Develop Reference

How to remove all but the first occurences of a character from a string?

I have a string of text and want to ensure that it contains at most one single occurrence of a specific character (,). Therefore I want to keep the first one, but simply remove all further occurrences of that character.
How could I do this the most elegant way using C#?

This works, but not the most elegant for sure :-)
string a = "12,34,56,789";
int pos = 1 + a.IndexOf(',');
return a.Substring(0, pos) + a.Substring(pos).Replace(",", string.Empty);

You could use a counter variable and a StringBuilder to create the new string efficiently:
var sb = new StringBuilder(text.Length);
int maxCount = 1;
int currentCount = 0;
char specialChar = ',';
foreach(char c in text)
if(c != specialChar || ++currentCount <= maxCount)
sb.Append(c);
text = sb.ToString();
This approach is not the shortest but it's efficient and you can specify the char-count to keep.
Here's a more "elegant" way using LINQ:
int commasFound = 0; int maxCommas = 1;
text = new string(text.Where(c => c != ',' || ++commasFound <= maxCommas).ToArray());
I don't like it because it requires to modify a variable from a query, so it's causing a side-effect.

Regular expressions are elegant, right?
Regex.Replace("Eats, shoots, and leaves.", #"(?<=,.*),", "");
This replaces every comma, as long as there is a comma before it, with nothing.
(Actually, it's probably not elegant - it may only be one line of code, but it may also be O(n^2)...)

If you don't deal with large strings and you reaaaaaaly like Linq oneliners:
public static string KeepFirstOccurence (this string #string, char #char)
{
var index = #string.IndexOf(#char);
return String.Concat(String.Concat(#string.TakeWhile(x => #string.IndexOf(x) < index + 1)), String.Concat(#string.SkipWhile(x=>#string.IndexOf(x) < index)).Replace(#char.ToString(), ""));
}

You could write a function like the following one that would split the string into two sections based on the location of what you were searching (via the String.Split() method) for and it would only remove matches from the second section (using String.Replace()) :
public static string RemoveAllButFirst(string s, string stuffToRemove)
{
// Check if the stuff to replace exists and if not, return the original string
var locationOfStuff = s.IndexOf(stuffToRemove);
if (locationOfStuff < 0)
{
return s;
}
// Calculate where to pull the first string from and then replace the rest of the string
var splitLocation = locationOfStuff + stuffToRemove.Length;
return s.Substring(0, splitLocation) + (s.Substring(splitLocation)).Replace(stuffToRemove,"");
}
You could simply call it by using :
var output = RemoveAllButFirst(input,",");
A prettier approach might actually involve building an extension method that handled this a bit more cleanly :
public static class StringExtensions
{
public static string RemoveAllButFirst(this string s, string stuffToRemove)
{
// Check if the stuff to replace exists and if not, return the
// original string
var locationOfStuff = s.IndexOf(stuffToRemove);
if (locationOfStuff < 0)
{
return s;
}
// Calculate where to pull the first string from and then replace the rest of the string
var splitLocation = locationOfStuff + stuffToRemove.Length;
return s.Substring(0, splitLocation) + (s.Substring(splitLocation)).Replace(stuffToRemove,"");
}
}
which would be called via :
var output = input.RemoveAllButFirst(",");
You can see a working example of it here.

static string KeepFirstOccurance(this string str, char c)
{
int charposition = str.IndexOf(c);
return str.Substring(0, charposition + 1) +
str.Substring(charposition, str.Length - charposition)
.Replace(c, ' ').Trim();
}

Pretty short with Linq; split string into chars, keep distinct set and join back to a string.
text = string.Join("", text.Select(c => c).Distinct());

How to replace part of string by position?

I have this string: ABCDEFGHIJ
I need to replace from position 4 to position 5 with the string ZX
It will look like this: ABCZXFGHIJ
But not to use with string.replace("DE","ZX") - I need to use with position
How can I do it?

string s = "ABCDEFGH";
s= s.Remove(3, 2).Insert(3, "ZX");

The easiest way to add and remove ranges in a string is to use the StringBuilder.
var theString = "ABCDEFGHIJ";
var aStringBuilder = new StringBuilder(theString);
aStringBuilder.Remove(3, 2);
aStringBuilder.Insert(3, "ZX");
theString = aStringBuilder.ToString();
An alternative is to use String.Substring, but I think the StringBuilder code gets more readable.

ReplaceAt(int index, int length, string replace)
Here's an extension method that doesn't use StringBuilder or Substring. This method also allows the replacement string to extend past the length of the source string.
//// str - the source string
//// index- the start location to replace at (0-based)
//// length - the number of characters to be removed before inserting
//// replace - the string that is replacing characters
public static string ReplaceAt(this string str, int index, int length, string replace)
{
return str.Remove(index, Math.Min(length, str.Length - index))
.Insert(index, replace);
}
When using this function, if you want the entire replacement string to replace as many characters as possible, then set length to the length of the replacement string:
"0123456789".ReplaceAt(7, 5, "Hello") = "0123456Hello"
Otherwise, you can specify the amount of characters that will be removed:
"0123456789".ReplaceAt(2, 2, "Hello") = "01Hello456789"
If you specify the length to be 0, then this function acts just like the insert function:
"0123456789".ReplaceAt(4, 0, "Hello") = "0123Hello456789"
I guess this is more efficient since the StringBuilder class need not be initialized and since it uses more basic operations. Please correct me if I am wrong. :)

If you care about performance, then the thing you want to avoid here are allocations. And if you're on .Net Core 2.1+, then you can, by using the string.Create method:
public static string ReplaceAt(this string str, int index, int length, string replace)
{
return string.Create(str.Length - length + replace.Length, (str, index, length, replace),
(span, state) =>
{
state.str.AsSpan().Slice(0, state.index).CopyTo(span);
state.replace.AsSpan().CopyTo(span.Slice(state.index));
state.str.AsSpan().Slice(state.index + state.length).CopyTo(span.Slice(state.index + state.replace.Length));
});
}
This approach is harder to understand than the alternatives, but it's the only one that will allocate only one object per call: the newly created string.

Use String.Substring() (details here) to cut left part, then your replacement, then right part. Play with indexes until you get it right :)
Something like:
string replacement=original.Substring(0,start)+
rep+original.Substring(start+rep.Length);

string s = "ABCDEFG";
string t = "st";
s = s.Remove(4, t.Length);
s = s.Insert(4, t);

As an extension method.
public static class StringBuilderExtension
{
public static string SubsituteString(this string OriginalStr, int index, int length, string SubsituteStr)
{
return new StringBuilder(OriginalStr).Remove(index, length).Insert(index, SubsituteStr).ToString();
}
}

You could try something link this:
string str = "ABCDEFGHIJ";
str = str.Substring(0, 2) + "ZX" + str.Substring(5);

Like other have mentioned the Substring() function is there for a reason:
static void Main(string[] args)
{
string input = "ABCDEFGHIJ";
string output = input.Overwrite(3, "ZX"); // 4th position has index 3
// ABCZXFGHIJ
}
public static string Overwrite(this string text, int position, string new_text)
{
return text.Substring(0, position) + new_text + text.Substring(position + new_text.Length);
}
Also I timed this against the StringBuilder solution and got 900 tics vs. 875. So it is slightly slower.

Yet another
public static string ReplaceAtPosition(this string self, int position, string newValue)
{
return self.Remove(position, newValue.Length).Insert(position, newValue);
}

With the help of this post, I create following function with additional length checks
public string ReplaceStringByIndex(string original, string replaceWith, int replaceIndex)
{
if (original.Length >= (replaceIndex + replaceWith.Length))
{
StringBuilder rev = new StringBuilder(original);
rev.Remove(replaceIndex, replaceWith.Length);
rev.Insert(replaceIndex, replaceWith);
return rev.ToString();
}
else
{
throw new Exception("Wrong lengths for the operation");
}
}

All others answers don't work if the string contains Unicode char (like Emojis) because an Unicode char weight more bytes than a char.
Example : the emoji '🎶' converted to bytes, will weight the equivalent of 2 chars. So, if the unicode char is placed at the beginning of your string, offset parameter will be shifted).
With this topic, i extend the StringInfo class to Replace by position keeping the Nick Miller's algorithm to avoid that :
public static class StringInfoUtils
{
public static string ReplaceByPosition(this string str, string replaceBy, int offset, int count)
{
return new StringInfo(str).ReplaceByPosition(replaceBy, offset, count).String;
}
public static StringInfo ReplaceByPosition(this StringInfo str, string replaceBy, int offset, int count)
{
return str.RemoveByTextElements(offset, count).InsertByTextElements(offset, replaceBy);
}
public static StringInfo RemoveByTextElements(this StringInfo str, int offset, int count)
{
return new StringInfo(string.Concat(
str.SubstringByTextElements(0, offset),
offset + count < str.LengthInTextElements
? str.SubstringByTextElements(offset + count, str.LengthInTextElements - count - offset)
: ""
));
}
public static StringInfo InsertByTextElements(this StringInfo str, int offset, string insertStr)
{
if (string.IsNullOrEmpty(str?.String))
return new StringInfo(insertStr);
return new StringInfo(string.Concat(
str.SubstringByTextElements(0, offset),
insertStr,
str.LengthInTextElements - offset > 0 ? str.SubstringByTextElements(offset, str.LengthInTextElements - offset) : ""
));
}
}

string myString = "ABCDEFGHIJ";
string modifiedString = new StringBuilder(myString){[3]='Z', [4]='X'}.ToString();
Let me explain my solution.
Given the problem statement of altering a string in its two specific position (“position 4 to position 5”) with two character ‘Z’ and ‘X’ and the ask is to use the position index to alter the string and not string Replace() method(may be because of the possibility of repetition of some characters in the actual string), I would prefer to use minimalist approach to achieve the goal over using Substring() and string Concat() or string Remove() and Insert() approach. Though all those solutions will serve the purpose in attaining the same goal, but it just depends on personal choice and philosophy of settling with minimalist approach.
Coming back to my solution mention above, if we take a closer look of string and StringBuilder, both of them internally treats a given string as an array of characters. If we look at the implementation of StringBuilder it maintains an internal variable something like “internal char[] m_ChunkChars;” to capture the given string. Now since this is an internal variable, we cannot directly access the same. For external world, to be able to access and alter that character array, StringBuilder exposes them through indexer property which looks something like below
[IndexerName("Chars")]
public char this[int index]
{
get
{
StringBuilder stringBuilder = this;
do
{
// … some code
return stringBuilder.m_ChunkChars[index1];
// … some more code
}
}
set
{
StringBuilder stringBuilder = this;
do
{
//… some code
stringBuilder.m_ChunkChars[index1] = value;
return;
// …. Some more code
}
}
}
My solution mentioned above leverage this indexer capability to directly alter the internally maintained character array which IMO is efficient and minimalist.
BTW; we can rewrite the above solution more elaborately something like below
string myString = "ABCDEFGHIJ";
StringBuilder tempString = new StringBuilder(myString);
tempString[3] = 'Z';
tempString[4] = 'X';
string modifiedString = tempString.ToString();
In this context also would like to mention that in case of string it also have indexer property as a means to expose its internal character array, but in this case it only has Getter property (and no Setter) as string is immutable in nature. And that is why we need to use StringBuilder to alter the character array.
[IndexerName("Chars")]
public extern char this[int index] { [SecuritySafeCritical, __DynamicallyInvokable, MethodImpl(MethodImplOptions.InternalCall)] get; }
And last but not the least this solution is only best fit for this specific problem where the ask is to replace only few characters with a known position index upfront. It may not be the best fit when the requirement is to alter a fairly lengthy string i.e. number of characters to alter are large in numbers.

I was looking for a solution with following requirements:
use only a single, one-line expression
use only system builtin methods (no custom implemented utility)
Solution 1
The solution that best suits me is this:
// replace `oldString[i]` with `c`
string newString = new StringBuilder(oldString).Replace(oldString[i], c, i, 1).ToString();
This uses StringBuilder.Replace(oldChar, newChar, position, count)
Solution 2
The other solution that satisfies my requirements is to use Substring with concatenation:
string newString = oldStr.Substring(0, i) + c + oldString.Substring(i+1, oldString.Length);
This is OK too. I guess it's not as efficient as the first one performance wise (due to unnecessary string concatenation). But premature optimization is the root of all evil.
So pick the one that you like the most :)

hello this code helps me:
var theString = "ABCDEFGHIJ";
var aStringBuilder = new StringBuilder(theString);
aStringBuilder.Remove(3, 2);
aStringBuilder.Insert(3, "ZX");
theString = aStringBuilder.ToString();

It's better to use the String.substr().
Like this:
ReplString = GivenStr.substr(0, PostostarRelStr)
+ GivenStr(PostostarRelStr, ReplString.lenght());

String timestamp = "2019-09-18 21.42.05.000705";
String sub1 = timestamp.substring(0, 19).replace('.', ':');
String sub2 = timestamp.substring(19, timestamp.length());
System.out.println("Original String "+ timestamp);
System.out.println("Replaced Value "+ sub1+sub2);

Here is a simple extension method:
public static class StringBuilderExtensions
{
public static StringBuilder Replace(this StringBuilder sb, int position, string newString)
=> sb.Replace(position, newString.Length, newString);
public static StringBuilder Replace(this StringBuilder sb, int position, int length, string newString)
=> (newString.Length <= length)
? sb.Remove(position, newString.Length).Insert(position, newString)
: sb.Remove(position, length).Insert(position, newString.Substring(0, length));
}
Use it like this:
var theString = new string(' ', 10);
var sb = new StringBuilder(theString);
sb.Replace(5, "foo");
return sb.ToString();

I do this
Dim QTT As Double
If IsDBNull(dr.Item(7)) Then
QTT = 0
Else
Dim value As String = dr.Item(7).ToString()
Dim posicpoint As Integer = value.LastIndexOf(".")
If posicpoint > 0 Then
Dim v As New Text.StringBuilder(value)
v.Remove(posicpoint, 1)
v.Insert(posicpoint, ",")
QTT = Convert.ToDouble(v.ToString())
Else
QTT = Convert.ToDouble(dr.Item(7).ToString())
End If
Console.WriteLine(QTT.ToString())
End If

Suppose we know the index of string to be replace.
string s = "ABCDEFGDEJ";
string z = "DE";
int i = s.IndexOf(z);
if(i == 3)
s = s.Remove(3,z.Length).Insert(3,"ZX");
//s = ABCZXFGDEJ

I believe the simplest way would be this:(without stringbuilder)
string myString = "ABCDEFGHIJ";
char[] replacementChars = {'Z', 'X'};
byte j = 0;
for (byte i = 3; i <= 4; i++, j++)
{
myString = myString.Replace(myString[i], replacementChars[j]);
}
This works because a variable of type string can be treated as an array of char variables.
You can, for example refer to the second character of a string variable with name "myString" as myString[1]

Edit characters in a String in C#

What's the cleanest way of editing the characters in a string in C#?
What's the C# equivalent of this in C++:
std::string myString = "boom";
myString[0] = "d";

Use a StringBuilder instead.
string is immutable as described by MSDN:
Strings are immutable--the contents of
a string object cannot be changed
after the object is created, although
the syntax makes it appear as if you
can do this.
So you want something like:
StringBuilder sb = new StringBuilder("Bello World!");
sb[0] = 'H';
string str = sb.ToString();

Decided to time what I felt where the two most canonical approaches, plus one I threw in as unrepresented; here's what I found (Release build):
ReplaceAtChars: 86ms
ReplaceAtSubstring: 258ms
ReplaceAtStringBuilder: 161ms
Clearly the Char array approach is by far best optimized by the runtime. Which actually suggests that the current leading answer (StringBuilder) is likely not the best answer.
And here was the test I used:
class Program
{
static void Main(string[] args)
{
Console.WriteLine("ReplaceAtChars: " + new Stopwatch().Time(() => "test".ReplaceAtChars(1, 'E').ReplaceAtChars(3, 'T'), 1000000) + "ms");
Console.WriteLine("ReplaceAtSubstring: " + new Stopwatch().Time(() => "test".ReplaceAtSubstring(1, 'E').ReplaceAtSubstring(3, 'T'), 1000000) + "ms");
Console.WriteLine("ReplaceAtStringBuilder: " + new Stopwatch().Time(() => "test".ReplaceAtStringBuilder(1, 'E').ReplaceAtStringBuilder(3, 'T'), 1000000) + "ms");
}
}
public static class ReplaceAtExtensions
{
public static string ReplaceAtChars(this string source, int index, char replacement)
{
var temp = source.ToCharArray();
temp[index] = replacement;
return new String(temp);
}
public static string ReplaceAtStringBuilder(this string source, int index, char replacement)
{
var sb = new StringBuilder(source);
sb[index] = replacement;
return sb.ToString();
}
public static string ReplaceAtSubstring(this string source, int index, char replacement)
{
return source.Substring(0, index) + replacement + source.Substring(index + 1);
}
}
public static class StopwatchExtensions
{
public static long Time(this Stopwatch sw, Action action, int iterations)
{
sw.Reset();
sw.Start();
for (int i = 0; i < iterations; i++)
{
action();
}
sw.Stop();
return sw.ElapsedMilliseconds;
}
}

Strings in .Net are immutable. That means you can't edit them. All you can do is make new ones. You have to do something like this:
string myString = "boom";
myString = "d" + myString.Substring(1);

Strings are immutable, so you can't just change it.
You could get a char[] from the string, make your changes, and make a new string from the altered array.
You could use things like replace, although they don't give you control over the per-char basis as C like. They also will make a new string with every change, where the char[] style allows you to make all changes and then make a new string from it.

You can't since strings are immutable.
You could always roll your own extension method to do something similar:
public static string ReplaceAtIndex(this string original,
int index, char character)
{
char[] temp = original.ToCharArray();
temp[index] = character;
return new String(temp);
}
And call it like:
string newString = myString.ReplaceAtIndex(0, 'd');

As Brian said, use a StringBuilder, instead:
string myString = "boom";
StringBuilder myStringBuilder = new StringBuilder(myString);
myStringBuilder[0] = 'd';
// Whatever else you're going to do to it
Whenever you need the string again, call myStringBuilder.ToString()

string myString = "boom";
char[] arr = myString.ToCharArray();
arr[0] = 'd';
myString = new String(arr);

string myString = "boom";
char[] myChars = myString.ToCharArray();
myChars[0] = 'd';
myString = new String(myChars);

Strings are immutable in C#. Use the StringBuilder class or a character array.
StringBuilder sb = new StringBuilder("boom");
sb[0] = 'd';
IIRC, .NET uses what is called String Pooling. Every time a new string literal is created, it is stored in memory as part of the string pool. If you create a second string that matches a string in the string pool, both variables will reference the same memory.
When you try to do an operation like you did to replace the 'b' character with a 'd' character using strings in .NET, your program is actually creating a second string in the string pool with the value "doom", although from your perspective it does not appear as if this is happening at all. By reading the code, one would assume that the character is being replaced.
I brought this up because I encounter this question all the time, and people often ask why they should be using the StringBuilder when a string can do the same thing. Well technically it can't, but it's designed in a way to appear as if it can.

Here's a fun one I put together. Now, please bear in mind this is not very efficient, especially for simple replacements. However, it was fun to write and lends itself to a fairly readable usage pattern. It also highlights the little known fact that String implements IEnumerable.
public static class LinqToStrings
{
public static IQueryable<char> LinqReplace(this string source, int startIndex, int length, string replacement)
{
var querySource = source.AsQueryable();
return querySource.LinqReplace(startIndex, length, replacement);
}
public static IQueryable<char> LinqReplace(this IQueryable<char> source, int startIndex, int length, string replacement)
{
var querySource = source.AsQueryable();
return querySource.Take(startIndex).Concat(replacement).Concat(querySource.Skip(startIndex + length));
}
public static string AsString(this IQueryable<char> source)
{
return new string(source.ToArray());
}
}
And here's some example usage:
public void test()
{
var test = "test";
Console.WriteLine("Old: " + test);
Console.WriteLine("New: " + test.LinqReplace(0, 4, "SOMEPIG")
.LinqReplace(4, 0, "terrific")
.AsString());
}
Outputs:
Old: test
New: SOMEterrificPIG
Another version of that same approach, which is not so horrifically slow is straightforward using Substring:
public static string ReplaceAt(this string source, int startIndex, int length, string replacement)
{
return source.Substring(0, startIndex) + replacement + source.Substring(startIndex + length);
}
And in a wonderful example of why you should profile your code, and why you probably should not use my LinqToStrings implementation in production code, here's a timing test:
Console.WriteLine("Using LinqToStrings: " + new Stopwatch().Time(() => "test".LinqReplace(0, 4, "SOMEPIG").LinqReplace(4, 0, "terrific").AsString(), 1000));
Console.WriteLine("Using Substrings: " + new Stopwatch().Time(() => "test".ReplaceAt(0, 4, "SOMEPIG").ReplaceAt(4, 0, "terrific"), 1000));
Which measures timer ticks in 1,000 iterations, producing this output:
Using LinqToStrings: 3,818,953
Using Substrings: 1,157

Try using string.replace
http://msdn.microsoft.com/en-us/library/fk49wtc1.aspx
Probably your cleanest, easiest way.

Extract only right most n letters from a string

How can I extract a substring which is composed of the rightmost six letters from another string?
Ex: my string is "PER 343573". Now I want to extract only "343573".
How can I do this?

string SubString = MyString.Substring(MyString.Length-6);

Write an extension method to express the Right(n); function. The function should deal with null or empty strings returning an empty string, strings shorter than the max length returning the original string and strings longer than the max length returning the max length of rightmost characters.
public static string Right(this string sValue, int iMaxLength)
{
//Check if the value is valid
if (string.IsNullOrEmpty(sValue))
{
//Set valid empty string as string could be null
sValue = string.Empty;
}
else if (sValue.Length > iMaxLength)
{
//Make the string no longer than the max length
sValue = sValue.Substring(sValue.Length - iMaxLength, iMaxLength);
}
//Return the string
return sValue;
}

Probably nicer to use an extension method:
public static class StringExtensions
{
public static string Right(this string str, int length)
{
return str.Substring(str.Length - length, length);
}
}
Usage
string myStr = "PER 343573";
string subStr = myStr.Right(6);

using System;
public static class DataTypeExtensions
{
#region Methods
public static string Left(this string str, int length)
{
str = (str ?? string.Empty);
return str.Substring(0, Math.Min(length, str.Length));
}
public static string Right(this string str, int length)
{
str = (str ?? string.Empty);
return (str.Length >= length)
? str.Substring(str.Length - length, length)
: str;
}
#endregion
}
Shouldn't error, returns nulls as empty string, returns trimmed or base values. Use it like "testx".Left(4) or str.Right(12);

MSDN
String mystr = "PER 343573";
String number = mystr.Substring(mystr.Length-6);
EDIT: too slow...

if you are not sure of the length of your string, but you are sure of the words count (always 2 words in this case, like 'xxx yyyyyy') you'd better use split.
string Result = "PER 343573".Split(" ")[1];
this always returns the second word of your string.

This isn't exactly what you are asking for, but just looking at the example, it appears that you are looking for the numeric section of the string.
If this is always the case, then a good way to do it would be using a regular expression.
var regex= new Regex("\n+");
string numberString = regex.Match(page).Value;

Since you are using .NET, which all compiles to MSIL, just reference Microsoft.VisualBasic, and use Microsoft's built-in Strings.Right method:
using Microsoft.VisualBasic;
...
string input = "PER 343573";
string output = Strings.Right(input, 6);
No need to create a custom extension method or other work. The result is achieved with one reference and one simple line of code.
As further info on this, using Visual Basic methods with C# has been documented elsewhere. I personally stumbled on it first when trying to parse a file, and found this SO thread on using the Microsoft.VisualBasic.FileIO.TextFieldParser class to be extremely useful for parsing .csv files.

Use this:
String text = "PER 343573";
String numbers = text;
if (text.Length > 6)
{
numbers = text.Substring(text.Length - 6);
}

Guessing at your requirements but the following regular expression will yield only on 6 alphanumerics before the end of the string and no match otherwise.
string result = Regex.Match("PER 343573", #"[a-zA-Z\d]{6}$").Value;

var str = "PER 343573";
var right6 = string.IsNullOrWhiteSpace(str) ? string.Empty
: str.Length < 6 ? str
: str.Substring(str.Length - 6); // "343573"
// alternative
var alt_right6 = new string(str.Reverse().Take(6).Reverse().ToArray()); // "343573"
this supports any number of character in the str. the alternative code not support null string. and, the first is faster and the second is more compact.
i prefer the second one if knowing the str containing short string. if it's long string the first one is more suitable.
e.g.
var str = "";
var right6 = string.IsNullOrWhiteSpace(str) ? string.Empty
: str.Length < 6 ? str
: str.Substring(str.Length - 6); // ""
// alternative
var alt_right6 = new string(str.Reverse().Take(6).Reverse().ToArray()); // ""
or
var str = "123";
var right6 = string.IsNullOrWhiteSpace(str) ? string.Empty
: str.Length < 6 ? str
: str.Substring(str.Length - 6); // "123"
// alternative
var alt_right6 = new string(str.Reverse().Take(6).Reverse().ToArray()); // "123"

Another solution that may not be mentioned
S.Substring(S.Length < 6 ? 0 : S.Length - 6)

Use this:
string mystr = "PER 343573";
int number = Convert.ToInt32(mystr.Replace("PER ",""));

Null Safe Methods :
Strings shorter than the max length returning the original string
String Right Extension Method
public static string Right(this string input, int count) =>
String.Join("", (input + "").ToCharArray().Reverse().Take(count).Reverse());
String Left Extension Method
public static string Left(this string input, int count) =>
String.Join("", (input + "").ToCharArray().Take(count));

Here's the solution I use...
It checks that the input string's length isn't lower than the asked length. The solutions I see posted above don't take this into account unfortunately - which can lead to crashes.
/// <summary>
/// Gets the last x-<paramref name="amount"/> of characters from the given string.
/// If the given string's length is smaller than the requested <see cref="amount"/> the full string is returned.
/// If the given <paramref name="amount"/> is negative, an empty string will be returned.
/// </summary>
/// <param name="string">The string from which to extract the last x-<paramref name="amount"/> of characters.</param>
/// <param name="amount">The amount of characters to return.</param>
/// <returns>The last x-<paramref name="amount"/> of characters from the given string.</returns>
public static string GetLast(this string #string, int amount)
{
if (#string == null) {
return #string;
}
if (amount < 0) {
return String.Empty;
}
if (amount >= #string.Length) {
return #string;
} else {
return #string.Substring(#string.Length - amount);
}
}

This is the method I use: I like to keep things simple.
private string TakeLast(string input, int num)
{
if (num > input.Length)
{
num = input.Length;
}
return input.Substring(input.Length - num);
}

//s - your string
//n - maximum number of right characters to take at the end of string
(new Regex("^.*?(.{1,n})$")).Replace(s,"$1")

Just a thought:
public static string Right(this string #this, int length) {
return #this.Substring(Math.Max(#this.Length - length, 0));
}

Without resorting to the bit converter and bit shifting (need to be sure of encoding)
this is fastest method I use as an extension method 'Right'.
string myString = "123456789123456789";
if (myString > 6)
{
char[] cString = myString.ToCharArray();
Array.Reverse(myString);
Array.Resize(ref myString, 6);
Array.Reverse(myString);
string val = new string(myString);
}

I use the Min to prevent the negative situations and also handle null strings
// <summary>
/// Returns a string containing a specified number of characters from the right side of a string.
/// </summary>
public static string Right(this string value, int length)
{
string result = value;
if (value != null)
result = value.Substring(0, Math.Min(value.Length, length));
return result;
}

using Microsoft.visualBasic;
public class test{
public void main(){
string randomString = "Random Word";
print (Strings.right(randomString,4));
}
}
output is "Word"

//Last word of string :: -> sentence
var str ="A random sentence";
var lword = str.Substring(str.LastIndexOf(' ') + 1);
//Last 6 chars of string :: -> ntence
var n = 6;
var right = str.Length >n ? str.Substring(str.Length - n) : "";

How do I replace the *first instance* of a string in .NET?

I want to replace the first occurrence in a given string.
How can I accomplish this in .NET?

string ReplaceFirst(string text, string search, string replace)
{
int pos = text.IndexOf(search);
if (pos < 0)
{
return text;
}
return text.Substring(0, pos) + replace + text.Substring(pos + search.Length);
}
Example:
string str = "The brown brown fox jumps over the lazy dog";
str = ReplaceFirst(str, "brown", "quick");
EDIT: As #itsmatt mentioned, there's also Regex.Replace(String, String, Int32), which can do the same, but is probably more expensive at runtime, since it's utilizing a full featured parser where my method does one find and three string concatenations.
EDIT2: If this is a common task, you might want to make the method an extension method:
public static class StringExtension
{
public static string ReplaceFirst(this string text, string search, string replace)
{
// ...same as above...
}
}
Using the above example it's now possible to write:
str = str.ReplaceFirst("brown", "quick");

As itsmatt said Regex.Replace is a good choice for this however to make his answer more complete I will fill it in with a code sample:
using System.Text.RegularExpressions;
...
Regex regex = new Regex("foo");
string result = regex.Replace("foo1 foo2 foo3 foo4", "bar", 1);
// result = "bar1 foo2 foo3 foo4"
The third parameter, set to 1 in this case, is the number of occurrences of the regex pattern that you want to replace in the input string from the beginning of the string.
I was hoping this could be done with a static Regex.Replace overload but unfortunately it appears you need a Regex instance to accomplish it.

Take a look at Regex.Replace.

using System.Text.RegularExpressions;
RegEx MyRegEx = new RegEx("F");
string result = MyRegex.Replace(InputString, "R", 1);
will find first F in InputString and replace it with R.

Taking the "first only" into account, perhaps:
int index = input.IndexOf("AA");
if (index >= 0) output = input.Substring(0, index) + "XQ" +
input.Substring(index + 2);
?
Or more generally:
public static string ReplaceFirstInstance(this string source,
string find, string replace)
{
int index = source.IndexOf(find);
return index < 0 ? source : source.Substring(0, index) + replace +
source.Substring(index + find.Length);
}
Then:
string output = input.ReplaceFirstInstance("AA", "XQ");

C# extension method that will do this:
public static class StringExt
{
public static string ReplaceFirstOccurrence(this string s, string oldValue, string newValue)
{
int i = s.IndexOf(oldValue);
return s.Remove(i, oldValue.Length).Insert(i, newValue);
}
}

In C# syntax:
int loc = original.IndexOf(oldValue);
if( loc < 0 ) {
return original;
}
return original.Remove(loc, oldValue.Length).Insert(loc, newValue);

Assumes that AA only needs to be replaced if it is at the very start of the string:
var newString;
if(myString.StartsWith("AA"))
{
newString ="XQ" + myString.Substring(2);
}
If you need to replace the first occurrence of AA, whether the string starts with it or not, go with the solution from Marc.

And because there is also VB.NET to consider, I would like to offer up:
Private Function ReplaceFirst(ByVal text As String, ByVal search As String, ByVal replace As String) As String
Dim pos As Integer = text.IndexOf(search)
If pos >= 0 Then
Return text.Substring(0, pos) + replace + text.Substring(pos + search.Length)
End If
Return text
End Function

One of the overloads of Regex.Replace takes an int for "The maximum number of times the replacement can occur". Obviously, using Regex.Replace for plain text replacement may seem like overkill, but it's certainly concise:
string output = (new Regex("AA")).Replace(input, "XQ", 1);

For anyone that doesn't mind a reference to Microsoft.VisualBasic, there is the Replace Method:
string result = Microsoft.VisualBasic.Strings.Replace("111", "1", "0", 2, 1); // "101"

This example abstracts away the substrings (but is slower), but is probably much fast than a RegEx:
var parts = contents.ToString().Split(new string[] { "needle" }, 2, StringSplitOptions.None);
return parts[0] + "replacement" + parts[1];

Updated extension method utilizing Span to minimize new string creation
public static string ReplaceFirstOccurrence(this string source, string search, string replace) {
int index = source.IndexOf(search);
if (index < 0) return source;
var sourceSpan = source.AsSpan();
return string.Concat(sourceSpan.Slice(0, index), replace, sourceSpan.Slice(index + search.Length));
}

With ranges and C# 10 we can do:
public static string ReplaceFirst(this string text, string search, string replace)
{
int pos = text.IndexOf(search, StringComparison.Ordinal);
return pos < 0 ? text : string.Concat(text[..pos], replace, text.AsSpan(pos + search.Length));
}

string abc = "AAAAX1";
if(abc.IndexOf("AA") == 0)
{
abc.Remove(0, 2);
abc = "XQ" + abc;
}