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REGEX help for replacing a value

I have below Json which is having -INF in its value and I need to replace INF with "Infinity". Below is the sample JSON and REGEX which does the trick for me. I am looking for ways I can optimize the below method and if I can use a single Regex instead of two.
Sample Json : "Power_dB":[-INF,-1000,-1000,-1000,-INF,-INF,-INF,-INF]
My code:
var pattern = #"-(INF){1},";
var secondPattern = #"-(INF){1}\]";
string replacement = "-Infinity,";
string secondreplacement = "-Infinity]";
string result = Regex.Replace( json, pattern, replacement );
result = Regex.Replace( result, secondPattern, secondreplacement );
var settings = new JsonSerializerSettings() { FloatFormatHandling = FloatFormatHandling.Symbol };
var data = JsonConvert.DeserializeObject<JObject>( result, settings ) ;
Above code gives me desired output as below:
"Power_dB":["-INFINITY",-1000,-1000,-1000,"-INFINITY","-INFINITY","-INFINITY","-INFINITY"]

I would suggest changing your expression to this:
-INF(?=[,\]])
Breaking it down:
-INF - Match the literal "-INF"
(?=something) - Positive lookahead. The expression only matches if this also matches, but this doesn't capture.
[,\]] - Match , or ]. In this case we have no modifier after it so it will only match a single character.
Then you can simply replace the matched part with this:
-Infinity
C# code:
var regex = new Regex(#"-INF(?=[,\]])");
string input = "\"Power_dB\":[-INF,-1000,-1000,-1000,-INF,-INF,-INF,-INF]";
string result = regex.Replace(input, "-Infinity");
Console.WriteLine(result);
Try it online

Regular expression to replace string except in sqaure brackets

Need to replace all forward-slash (/) with > except for the ones in the square brackets
input string:
string str = "//div[1]/li/a[#href='https://www.facebook.com/']";
Tried pattern (did not work):
string regex = #"\/(?=$|[^]]+\||\[[^]]+\]\/)";
var pattern = Regex.Replace(str, regex, ">");
Expected Result:
">>div[1]>li>a[#href='https://www.facebook.com/']"

Your thinking was good with lookbehind but instead positive use negative.
(?<!\[[^\]]*)(\/)
Demo
After updating your c# code
string pattern = #"(?<!\[[^\]]*)(\/)";
string input = "//div[1]/li/a[#href='https://www.facebook.com/']";
var result = Regex.Replace(input, pattern, ">");
You will get
>>div[1]>li>a[#href='https://www.facebook.com/']

If you're willing to also use String.Replace you can do the following:
string input = "//div[1]/li/a[#href='https://www.facebook.com/']";
string expected = ">>div[1]>li>a[#href='https://www.facebook.com/']";
var groups = Regex.Match(input, #"^(.*)(\[.*\])$")
.Groups
.Cast<Group>()
.Select(g => g.Value)
.Skip(1);
var left = groups.First().Replace('/', '>');
var right = groups.Last();
var actual = left + right;
Assert.Equal(expected, actual);
What this does is split the string into two groups, where for the first group the / is replaced by > as you describe. The second group is appended as is. Basically, you don't care what is between square brackets.
(The Assert is from an xUnit unit test.)

You could either match from an opening till a closing square bracket or capture the / in a capturing group.
In the replacement replace the / with a <
Pattern
\[[^]]+\]|(/)
\[[^]]+\] Match from opening [ till closing ]
| Or
(/) Capture / in group 1
Regex demo | C# demo
For example
string str = "//div[1]/li/a[#href='https://www.facebook.com/']";
string regex = #"\[[^]]+\]|(/)";
str = Regex.Replace(str, regex, m => m.Groups[1].Success ? ">" : m.Value);
Console.WriteLine(str);
Output
>>div[1]>li>a[#href='https://www.facebook.com/']

c# regex replace everything including a word base64, with nothing and keeping rest of the string

I am wanting to take a string and find base64, and get rid of that and everything prior to that
example
"asdfjljlkjaldf_base64,234u0909230948098234082304802384023094"
Notice "base64," ... I want ONLY everything after "base64,"
Desired: "234u0909230948098234082304802384023094"
I was looking at this code
"string test = "hello, base64, matching";
string regexStrTest;
regexStrTest = #"test\s\w+";
MatchCollection m1 = Regex.Matches(base64,, regexStrTest);
//gets the second matched value
string value = m1[1].Value;
but that is not quite what I want..

Why regular expressions? IndexOf + Substring seems to be quite enough:
string source = "asdfjljlkjaldf_base64,234u0909230948098234082304802384023094";
string tag = "base64,";
string result = source.Substring(source.IndexOf(tag) + tag.Length);

You tried a regex that matches test, a whitespace, and 1+ word chars. The input string just did not match it.
You may use
var results = Regex.Matches(s, #"base64,(\w+)")
.Cast<Match>()
.Select(m => m.Groups[1].Value)
.ToList();
See the regex demo.
The pattern matches base64, substring and then captures into Group 1 one or more word chars with (\w+) pattern. The captured value is stored inside match.Groups[1].Value, just what you get with .Select(m => m.Groups[1].Value).

Some of the other answers are good. Here is a very simple regex
string yourData = "asdfjljlkjaldf_base64,234u0909230948098234082304802384023094";
var newString = Regex.Replace(yourData, "^.*base64,", "");

Extract string that contains only letters in C#

string input = "5991 Duncan Road";
var onlyLetters = new String(input.Where(Char.IsLetter).ToArray());
Output: DuncanRoad
But I am expecting output is Duncan Road. What need to change ?

For the input like yours, you do not need a regex, just skip all non-letter symbols at the beginning with SkipWhile():
Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements.
C# code:
var input = "5991 Duncan Road";
var onlyLetters = new String(input.SkipWhile(p => !Char.IsLetter(p)).ToArray());
Console.WriteLine(onlyLetters);
See IDEONE demo
A regx solution that will remove numbers that are not part of words and also adjoining whitespace:
var res = Regex.Replace(str, #"\s+(?<!\p{L})\d+(?!\p{L})|(?<!\p{L})\d+(?!\p{L})\s+", string.Empty); 

You can use this lookaround based regex:
repl = Regex.Replace(input, #"(?<![a-zA-Z])[^a-zA-Z]|[^a-zA-Z](?![a-zA-Z])", "");
//=> Duncan Road
(?<![a-zA-Z])[^a-zA-Z] matches a non-letter that is not preceded by another letter.
| is regex alternation
[^a-zA-Z](?![a-zA-Z]) matches a non-letter that is not followed by another letter.
RegEx Demo

You can still use LINQ filtering with Char.IsLetter || Char.IsWhiteSpace. To remove all leading and trailing whitespace chars you can call String.Trim:
string input = "5991 Duncan Road";
string res = String.Join("", input.Where(c => Char.IsLetter(c) || Char.IsWhiteSpace(c)))
.Trim();
Console.WriteLine(res); // Duncan Road

start from end of string till special character c#

This is probably a super easy question but I can't seem to figure it out. Do you know how to extract just the portion after the '/' in a string. So for like the following:
[HKEY_LOCAL_MACHINE\SOFTWARE\4YourSoul\Server\ReportEMailService\OrderConfirmation_SynergyWorldInc]
So I just want the 'OrderConfirmation_SynergyWorldInc' portion. I got 271 entries where I gotta extract just the end portion (the all have the portions before that in all of them, if that helps).
Thanks!!

IF YOU HAVE A SINGLE ENTRY...
You need to use LastIndexOf with Substring after a bit of trimming:
var s = #"[HKEY_LOCAL_MACHINE\SOFTWARE\4YourSoul\Server\ReportEMailService\OrderConfirmation_SynergyWorldInc]";
s = s.Trim('[',']');
Console.Write(s.Substring(s.LastIndexOf('\\') + 1));
Result: OrderConfirmation_SynergyWorldInc
IF YOU HAVE MULTIPLE RECORDS...
You can use a regex to extract multiple matches from a large text containing [...] substring:
[^\\\[\]]+(?=\])
See demo
For [HKEY_LOCAL_MACHINE\SOFTWARE\4YourSoul\Server\ReportEMailService\OrderConfirmation_SynergyWorldInc][SOMEENTRY] string, you will then get 2 results:
The regex matches
[^\\\[\]]+ - 1 or more characters other than ], [ and \
(?=\]) - before the end ] character.
C#:
var results = Regex.Matches(s, #"[^\\\[\]]+(?=\])").OfType<Match>().Select(p => p.Value).ToList();

var s = #"[HKEY_LOCAL_MACHINE\SOFTWARE\4YourSoul\Server\ReportEMailService\OrderConfirmation_SynergyWorldInc]";
Console.WriteLine (s.Trim(']').Split('\\').Last());
prints
OrderConfirmation_SynergyWorldInc

var key = #"[HKEY_LOCAL_MACHINE\SOFTWARE\4YourSoul\Server\ReportEMailService\OrderConfirmation_SynergyWorldInc]";
key = key.Replace("[", string.Empty);
key = key.Replace("]", string.Empty);
var splitkeys =key.Split('\\');
if (splitkeys.Any())
{
string result = splitkeys.Last();
}

Try:
string orderConfirmation = yourString.Split(new []{'\\\'}).Last();
You may also want to remove the last character if the brackets are included in the string.

string pattern = ".*\\(\w*)";
string value = "[HKEY_LOCAL_MACHINE\SOFTWARE\4YourSoul\Server\ReportEMailService\OrderConfirmation_SynergyWorldInc]"
Regex r = new Regex(pattern);
Match m = r.Match(value);