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Add prefix to special characters with Regular Expressions

I have a list of special characters that includes ^ $ ( ) % . [ ] * + - ?. I want put % in front of this special characters in a string value.
I need this to generate a Lua script to use in Redis.
For example Test$String? must be change to Test%$String%?.
Is there any way to do this with regular expressions in C#?

In C#, you just need a Regex.Replace:
var LuaEscapedString = Regex.Replace(input, #"[][$^()%.*+?-]", "%$&");
See the regex demo
The [][$^()%.*+?-] character class will match a single character, either a ], [, $, ^, (, ), %, ., *, +, ? or - and will reinsert it back with the $& backreference in the replacement pattern pre-pending with a % character.
A lookahead is just a redundant overhead here (or a show-off trick for your boss).

You can use lookaheads and replace with %
/(?=[]*+$?)[(.-])/
Regex Demo
(?=[]*+$?)[(.-]) Postive lookahead, checks if the character following any one from the altenation []. If yes, substitutes with %

You can use this regex: ([\\^$()%.\\[\\]*+\\-?])
It will match and capture characters inside the character class. Then you can use $1 to reference the captured character and insert % before it, like so: %$1.
Here is an example code and demo:
string input = "Test$String?";
string pattern = "([\\^$()%.\\[\\]*+\\-?])";
string replacement = "%$1";
Regex rgx = new Regex(pattern);
string result = rgx.Replace(input, replacement);
Console.WriteLine("Original String: {0}", input);
Console.WriteLine("Replacement String: {0}", result);

You can use (?=[\^\$()\%.[]*+-\?]) regex replaced String as "%"

Replacing special chars in a string with a single unique char

I have a string like so:
string inputStr = "Name*&^%LastName*##";
The following Regex will replace all the special chars with a '-'
Regex rgx = new Regex("[^a-zA-Z0-9 - _]");
someStr = rgx.Replace(someStr, "-");
That produces an output something like:
Name---LastName---
How do I replace '---' with a single '-' so the output looks like this:
Name-LastName
So the question is how do I replace all the special chars with a single '-'?
Regards.

Try this
Regex rgx = new Regex("[^a-zA-Z0-9 \- _]+");//note - character is escaped
or
Regex rgx = new Regex("[^a-zA-Z0-9 _-]+");//or use - as last character
But this will give Name-LastName- Is this okay or..?
If you don't need - at last position you can use the following code as well. Credit goes to
#MatthewStrawbridge. You can see in comments.
string someStr = rgx.Replace(inputStr, "-").TrimEnd('-');
will output Name-LastName.
Edit: As #pguardiario pointed in comments updated my answer to escape - since range([]) has special meaning for - character. If we need - as a literal we need to escape it or make it first or last character of the character class in order to behave as literal.

Regular expression query

I am looking for regular expression that evaluates below.
0-9 a-Z A-Z - / '

The C# version of this pattern is:
#"[0-9a-zA-Z/'-]"
Used in code:
var regex = new Regex(#"[0-9a-zA-Z/'-]");
or
var regex = new Regex(#"[0-9a-z/'-]", RegexOptions.IgnoreCase);
Note that the - is at the very end of the character class (the part in the brackets). For - to mean a literal hyphen inside a character class, it must be at the beginning or end of the class (i.e. [-blah] or [blah-]), or escaped with a backslash: [ab\-c] will match a, b, c, or -.
Note also the # at the beginning of the quoted string. This isn't important for this pattern, but it's a good habit to get into with C# regex. Regular expressions often contain backslashes, and the #"..." form will allow you to use backslashes in your pattern without having to escape them.

Use bellow code to validate(Regex patterns) Alphabetic and numbers:
String name="123ABCabc";
if(System.Text.RegularExpressions.Regex.Match(name, #"[0-9a-zA-Z_]") == true)
{
return true;
}
else
{
return false;
}

In the case you want to match digits, lower- and higher-case latin characters, "-", "/" and "'" then I would suggest the following:
[0-9a-zA-Z-\/\']

Replace any character before <usernameredacted#example.com> with an empty string

I have this string
AnyText: "jonathon" <usernameredacted#example.com>
Desired Output Using Regex
AnyText: <usernameredacted#example.com>
Omit anything in between !
I am still a rookie at regular expressions. Could anyone out there help me with the matching & replacing expression for the above scenario?

Try this:
string input = "jonathon <usernameredacted#example.com>";
string output = Regex.Match(input, #"<[^>]+>").Groups[0].Value;
Console.WriteLine(output); //<usernameredacted#example.com>

You could use the following regex to match all the characters that you want to replace with an empty string:
^[^<]*
The first ^ is an anchor to the beginning of the string. The ^ inside the character class means that the character class is a negation. ie. any character that isn't an < will match. The * is a greedy quantifier. So in summary, this regex will swallow up all characters from the beginning of the string until the first <.

Here is the way to do it in VBA flavor: Replace "^[^""]*" with "".
^ marks the start of the sentence.
[^""]* marks anything other than a
quote sign.
UPDATE:
Since in your additional comment you mentioned you wanted to grab the "From:" and the email address, but none of the junk in between or after, I figure instead of replace, extract would be better. Here is a VBA function written for Excel that will give you back all the subgroup matches (everything you put in parenthesis) and nothing else.
Function RegexExtract(ByVal text As String, _
ByVal extract_what As String) As String
Application.ScreenUpdating = False
Dim i As Long
Dim result As String
Dim allMatches As Object
Dim RE As Object
Set RE = CreateObject("vbscript.regexp")
RE.Pattern = extract_what
RE.Global = True
Set allMatches = RE.Execute(text)
For i = 0 To allMatches.Item(0).submatches.count - 1
result = result & allMatches.Item(0).submatches.Item(i)
Next
RegexExtract = result
Application.ScreenUpdating = True
End Function
Using this code, your regex call would be: "^(.+: ).+(<.+>).*"
^ denotes start of sentence
(.+: ) denotes first match group. .+ is one or more characters, followed by : and a space
.+ denotes one or more characters
(<.+>) denotes second match group.
< is <, then .+ for one or more characters, then the final >
.* denotes zero or more
characters.
So in excel you'd use (assuming cell is A1):
=RegexExtract(A1, "^(.+: ).+(<.+>).*")

Regex to match alphanumeric and spaces

What am I doing wrong here?
string q = "john s!";
string clean = Regex.Replace(q, #"([^a-zA-Z0-9]|^\s)", string.Empty);
// clean == "johns". I want "john s";

just a FYI
string clean = Regex.Replace(q, #"[^a-zA-Z0-9\s]", string.Empty);
would actually be better like
string clean = Regex.Replace(q, #"[^\w\s]", string.Empty);

This:
string clean = Regex.Replace(dirty, "[^a-zA-Z0-9\x20]", String.Empty);
\x20 is ascii hex for 'space' character
you can add more individual characters that you want to be allowed.
If you want for example "?" to be ok in the return string add \x3f.

I got it:
string clean = Regex.Replace(q, #"[^a-zA-Z0-9\s]", string.Empty);
Didn't know you could put \s in the brackets

The following regex is for space inclusion in textbox.
Regex r = new Regex("^[a-zA-Z\\s]+");
r.IsMatch(textbox1.text);
This works fine for me.

I suspect ^ doesn't work the way you think it does outside of a character class.
What you're telling it to do is replace everything that isn't an alphanumeric with an empty string, OR any leading space. I think what you mean to say is that spaces are ok to not replace - try moving the \s into the [] class.

There appear to be two problems.
You're using the ^ outside a [] which matches the start of the line
You're not using a * or + which means you will only match a single character.
I think you want the following regex #"([^a-zA-Z0-9\s])+"

bottom regex with space, supports all keyboard letters from different culture
string input = "78-selim güzel667.,?";
Regex regex = new Regex(#"[^\w\x20]|[\d]");
var result= regex.Replace(input,"");
//selim güzel

The circumflex inside the square brackets means all characters except the subsequent range. You want a circumflex outside of square brackets.

This regex will help you to filter if there is at least one alphanumeric character and zero or more special characters i.e. _ (underscore), \s whitespace, -(hyphen)
string comparer = "string you want to compare";
Regex r = new Regex(#"^([a-zA-Z0-9]+[_\s-]*)+$");
if (!r.IsMatch(comparer))
{
return false;
}
return true;
Create a set using [a-zA-Z0-9]+ for alphanumeric characters, "+" sign (a quantifier) at the end of the set will make sure that there will be at least one alphanumeric character within the comparer.
Create another set [_\s-]* for special characters, "*" quantifier is to validate that there can be special characters within comparer string.
Pack these sets into a capture group ([a-zA-Z0-9]+[_\s-]*)+ to say that the comparer string should occupy these features.

[RegularExpression(#"^[A-Z]+[a-zA-Z""'\s-]*$")]
Above syntax also accepts space