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Get substring between first occurance of "A" and last occurance of "B"

I have a string which has two tokens that bound a substring that I want to extract, but the substring may contain the tokens themselves, so I want between the 1st occurrence of token A and the last occurrence of token B. I also need to search for the tokens in a case-insensitive search.
Tried to wrap my head around using regex to get this, but can't seem to figure it out. Not sure the best approach here. String.split won't work.
I can't modify the casing of the data between the tokens in the string.

Try this, (I've made it into an extension method)
public static string Between(this string value, string a, string b)
{
int posA = value.IndexOf(a);
int posB = value.LastIndexOf(b);
if (posA == -1) || (posB == -1)
{
return "";
}
int adjustedPosA = posA + a.Length;
return (adjustedPosA >= posB) ? "" : value.Substring(adjustedPosA, posB - adjustedPosA);
}
Usage would be:
var myString = "hereIsAToken_andThisIsWhatIwant_andSomeOtherToken";
var whatINeed = myString.Between("hereIsAToken_", "_andSomeOtherToken");

An easy way to approach this problem is the use of the indexOf function provided by the string class. IndexOf returns the first occurence, lastIndexOf as the name suggests, the last one.
string data;
string token1;
string token2;
int start = data.IndexOf(token1)+token1.Length;
int end = data.LastIndexOf(token2);
string result = data.Substring(start, end-start);
From my personal point of view, regex might be a bit overkill here, just try my example :)

Substring, if the string is not long enough replace chars with spaces

I'm trying to compare first 3 chars of a string, i'm trying to use substring then compare.
The strings are read from an input file, and the string may not be 3 chars long. if an string is not 3 chars long i want the substring method to replace the empty chars with spaces.
How would i go about doing that.
Current code throws an exeption when the string is not long enough.

Use String.PadRight
myString.PadRight(3, ' ');
// do SubString here..
You could also create a .Left extension method that doesn't throw an exception when the string isn't big enough:
public static string Left(this string s, int len)
{
if (len == 0 || s.Length == 0)
return "";
else if (s.Length <= len)
return s;
else
return s.Substring(0, len);
}
Usage:
myString.Left(3);

Use one of the String.PadRight() methods before calling Substring():
string subString = myString.PadRight(3).Substring(0,3);
If you use the overload with one parameter like I did above, it will insert spaces.
string subString1 = string1.PadRight(3).Substring(0,3);
string subString2 = string2.PadRight(3).Substring(0,3);
if (String.Compare(subString1, subString2) == 0)
{
// if equal
}
else
{
// not equal
}
I used separate variables because it's a bit more readable, but you could in-line them in the if statement if you wanted to.

You can use this dirty hack:
var res = (myStr+" ").Substring(...);

How can I truncate my strings with a "..." if they are too long?

Hope somebody has a good idea. I have strings like this:
abcdefg
abcde
abc
What I need is for them to be trucated to show like this if more than a specified lenght:
abc ..
abc ..
abc
Is there any simple C# code I can use for this?

Here is the logic wrapped up in an extension method:
public static string Truncate(this string value, int maxChars)
{
return value.Length <= maxChars ? value : value.Substring(0, maxChars) + "...";
}
Usage:
var s = "abcdefg";
Console.WriteLine(s.Truncate(3));

All very good answers, but to clean it up just a little, if your strings are sentences, don't break your string in the middle of a word.
private string TruncateForDisplay(this string value, int length)
{
if (string.IsNullOrEmpty(value)) return string.Empty;
var returnValue = value;
if (value.Length > length)
{
var tmp = value.Substring(0, length) ;
if (tmp.LastIndexOf(' ') > 0)
returnValue = tmp.Substring(0, tmp.LastIndexOf(' ') ) + " ...";
}
return returnValue;
}

public string TruncString(string myStr, int THRESHOLD)
{
if (myStr.Length > THRESHOLD)
return myStr.Substring(0, THRESHOLD) + "...";
return myStr;
}
Ignore the naming convention it's just in case he actually needs the THRESHOLD variable or if it's always the same size.
Alternatively
string res = (myStr.Length > THRESHOLD) ? myStr.Substring(0, THRESHOLD) + ".." : myStr;

Here's a version that accounts for the length of the ellipses:
public static string Truncate(this string value, int maxChars)
{
const string ellipses = "...";
return value.Length <= maxChars ? value : value.Substring(0, maxChars - ellipses.Length) + ellipses;
}

There isn't a built in method in the .NET Framework which does this, however this is a very easy method to write yourself. Here are the steps, try making it yourself and let us know what you come up with.
Create a method, perhaps an extension method public static void TruncateWithEllipsis(this string value, int maxLength)
Check to see if the passed in value is greater than the maxLength specified using the Length property. If the value not greater than maxLength, just return the value.
If we didn't return the passed in value as is, then we know we need to truncate. So we need to get a smaller section of the string using the SubString method. That method will return a smaller section of a string based on a specified start and end value. The end position is what was passed in by the maxLength parameter, so use that.
Return the sub section of the string plus the ellipsis.
A great exercise for later would be to update the method and have it break only after full words. You can also create an overload to specify how you would like to show a string has been truncated. For example, the method could return " (click for more)" instead of "..." if your application is set up to show more detail by clicking.

Code behind:
string shorten(sting s)
{
//string s = abcdefg;
int tooLongInt = 3;
if (s.Length > tooLongInt)
return s.Substring(0, tooLongInt) + "..";
return s;
}
Markup:
<td><%= shorten(YOUR_STRING_HERE) %></td>

Maybe it is better to implement a method for that purpose:
string shorten(sting yourStr)
{
//Suppose you have a string yourStr, toView and a constant value
string toView;
const int maxView = 3;
if (yourStr.Length > maxView)
toView = yourStr.Substring(0, maxView) + " ..."; // all you have is to use Substring(int, int) .net method
else
toView = yourStr;
return toView;
}

I found this question after searching for "C# truncate ellipsis". Using various answers, I created my own solution with the following features:
An extension method
Add an ellipsis
Make the ellipsis optional
Validate that the string is not null or empty before attempting to truncate it.
public static class StringExtensions
{
public static string Truncate(this string value,
int maxLength,
bool addEllipsis = false)
{
// Check for valid string before attempting to truncate
if (string.IsNullOrEmpty(value)) return value;
// Proceed with truncating
var result = string.Empty;
if (value.Length > maxLength)
{
result = value.Substring(0, maxLength);
if (addEllipsis) result += "...";
}
else
{
result = value;
}
return result;
}
}
I hope this helps someone else.

string s = "abcdefg";
if (s.length > 3)
{
s = s.substring(0,3);
}
You can use the Substring function.

Refactor with new C# features just for disclosure:
Nullables (C# 7)
Expression-bodied members (C# 6 and 7)
Ranges on strings (C# 8)
// public static class StringExtensions { ...
private static string? Truncate(this string? value, int maxChars)
=>
string.IsNullOrEmpty(value) ? value :
value.Length <= maxChars ? value :
value[..maxChars] + "...";
Checked as "Community wiki", be free to improve answer.

Sure, here is some sample code:
string str = "abcdefg";
if (str.Length > X){
str = str.Substring(0, X) + "...";
}

I has this problem recently. I was storing a "status" message in a nvarcharMAX DB field which is 4000 characters. However my status messages were building up and hitting the exception.
But it wasn't a simple case of truncation as an arbitrary truncation would orphan part of a status message, so I really needed to "truncate" at a consistent part of the string.
I solved the problem by converting the string to a string array, removing the first element and then restoring to a string. Here is the code ("CurrentStatus" is the string holding the data)...
if (CurrentStatus.Length >= 3750)
{
// perform some truncation to free up some space.
// Lets get the status messages into an array for processing...
// We use the period as the delimiter, then skip the first item and re-insert into an array.
string[] statusArray = CurrentStatus.Split(new string[] { "." }, StringSplitOptions.None)
.Skip(1).ToArray();
// Next we return the data to a string and replace any escaped returns with proper one.
CurrentStatus = (string.Join(".", statusArray))
.Replace("\\r\\n", Environment.NewLine);
}
Hope it helps someone out.

string manipulation check and replace with fastest method

I have some strings like below:
string num1 = "D123_1";
string num2 = "D123_2";
string num3 = "D456_11";
string num4 = "D456_22";
string num5 = "D_123_D";
string num5 = "_D_123";
I want to make a function that will do the following actions:
1- Checks if given string DOES HAVE an Underscore in it, and this underscore should be after some Numbers and Follow with some numbers: in this case 'num5' and 'num6' are invalid!
2- Replace the numbers after the last underscore with any desired string, for example I want 'num1 = "D123_1"' to be changed into 'D123_2'
So far I came with this idea but it is not working :( First I dont know how to check for criteria 1 and second the replace statement is not working:
private string CheckAndReplace(string given, string toAdd)
{
var changedString = given.Split('_');
return changedString[changedString.Length - 1] + toAdd;
}
Any help and tips will be appriciated

What you are looking for is a regular expression. This is (mostly) from the top of my head. But it should easily point you in the right direction. The regular expression works fine.
public static Regex regex = new Regex("(?<character>[a-zA-Z]+)(?<major>\\d+)_(?<minor>\\d+)",RegexOptions.CultureInvariant | RegexOptions.Compiled);
Match m = regex.Match(InputText);
if (m.Succes)
{
var newValue = String.Format("{0}{1}_{2}"m.Groups["character"].Value, m.Groups["major"].Value, m.Groups["minor"].Value);
}

In your code you split the String into an array of strings and then access the wrong index of the array, so it isn't doing what you want.
Try working with a substring instead. Find the index of the last '_' and then get the substring:
private string CheckAndReplace(string given, string toAdd) {
int index = given.LastIndexOf('_')+1;
return given.Substring(0,index) + toAdd;
}
But before that check the validity of the string (see other answers). This code fragment will break when there's no '_' in the string.

You could use a regular expression (this is not a complete implementation, only a hint):
private string CheckAndReplace(string given, string toAdd)
{
Regex regex = new Regex("([A-Z]*[0-9]+_)[0-9]+");
if (regex.IsMatch(given))
{
return string.Concat(regex.Match(given).Groups[1].Value, toAdd);
}
else
{
... do something else
}
}

Use a good regular expression implementation. .NET has standard implementation of them

BestPractice - Transform first character of a string into lower case

I'd like to have a method that transforms the first character of a string into lower case.
My approaches:
1.
public static string ReplaceFirstCharacterToLowerVariant(string name)
{
return String.Format("{0}{1}", name.First().ToString().ToLowerInvariant(), name.Substring(1));
}
2.
public static IEnumerable<char> FirstLetterToLowerCase(string value)
{
var firstChar = (byte)value.First();
return string.Format("{0}{1}", (char)(firstChar + 32), value.Substring(1));
}
What would be your approach?

I would use simple concatenation:
Char.ToLowerInvariant(name[0]) + name.Substring(1)
The first solution is not optimized because string.Format is slow and you don't need it if you have a format that will never change. It also generates an extra string to covert the letter to lowercase, which is not needed.
The approach with "+ 32" is ugly / not maintainable as it requires knowledge of ASCII character value offsets. It will also generate incorrect output with Unicode data and ASCII symbol characters.

Depending on the situation, a little defensive programming might be desirable:
public static string FirstCharacterToLower(string str)
{
if (String.IsNullOrEmpty(str) || Char.IsLower(str, 0))
return str;
return Char.ToLowerInvariant(str[0]) + str.Substring(1);
}
The if statement also prevents a new string from being built if it's not going to be changed anyway. You might want to have the method fail on null input instead, and throw an ArgumentNullException.
As people have mentioned, using String.Format for this is overkill.

Just in case it helps anybody who happens to stumble across this answer.
I think this would be best as an extension method, then you can call it with yourString.FirstCharacterToLower();
public static class StringExtensions
{
public static string FirstCharacterToLower(this string str)
{
if (String.IsNullOrEmpty(str) || Char.IsLower(str, 0))
{
return str;
}
return Char.ToLowerInvariant(str[0]) + str.Substring(1);
}
}

The fastest solution I know without abusing c#:
public static string LowerCaseFirstLetter(string value)
{
if (value?.Length > 0)
{
var letters = value.ToCharArray();
letters[0] = char.ToLowerInvariant(letters[0]);
return new string(letters);
}
return value;
}

Mine is
if (!string.IsNullOrEmpty (val) && val.Length > 0)
{
return val[0].ToString().ToLowerInvariant() + val.Remove (0,1);
}

I like the accepted answer, but beside checking string.IsNullOrEmpty I would also check if Char.IsLower(name[1]) in case you are dealing with abbreviation. E.g. you would not want "AIDS" to become "aIDS".

If you care about performance I would go with
public static string FirstCharToLower(this string str)
=> string.Create(str.Length, str, (output, input) =>
{
input.CopyTo(output);
output[0] = char.ToLowerInvariant(input[0]);
});
I did some quick benchmarking and it seems to be at least twice as fast as the fastest solution posted here and 3.5 times faster than the worst one across multiple input lengths.
There is no input checking as it should be the responsibility of the caller. Allowing you to prepare your data in advance and do fast bulk processing not being slowed down by having branches in your hot path if you ever need it.

With range operator C# 8.0 or above you can do this:
Char.ToLowerInvariant(name[0]) + name[1..];

Combined a few and made it a chainable extension. Added short-circuit on whitespace and non-letter.
public static string FirstLower(this string input) =>
(!string.IsNullOrWhiteSpace(input) && input.Length > 0
&& char.IsLetter(input[0]) && !char.IsLower(input[0]))
? input[0].ToString().ToLowerInvariant() + input.Remove(0, 1) : input;

This is a little extension method using latest syntax and correct validations
public static class StringExtensions
{
public static string FirstCharToLower(this string input)
{
switch (input)
{
case null: throw new ArgumentNullException(nameof(input));
case "": throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input));
default: return input.First().ToString().ToLower() + input.Substring(1);
}
}
}

Use This:
string newName= name[0].ToString().ToLower() + name.Substring(1);

If you don't want to reference your string twice in your expression you could do this using System.Linq.
new string("Hello World".Select((c, i) => i == 0 ? char.ToLower(c) : c).ToArray())
That way if your string comes from a function, you don't have to store the result of that function.
new string(Console.ReadLine().Select((c, i) => i == 0 ? char.ToLower(c) : c).ToArray())

It is better to use String.Concat than String.Format if you know that format is not change data, and just concatenation is desired.