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how to add whitespace to separate merged words with punctuation mark if it is between and keep it with whitespace

I need add white space if my input string, looks like this:
"hello,world"
to pass it to write in text document like this:
"hello, world" with white-space and keep the punctuation mark at the place.
In other words I need to add one white-space after punctuation mark, if next word is merged with the previous word punctuation marks. And I need it for all, comma, dot, exclamation mark and dash.
So I'm not sure, if I can use this:
string input = "hello,world,world,world";
string pattern = #",(\S)";
string substitution = #", ";
Regex regex = new Regex(pattern);
string result = regex.Replace(input, substitution);
but in result it cuts first character of word after punctuation mark:
hello, orld, orld, orld
and desired result should be:
"hello, world, world, world"

Use the Regex.Replace overload which gets a MatchEvaluator delegate:
string input = "hello!world.world-world";
var result = Regex.Replace(input, #"[\,\.\-\!]", (m) => m + " ");
// hello! world. world- world
For more about the MatchEvaluator see: How does MatchEvaluator in Regex.Replace work?

Regex & C#: Replace all Special Characters except Emojis

I need to replace all special characters in a string except the following (which includes alphabetic characters):
:)
:P
;)
:D
:(
This is what I have now:
string input = "Hi there!!! :)";
string output = Regex.Replace(input, "[^0-9a-zA-Z]+", "");
This replaces all special characters. How can I modify this to not replace mentioned characters (emojis) but replace any other special character?

You may use a known technique: match and capture what you need and match only what you want to remove, and replace with the backreference to Group 1:
(:(?:[D()P])|;\))|[^0-9a-zA-Z\s]
Replace with $1. Note I added \s to the character class, but in case you do not need spaces, remove it.
See the regex demo
Pattern explanation:
(:(?:[D()P])|;\)) - Group 1 (what we need to keep):
:(?:[D()P]) - a : followed with either D, (, ) or P
| - or
;\) - a ;) substring
(here, you may extend the capture group with more |-separated branches).
| - or ...
[^0-9a-zA-Z\s] - match any char other than ASCII digits, letters (and whitespace, but as I mentioned, you may remove \s if you do not need to keep spaces).

I would use a RegEx to match all emojis and select them out of the text
string input = "Hi there!!! :)";
string output = string.Concat(Regex.Matches(input, "[;|:][D|P|)|(]+").Cast<Match>().Select(x => x.Value));
Pattern [;|:][D|P|)|(]+
[;|:] starts with : or ;
[D|P|)|(] ends with D, P, ) or (
+ one or more

How can I remove all special chars from UTF8 text in c#?

I would like to remove all special characters from my UTF8 text, but I can't find any matching regular expression.
My text like this:
ASDÉÁPŐÓÖŰ_->,.!"%=%!HMHF
I would like to remove only these chars: _->,.!"%=%!
I tried this regex:
result = Regex.Replace(text, #"([^a-zA-Z0-9_]|^\s)", "");
But it removes my uft8 chars also.
I don't want to remove the accented characters, but I want to remove all glyph.

Regex.Replace(text, #"([^\w]|_)", "")

you want only numbers and letters?
then this is your solution:
result = Regex.Replace(text, "[^0-9a-zA-Z]+", "");
you could also try to specify a range in the ASCII table if you want a custom way of things stay in your string:
result = Regex.Replace(text, "[^\x00-\x80]+", "");

How to replace words following certain character and extract rest with REGEX

Assume that i have the following sentence
select PathSquares from tblPathFinding where RouteId=470
and StartingSquareId=267 and ExitSquareId=13
Now i want to replace words followed by = and get the rest of the sentence
Lets say i want to replace following word of = with %
Words are separated with space character
So this sentence would become
select PathSquares from tblPathFinding where RouteId=%
and StartingSquareId=% and ExitSquareId=%
With which regex i can achieve this ?
.net 4.5 C#

Use a lookbehind to match all the non-space or word chars which are just after to = symbol . Replacing the matched chars with % wiil give you the desired output.
#"(?<==)\S+"
OR
#"(?<==)\w+"
Replacement string:
%
DEMO
string str = #"select PathSquares from tblPathFinding where RouteId=470
and StartingSquareId=267 and ExitSquareId=13";
string result = Regex.Replace(str, #"(?<==)\S+", "%");
Console.WriteLine(result);
IDEONE
Explanation:
(?<==) Asserts that the match must be preceded by an = symbol.
\w+ If yes, then match the following one or more word characters.

Replace special character with white space through regex

I have a function which replace character.
public static string Replace(string value)
{
value = Regex.Replace(value, "[\n\r\t]", " ");
return value;
}
value="abc\nbcd abcd abcd\ " if in string there is any unwanted white space they are also remove.Means I want result like this
value="abcabcdabcd".Help to change Regex Pattern to get desire result.Thanks a lot.

If you need to remove any number of whitespace characters from the string, probably you're looking for something like this:
value = Regex.Replace(value, #"\s+", "");
where \s matches any whitespace character and + means one or more times.

Instead of replacing your newline, tab, etc. characters with a space, just replace all whitespace with nothing:
public static string RemoveWhitespace(string value)
{
return Regex.Replace(value, "\\s", "");
}
\s is a special character group that matches all whitespace characters. (The backslash is doubled because the backslash has a special meaning in C# strings as well.) The following MSDN link contains the exact definition of that character group:
Character Classes: White-Space Character: \s

You may want to try \s indicating white spaces. With the statement Regex.Replace(value, #"\s", ""), the output will be "abcabcdabcd".