Develop Reference

Replace the nth index of a character

How can I replace the nth index of a character using only Regex.
string input = "%fdfdfdfdfdfdfdfdfdfdfdffd";
string result = Regex.Replace(input, "^%", "");
The above code, replaces the first character with an empty string, But, I want to specify an index: like nth index, so that character gets replaced with an empty string.
Can someone help me out here.

It's possible to create a regex pattern that captures all characters before and after the replaced character and then replace the whole string with the two captures separated by the new character. For example:
Regex.Replace("abcdefgh", #"^(.{4}).(.*)$", #"$1E$2") // returns "abcdEfgh"
You could then create a method that replaces the character at a specific index:
string ReplaceCharacter(string text, int index, char value)
=> Regex.Replace(text, $#"^(.{{{index}}}).(.*)$", $#"${{1}}{value}${{2}}");
// Usage:
ReplaceCharacter("Foo-bar", 3, 'l') // returns "Foolbar"

As Johan Wentholt said in the comments, you can perfectly use Regex.Replace to match a number of characters from the start of the line and replace it with a capture group that's one character less than the full matched piece:
String result = Regex.Replace(input, "^(.{" + index + "}).", "$1");
This matches "index times any character, followed by another character, at the start of the string", but replaces it by only the "index times any character" without that last character, since that last dot is outside of the capture group.
If you want to replace by something else than an empty string, you just concatenate it to the end of the "$1" replacement string. Though to be safe then, you should replace it with "${1}" to avoid problems if the piece you add behind it starts with a number, since that would change the capture group number.

What you want to do may not be possible with Regex alone. This is sort of a cheat:
var input = "%fdfd678dfdfdfdfdfdfdfdffd";
var result = Regex.Replace(input, "^.{7}", input.Substring(0,6));
Console.WriteLine($"result = {result}");

Replace one character but not two in a string

I want to replace single occurrences of a character but not two in a string using C#.
For example, I want to replace & by an empty string but not when the ocurrence is &&. Another example, a&b&&c would become ab&&c after the replacement.
If I use a regex like &[^&], it will also match the character after the & and I don't want to replace it.
Another solution I found is to iterate over the string characters.
Do you know a cleaner solution to do that?

To only match one & (not preceded or followed by &), use look-arounds (?<!&) and (?!&):
(?<!&)&(?!&)
See regex demo
You tried to use a negated character class that still matches a character, and you need to use a look-ahead/look-behind to just check for some character absence/presence, without consuming it.
See regular-expressions.info:
Negative lookahead is indispensable if you want to match something not followed by something else. When explaining character classes, this tutorial explained why you cannot use a negated character class to match a q not followed by a u. Negative lookahead provides the solution: q(?!u).
Lookbehind has the same effect, but works backwards. It tells the regex engine to temporarily step backwards in the string, to check if the text inside the lookbehind can be matched there. (?<!a)b matches a "b" that is not preceded by an "a", using negative lookbehind. It doesn't match cab, but matches the b (and only the b) in bed or debt.

You can match both & and && (or any number of repetition) and only replace the single one with an empty string:
str = Regex.Replace(str, "&+", m => m.Value.Length == 1 ? "" : m.Value);

You can use this regex: #"(?<!&)&(?!&)"
var str = Regex.Replace("a&b&&c", #"(?<!&)&(?!&)", "");
Console.WriteLine(str); // ab&&c

You can go with this:
public static string replacement(string oldString, char charToRemove)
{
string newString = "";
bool found = false;
foreach (char c in oldString)
{
if (c == charToRemove && !found)
{
found = true;
continue;
}
newString += c;
}
return newString;
}
Which is as generic as possible

I would use something like this, which IMO should be better than using Regex:
public static class StringExtensions
{
public static string ReplaceFirst(this string source, char oldChar, char newChar)
{
if (string.IsNullOrEmpty(source)) return source;
int index = source.IndexOf(oldChar);
if (index < 0) return source;
var chars = source.ToCharArray();
chars[index] = newChar;
return new string(chars);
}
}

I'll contribute to this statement from the comments:
in this case, only the substring with odd number of '&' will be replaced by all the "&" except the last "&" . "&&&" would be "&&" and "&&&&" would be "&&&&"
This is a pretty neat solution using balancing groups (though I wouldn't call it particularly clean nor easy to read).
Code:
string str = "11&222&&333&&&44444&&&&55&&&&&";
str = Regex.Replace(str, "&((?:(?<2>&)(?<-2>&)?)*)", "$1$2");
Output:
11222&&333&&44444&&&&55&&&&
ideone demo
It always matches the first & (not captured).
If it's followed by an even number of &, they're matched and stored in $1. The second group is captured by the first of the pair, but then it's substracted by the second.
However, if there's there's an odd number of of &, the optional group (?<-2>&)? does not match, and the group is not substracted. Then, $2 will capture an extra &
For example, matching the subject "&&&&", the first char is consumed and it isn't captured (1). The second and third chars are matched, but $2 is substracted (2). For the last char, $2 is captured (3). The last 3 chars were stored in $1, and there's an extra & in $2.
Then, the substitution "$1$2" == "&&&&".

Replace all characters and first 0's (zeroes)

I am trying to replace all characters inside a Regular Expression expect the number, but the number should not start with 0
How can I achieve this using Regular Expression?
I have tried multiple things like #"^([1-9]+)(0+)(\d*)"and "(?<=[1-9])0+", but those does not work
Some examples of the text could be hej:\\\\0.0.0.22, hej:22, hej:\\\\?022 and hej:\\\\?22, and the result should in all places be 22

Rather than replace, try and match against [1-9][0-9]*$ on your string. Grab the matched text.
Note that as .NET regexes match Unicode number characters if you use \d, here the regex restricts what is matched to a simple character class instead.
(note: regex assumes matches at end of line only)

According to one of your comments hej:\\\\0.011.0.022 should yield 110022. First select the relevant string part from the first non zero digit up to the last number not being zero:
([1-9].*[1-9]\d*)|[1-9]
[1-9] is the first non zero digit
.* are any number of any characters
[1-9]\d* are numbers, starting at the first non-zero digit
|[1-9] includes cases consisting of only one single non zero digit
Then remove all non digits (\D)
Match match = Regex.Match(input, #"([1-9].*[1-9]\d*)|[1-9]");
if (match.Success) {
result = Regex.Replace(match.Value, "\D", "");
} else {
result = "";
}

Use following
[1-9][0-9]*$
You don't need to do any recursion, just match that.

Here is something that you can try The87Boy you can play around with or add to the pattern as you like.
string strTargetString = #"hej:\\\\*?0222\";
string pattern = "[\\\\hej:0.?*]";
string replacement = " ";
Regex regEx = new Regex(pattern);
string newRegStr = Regex.Replace(regEx.Replace(strTargetString, replacement), #"\s+", " ");
Result from the about Example = 22

Is there a better way to trim whitespace and other characters from a string?

For example, if I want to remove whitespace and trailing commas from a string, I can do this:
String x = "abc,\n";
x.Trim().Trim(new char[] { ',' });
which outputs abc correctly. I could easily wrap this in an extension method, but I'm wondering if there is an in-built way of doing this with a single call to Trim() that I'm missing. I'm used to Python, where I could do this:
import string
x = "abc,\n"
x.strip(string.whitespace + ",")
The documentation states that all Unicode whitespace characters, with a few exceptions, are stripped (see Notes to Callers section), but I'm wondering if there is a way to do this without manually defining a character array in an extension method.
Is there an in-built way to do this? The number of non-whitespace characters I want to strip may vary and won't necessarily include commas, and I want to remove all whitespace, not just \n.

Yes, you can do this:
x.Trim(new char[] { '\n', '\t', ' ', ',' });
Because newline is technically a character, you can add it to the array and avoid two calls to Trim.
EDIT
.NET 4.0 uses this method to determine if a character is considered whitespace. Earlier versions maintain an internal list of whitespace characters (Source).
If you really want to only use one Trim call, then your application could do the following:
On startup, scan the range of Unicode whitespace characters, calling Char.IsWhiteSpace on each character.
If the method call returns true, then push the character onto an array.
Add your custom characters to the array as well
Now you can use a single Trim call, by passing the array you constructed.
I'm guessing that Char.IsWhiteSpace depends on the current locale, so you'll have to pay careful attention to locale.

Using regex makes this simple:
text = Regex.Replace(text, #"^[\s,]+|[\s,]+$", "");
This will match Unicode whitespace characters as well.

You can have following Strip Extension method
public static class ExtensionMethod
{
public static string Strip(this string str, char[] otherCharactersToRemove)
{
List<char> charactersToRemove = (from s in str
where char.IsWhiteSpace(s)
select s).ToList();
charactersToRemove.AddRange(otherCharactersToRemove);
string str2 = str.Trim(charactersToRemove.ToArray());
return str2;
}
}
And then you can call it like:
static void Main(string[] args)
{
string str = "abc\n\t\r\n , asdfadf , \n \r \t";
string str2 = str.Strip(new char[]{','});
}
Out put would be:
str2 = "abc\n\t\r\n , asdfadf"
The Strip Extension method will first get all the WhiteSpace characters from the string in a list. Add other characters to remove in the list and then call trim on it.

How do i strip special characters from the end of a string?

I need to strip unknown characters from the end of a string returned from an SQL database. I also need to log when a special character occurs in the string.
What's the best way to do this?

You can use the Trim() method to trim blanks or specific characters from the end of a string. If you need to trim a certain number of characters you can use the Substring() method. You can use Regexs (System.Text.RegularExpressions namespace) to match patterns in a string and detect when they occur. See MSDN for more info.
If you need more help you'll need to provide a bit more info on what exactly you're trying to do.

First define what are unknown characters (chars other than 0-9, a to z and A to Z ?) and put them in an array
Loop trough the characters of a string and check if the char occurs, if so remove.
you can also to a String.Replace with as param the unknown char, and replaceparam ''.

Since you've specified that the legal characters are only alphanumeric, you could do something like this:
Match m = Regex.Match(original, "^([0-9A-Za-z]*)(.*)$");
string good = m.Groups[1].Value;
string bad = m.Groups[2].Value;
if (bad.Length > 0)
{
// log bad characters
}
Console.WriteLine(good);

Your definition of the problem is not precise yet this is a fast trick to do so:
string input;
...
var trimed = input.TrimEnd(new[] {'#','$',...} /* array of unwanted characters */);
if(trimed != input)
myLogger.Log(input.Replace(trimed, ""));

check out the Regex.Replace methods...there are lots of overloads. You can use the Match methods for the logging to identify all matches.
String badString = "HELLO WORLD!!!!";
Regex regex = new Regex("!{1,}$" );
String newString = regex.Replace(badString, String.Empty);