Develop Reference

Method that takes a message and index, creates a substring using the index

Problem: I want to write a method that takes a message/index pair like this:
("Hello, I am *Name1, how are you doing *Name2?", 2)
The index refers to the asterisk delimited name in the message. So if the index is 1, it should refer to *Name1, if it's 2 it should refer to *Name2.
The method should return just the name with the asterisk (*Name2).
I have attempted to play around with substrings, taking the first delimited * and ending when we reach a character that isn't a letter, number, underscore or hyphen, but the logic just isn't setting in.
I know this is similar to a few problems on SO but I can't find anything this specific. Any help is appreciated.
This is what's left of my very vague attempt so far. Based on this thread:
public string GetIndexedNames(string message, int index)
{
int strStart = message.IndexOf("#") + "#".Length;
int strEnd = message.LastIndexOf(" ");
String result = message.Substring(strStart, strEnd - strStart);
}

If you want to do it the old school way, then something like:
public static void Main(string[] args)
{
string message = "Hello, I am *Name1, how are you doing *Name2?";
string name1 = GetIndexedNames(message, "*", 1);
string name2 = GetIndexedNames(message, "*", 2);
Console.WriteLine(message);
Console.WriteLine(name1);
Console.WriteLine(name2);
Console.ReadLine();
}
public static string GetIndexedNames(string message, string singleCharDelimiter, int index)
{
string valid = "abcdefghijlmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_-";
string[] parts = message.Split(singleCharDelimiter.ToArray());
if (parts.Length >= index)
{
StringBuilder sb = new StringBuilder();
for(int i = 0; i < parts[index].Length; i++)
{
string character = parts[index].Substring(i, 1);
if (valid.Contains(character))
{
sb.Append(character);
}
else
{
return sb.ToString();
}
}
return sb.ToString();
}
return "";
}

You can try using regular expressions to match the names. Assuming that name is a sequence of word characters (letters or digits):
using System.Linq;
using System.Text.RegularExpressions;
...
// Either name with asterisk *Name or null
// index is 1-based
private static ObtainName(string source, int index) => Regex
.Matches(source, #"\*\w+")
.Cast<Match>()
.Select(match => match.Value)
.Distinct() // in case the same name repeats several times
.ElementAtOrDefault(index - 1);
Demo:
string name = ObtainName(
"Hello, I am *Name1, how are you doing *Name2?", 2);
Console.Write(name);
Outcome:
*Name2

Perhaps not the most elegant solution, but if you want to use IndexOf, use a loop:
public static string GetIndexedNames(string message, int index, char marker='*')
{
int lastFound = 0;
for (int i = 0; i < index; i++) {
lastFound = message.IndexOf(marker, lastFound+1);
if (lastFound == -1) return null;
}
var space = message.IndexOf(' ', lastFound);
return space == -1 ? message.Substring(lastFound) : message.Substring(lastFound, space - lastFound);
}

Get string between two strings in a string

I have a string like:
"super example of string key : text I want to keep - end of my string"
I want to just keep the string which is between "key : " and " - ". How can I do that? Must I use a Regex or can I do it in another way?

Perhaps, a good way is just to cut out a substring:
String St = "super exemple of string key : text I want to keep - end of my string";
int pFrom = St.IndexOf("key : ") + "key : ".Length;
int pTo = St.LastIndexOf(" - ");
String result = St.Substring(pFrom, pTo - pFrom);

string input = "super exemple of string key : text I want to keep - end of my string";
var match = Regex.Match(input, #"key : (.+?)-").Groups[1].Value;
or with just string operations
var start = input.IndexOf("key : ") + 6;
var match2 = input.Substring(start, input.IndexOf("-") - start);

You can do it without regex
input.Split(new string[] {"key :"},StringSplitOptions.None)[1]
.Split('-')[0]
.Trim();

Here is the way how i can do that
public string Between(string STR , string FirstString, string LastString)
{
string FinalString;
int Pos1 = STR.IndexOf(FirstString) + FirstString.Length;
int Pos2 = STR.IndexOf(LastString);
FinalString = STR.Substring(Pos1, Pos2 - Pos1);
return FinalString;
}

Depending on how robust/flexible you want your implementation to be, this can actually be a bit tricky. Here's the implementation I use:
public static class StringExtensions {
/// <summary>
/// takes a substring between two anchor strings (or the end of the string if that anchor is null)
/// </summary>
/// <param name="this">a string</param>
/// <param name="from">an optional string to search after</param>
/// <param name="until">an optional string to search before</param>
/// <param name="comparison">an optional comparison for the search</param>
/// <returns>a substring based on the search</returns>
public static string Substring(this string #this, string from = null, string until = null, StringComparison comparison = StringComparison.InvariantCulture)
{
var fromLength = (from ?? string.Empty).Length;
var startIndex = !string.IsNullOrEmpty(from)
? #this.IndexOf(from, comparison) + fromLength
: 0;
if (startIndex < fromLength) { throw new ArgumentException("from: Failed to find an instance of the first anchor"); }
var endIndex = !string.IsNullOrEmpty(until)
? #this.IndexOf(until, startIndex, comparison)
: #this.Length;
if (endIndex < 0) { throw new ArgumentException("until: Failed to find an instance of the last anchor"); }
var subString = #this.Substring(startIndex, endIndex - startIndex);
return subString;
}
}
// usage:
var between = "a - to keep x more stuff".Substring(from: "-", until: "x");
// returns " to keep "

I think this works:
static void Main(string[] args)
{
String text = "One=1,Two=2,ThreeFour=34";
Console.WriteLine(betweenStrings(text, "One=", ",")); // 1
Console.WriteLine(betweenStrings(text, "Two=", ",")); // 2
Console.WriteLine(betweenStrings(text, "ThreeFour=", "")); // 34
Console.ReadKey();
}
public static String betweenStrings(String text, String start, String end)
{
int p1 = text.IndexOf(start) + start.Length;
int p2 = text.IndexOf(end, p1);
if (end == "") return (text.Substring(p1));
else return text.Substring(p1, p2 - p1);
}

Regex is overkill here.
You could use string.Split with the overload that takes a string[] for the delimiters but that would also be overkill.
Look at Substring and IndexOf - the former to get parts of a string given and index and a length and the second for finding indexed of inner strings/characters.

A working LINQ solution:
string str = "super example of string key : text I want to keep - end of my string";
string res = new string(str.SkipWhile(c => c != ':')
.Skip(1)
.TakeWhile(c => c != '-')
.ToArray()).Trim();
Console.WriteLine(res); // text I want to keep

In C# 8.0 and above, you can use the range operator .. as in
var s = "header-THE_TARGET_STRING.7z";
var from = s.IndexOf("-") + "-".Length;
var to = s.IndexOf(".7z");
var versionString = s[from..to]; // THE_TARGET_STRING
See documentation for details.

string str="super exemple of string key : text I want to keep - end of my string";
int startIndex = str.IndexOf("key") + "key".Length;
int endIndex = str.IndexOf("-");
string newString = str.Substring(startIndex, endIndex - startIndex);

or, with a regex.
using System.Text.RegularExpressions;
...
var value =
Regex.Match(
"super exemple of string key : text I want to keep - end of my string",
"key : (.*) - ")
.Groups[1].Value;
with a running example.
You can decide if its overkill.
or
as an under validated extension method
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
var value =
"super exemple of string key : text I want to keep - end of my string"
.Between(
"key : ",
" - ");
Console.WriteLine(value);
}
}
public static class Ext
{
static string Between(this string source, string left, string right)
{
return Regex.Match(
source,
string.Format("{0}(.*){1}", left, right))
.Groups[1].Value;
}
}

Since the : and the - are unique you could use:
string input;
string output;
input = "super example of string key : text I want to keep - end of my string";
output = input.Split(new char[] { ':', '-' })[1];

I used the code snippet from Vijay Singh Rana which basically does the job. But it causes problems if the firstString does already contain the lastString. What I wanted was extracting a access_token from a JSON Response (no JSON Parser loaded). My firstString was \"access_token\": \" and my lastString was \". I ended up with a little modification
string Between(string str, string firstString, string lastString)
{
int pos1 = str.IndexOf(firstString) + firstString.Length;
int pos2 = str.Substring(pos1).IndexOf(lastString);
return str.Substring(pos1, pos2);
}

You can use the extension method below:
public static string GetStringBetween(this string token, string first, string second)
{
if (!token.Contains(first)) return "";
var afterFirst = token.Split(new[] { first }, StringSplitOptions.None)[1];
if (!afterFirst.Contains(second)) return "";
var result = afterFirst.Split(new[] { second }, StringSplitOptions.None)[0];
return result;
}
Usage is:
var token = "super exemple of string key : text I want to keep - end of my string";
var keyValue = token.GetStringBetween("key : ", " - ");

var matches = Regex.Matches(input, #"(?<=key :)(.+?)(?=-)");
This returns only the value(s) between "key :" and the following occurance of "-"

If you are looking for a 1 line solution, this is it:
s.Substring(s.IndexOf("eT") + "eT".Length).Split("97".ToCharArray()).First()
The whole 1 line solution, with System.Linq:
using System;
using System.Linq;
class OneLiner
{
static void Main()
{
string s = "TextHereTisImortant973End"; //Between "eT" and "97"
Console.WriteLine(s.Substring(s.IndexOf("eT") + "eT".Length)
.Split("97".ToCharArray()).First());
}
}

private string gettxtbettwen(string txt, string first, string last)
{
StringBuilder sb = new StringBuilder(txt);
int pos1 = txt.IndexOf(first) + first.Length;
int len = (txt.Length ) - pos1;
string reminder = txt.Substring(pos1, len);
int pos2 = reminder.IndexOf(last) - last.Length +1;
return reminder.Substring(0, pos2);
}

When questions are stated in terms of a single example ambiguities are inevitably present. This question is no exception.
For the example given in the question the desired string is clear:
super example of string key : text I want to keep - end of my string
^^^^^^^^^^^^^^^^^^^
However, this string is but an example of strings and boundary strings for which certain substrings are to be identified. I will consider a generic string with generic boundary strings, represented as follows.
abc FF def PP ghi,PP jkl,FF mno PP pqr FF,stu FF vwx,PP yza
^^^^^^^^^^^^ ^^^^^
PP is the preceding string, FF is the following string and the party hats indicate which substrings are to be matched. (In the example given in the question key : is the preceding string and - is the following string.) I have assumed that PP and FF are preceded and followed by word boundaries (so that PPA and FF8 are not matched).
My assumptions, as reflected by the party hats, are as follows:
The first substring PP may be preceded by one (or more) FF substrings, which, if present, are disregarded;
If PP is followed by one or more PPs before FF is encountered, the following PPs are part of the substring between the preceding and following strings;
If PP is followed by one or more FFs before a PP is encounter, the first FF following PP is considered to be the following string.
Note that many of the answers here deal with only strings of the form
abc PP def FF ghi
^^^^^
or
abc PP def FF ghi PP jkl FF mno
^^^^^ ^^^^^
One may use a regular expression, code constructs, or a combination of the two to identify the substrings of interest. I make no judgement as to which approach is best. I will only present the following regular expression that will match the substrings of interest.
(?<=\bPP\b)(?:(?!\bFF\b).)*(?=\bFF\b)
Start your engine!1
I tested this with the PCRE (PHP) regex engine, but as the regex is not at all exotic, I am sure it will work with the .NET regex engine (which is very robust).
The regex engine performs the following operations:
(?<= : begin a positive lookbehind
\bPP\b : match 'PP'
) : end positive lookbehind
(?: : begin a non-capture group
(?! : begin a negative lookahead
\bFF\b : match 'FF'
) : end negative lookahead
. : match any character
) : end non-capture group
* : execute non-capture group 0+ times
(?= : begin positive lookahead
\bFF\b : match 'FF'
) : end positive lookahead
This technique, of matching one character at a time, following the preceding string, until the character is F and is followed by F (or more generally, the character beings the string that constitutes the following string), is called Tempered Greedy Token Solution.
Naturally, the regex would have to be modified (if possible) if the assumptions I set out above are changed.
1. Move the cursor around for detailed explanations.

If you want to handle multiple occurrences of substring pairs, it won't be easy without RegEx:
Regex.Matches(input ?? String.Empty, "(?=key : )(.*)(?<= - )", RegexOptions.Singleline);
input ?? String.Empty avoids argument null exception
?= keeps 1st substring and ?<= keeps 2nd substring
RegexOptions.Singleline allows newline between substring pair
If order & occurrence count of substrings doesn't matter, this quick & dirty one may be an option:
var parts = input?.Split(new string[] { "key : ", " - " }, StringSplitOptions.None);
string result = parts?.Length >= 3 ? result[1] : input;
At least it avoids most exceptions, by returning the original string if none/single substring match.

For get string between string's, I'm using this method:
public static class Extension
{
/// <summary>
/// Gets currently string between
/// </summary>
/// <param name="word">Currently string</param>
/// <param name="start">String left</param>
/// <param name="end">String right</param>
/// <returns>String between start and end</returns>
/// <example>The string "value (4815162342)" use Between("(",")") generates in method: "4815162342"</example>
public static string Between(this string word, string start, string end)
{
if (start.Equals(end))
throw new ArgumentException("Start string can't equals a end string.");
int startIndex = word.LastIndexOf(start) + 1;
int endIndex = word.LastIndexOf(end) - 1 - word.LastIndexOf(start);
return word.Substring(startIndex, endIndex);
}
}

You already have some good answers and I realize the code I am providing is far from the most efficient and clean. However, I thought it might be useful for educational purposes. We can use pre-built classes and libraries all day long. But without understanding the inner-workings, we are simply mimicking and repeating and will never learn anything. This code works and is more basic or "virgin" than some of the others:
char startDelimiter = ':';
char endDelimiter = '-';
Boolean collect = false;
string parsedString = "";
foreach (char c in originalString)
{
if (c == startDelimiter)
collect = true;
if (c == endDelimiter)
collect = false;
if (collect == true && c != startDelimiter)
parsedString += c;
}
You end up with your desired string assigned to the parsedString variable. Keep in mind that it will also capture proceeding and preceding spaces. Remember that a string is simply an array of characters that can be manipulated like other arrays with indices etc.
Take care.

As I always say nothing is impossible:
string value = "super exemple of string key : text I want to keep - end of my string";
Regex regex = new Regex(#"(key \: (.*?) _ )");
Match match = regex.Match(value);
if (match.Success)
{
Messagebox.Show(match.Value);
}
Remeber that should add reference of System.Text.RegularExpressions
Hope That I Helped.

Something like this perhaps
private static string Between(string text, string from, string to)
{
return text[(text.IndexOf(from)+from.Length)..text.IndexOf(to, text.IndexOf(from))];
}

getStringBetween(startStr, endStr, fullStr) {
string startIndex = fullStr.indexOf(startStr);
string endIndex= fullStr.indexOf(endStr);
return fullStr.substring(startIndex + startStr.length, endIndex);
}

Here it is;
/// <summary>
///
/// </summary>
/// <param name="line"></param>
/// <param name="begin_tag"></param>
/// <param name="end_tag"></param>
/// <param name="lastIndexOfEndTag"></param>
/// <returns></returns>
private string getBetween(string line, string begin_tag, string end_tag, bool lastIndexOfEndTag = false, bool returnNullIfTagsNotExists = false)
{
if (!string.IsNullOrEmpty(line) && !string.IsNullOrEmpty(begin_tag) && !string.IsNullOrEmpty(end_tag))
{
// 1 2 3 4 5 6 7
//0123456789012345678901234567890123456789012345678901234567890123456789012
//StdErrorData: Duration: 01:59:54.88, start: 0.000000, bitrate: 557 kb/s
int startIndex = line.IndexOf(begin_tag);
if (startIndex >= 0)
{
startIndex += begin_tag.Length;
}
else
{
if (returnNullIfTagsNotExists)
{
return null;
}
else
{
startIndex = 0;
}
}
int endIndex = lastIndexOfEndTag ?
line.LastIndexOf(end_tag, startIndex)
: line.IndexOf(end_tag, startIndex);
if (endIndex > startIndex)
{
return line.Substring(startIndex, endIndex - startIndex);
}
else
{
if (returnNullIfTagsNotExists)
{
return null;
}
else
{
return line.Substring(startIndex);
}
}
}
return null;
}
Test;
string r = getBetween("StdErrorData: Duration: 01:59:54.88, start: 0.000000, bitrate: 557 kb/s", "Duration:", ",");
Console.WriteLine($"<{r}>");
//< 01:59:54.88>

Here is the extension method in case anyone interested in keeping the start and end text as well.
public static string SubstringBetween(this string text, string start, string end, bool keepStartEndText = false)
{
var startIndex = text.IndexOf(start);
var endIndex = text.LastIndexOf(end);
if (keepStartEndText)
return text.Substring(startIndex, (endIndex + end.Length) - startIndex);
else
return text.Substring(startIndex + start.Length, endIndex - (startIndex + start.Length));
}

public static string ExtractBetweenTwoStrings(string FullText, string StartString, string EndString, bool IncludeStartString, bool IncludeEndString)
{
try { int Pos1 = FullText.IndexOf(StartString) + StartString.Length; int Pos2 = FullText.IndexOf(EndString, Pos1); return ((IncludeStartString) ? StartString : "")
+ FullText.Substring(Pos1, Pos2 - Pos1) + ((IncludeEndString) ? EndString : ""); } catch (Exception ex) { return ex.ToString(); } //return ""; }
}
credit to: https://www.c-sharpcorner.com/blogs/how-to-extract-a-string-lies-between-two-strings-in-c-sharpnet1

the easiest way to get number from string

i want to retrive some number from string, for example "valueID = 234232" or "valueID = 2" or "valueID=23" or "valueID= 2" or "valueID =234", so what is the easiest way to get this number assuming that after this number is space " " or end of line?

string input = "valueID = 234232";
var split = input.Split(new char[] {'='});
int value = int.Parse(split[1].Trim());

As long as your sure there will be a number in there
int.parse( Regex.match(String, #"\d+").value)

How about using a regex:
Match match = Regex.Match(input, #"\d+");
if (match.Success)
{
number = int.Parse(match.Value);
}

Here's some flexible code I use all the time for this sort of thing:
public const string FilterDigits = "0123456789";
public const string FilterLetters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
/// <summary>
/// Strips away any characters not defined in validChars and returns the result.
/// </summary>
public static string FilterTo(string text, string validChars)
{
StringBuilder Result = new StringBuilder();
for (int i = 1; i <= text.Length; i++) {
char CurrentChar = Convert.ToChar(Strings.Mid(text, i, 1));
if (validChars.Contains(CurrentChar)) {
Result.Append(CurrentChar);
}
}
return Result.ToString;
}
Then you can just call:
int.parse(FilterTo("valueID=23", FilterDigits));
It could also be turned into a handy extension method, then you could do:
int.parse("valueID=23".FilterTo(FilterDigits));

Once you've parsed out your potential number (through regex, split, substr, etc), TryParse is a great improvement (since .NET 3.5 at least):
string myPotentialNumber = "1";
int myNumber;
bool isNumber = int.TryParse(myPotentialNumber, out myNumber);
if( isNumber )
{
//do something
}
else
{
//throw error
}
TryParse() is exception safe, which makes it a bit faster and cleaner for many types of conversions.

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;
}