shown, use Regex.Match with Capturing Groups (see Matched Subexpressions). Split is useful for dividing up a repeating sequence with unbound multiplicity; the input does not represent such a sequence but rather a fixed set of fields/values.
var m = Regex.Match(inp, #"ID:\s+(\d+)\s+(.*)\s+");
if (m.Success) {
var number = m.Groups[1].Value;
var rest = m.Groups[2].Value;
} else {
// Failed to match.
}
Alternatively, one could use Named Groups and have a read through the Regular Expression Language quick-reference.

Regular Expression Pattern C#

I have the following string that would require me to parse it via Regex in C#.
Format: rec_mnd.rate.current_rate.sum.QWD.RET : 214345
I would like to extract our the bold chars as group objects in a groupcollection.
QWD = 1 group
RET = 1 group
214345 = 1 group
what would the message pattern be like?

It would be something like this:
string s = "Format: rec_mnd.rate.current_rate.sum.QWD.RET : 214345";
Match m = Regex.Match(s, #"^Format: rec_mnd\.rate\.current_rate\.sum\.(.+?)\.(.+?) : (\d+)$");
if( m.Success )
{
Console.WriteLine(m.Groups[1].Value);
Console.WriteLine(m.Groups[2].Value);
Console.WriteLine(m.Groups[3].Value);
}
The question mark in the first two groups make that quantifier lazy: it will capture the least possible amount of characters. In other words, it captures until the first . it sees. Alternatively, you could use ([^.]+) in those groups, which explicitly captures everything except a period.
The last group explicitly only captures decimal digits. If your expression can have other values on the right side of the : you'd have to change that to .+ as well.

Please, make it a lot easier on yourself and label your groups to make it easier to understand what is going on in code.
RegEx myRegex = new Regex(#"rec_mnd\.rate\.current_rate\.sum\.(?<code>[A-Z]{3})\.(?<subCode>[A-Z]{3})\s*:\s*(?<number>\d+)");
var matches = myRegex.Matches(sourceString);
foreach(Match match in matches)
{
//do stuff
Console.WriteLine("Match");
Console.WriteLine("Code: " + match.Groups["code"].Value);
Console.WriteLine("SubCode: " + match.Groups["subCode"].Value);
Console.WriteLine("Number: " + match.Groups["number"].Value);
}

This should give you what you want regardless of what's between the .'s.
#"(?:.+\.){4}(.\w+)\.(\w+)\s?:\s?(\d+)"

Regex to match and return group names

I need to match the following strings and returns the values as groups:
abctic
abctac
xyztic
xyztac
ghhtic
ghhtac
Pattern is wrote with grouping is as follows:
(?<arch>[abc,xyz,ghh])(?<flavor>[tic,tac]$)
The above returns only parts of group names. (meaning match is not correct).
If I use * in each sub pattern instead of $ at the end, groups are correct, but that would mean that abcticff will also match.
Please let me know what my correct regex should be.

Your pattern is incorrect because a pipe symbol | is used to specify alternate matches, not a comma in brackets as you were using, i.e., [x,y].
Your pattern should be: ^(?<arch>abc|xyz|ghh)(?<flavor>tic|tac)$
The ^ and $ metacharacters ensures the string matches from start to end. If you need to match text in a larger string you could replace them with \b to match on a word boundary.
Try this approach:
string[] inputs = { "abctic", "abctac", "xyztic", "xyztac", "ghhtic", "ghhtac" };
string pattern = #"^(?<arch>abc|xyz|ghh)(?<flavor>tic|tac)$";
foreach (var input in inputs)
{
var match = Regex.Match(input, pattern);
if (match.Success)
{
Console.WriteLine("Arch: {0} - Flavor: {1}",
match.Groups["arch"].Value,
match.Groups["flavor"].Value);
}
else
Console.WriteLine("No match for: " + input);
}