C# Online Compiler | .NET Fiddle

<no name> by Anonymous

using System;
					
public class Program
{
	public static void Main()
	{
		string input = "L10113540	VD44444		VD2002100234949		000116161		04201261\r\n";
		input += "	KN00010000000129000LPEUR003000001*1003A.Muller-Schulz*1017Bastei\r\n";
		input += "Lubbe.61204 Laund.Meine Schuld*1019KL*102990300*1030NO*1032EUR*1131KT";
		
		int start = input.IndexOf("*1003");
		int end = input.IndexOf("*", start + 1);
		string result = input.Substring(start + 5, end - start - 5);
		
		Console.WriteLine(result);
		
		// Your code
		start = input.IndexOf("*1003") + 5;
		end = input.IndexOf("*1003") - 5;
		result = input.Substring(start, input.IndexOf("*", start) - end);
		
		Console.WriteLine(result);
		
	}
}

 
using System;
                    
public class Program
{
    public static void Main()
    {
        string input = "L10113540   VD44444     VD2002100234949     000116161       04201261\r\n";
        input += "  KN00010000000129000LPEUR003000001*1003A.Muller-Schulz*1017Bastei\r\n";
        input += "Lubbe.61204 Laund.Meine Schuld*1019KL*102990300*1030NO*1032EUR*1131KT";
        
        int start = input.IndexOf("*1003");
        int end = input.IndexOf("*", start + 1);
        string result = input.Substring(start + 5, end - start - 5);
        
        Console.WriteLine(result);
        
        // Your code
        start = input.IndexOf("*1003") + 5;
        end = input.IndexOf("*1003") - 5;
        result = input.Substring(start, input.IndexOf("*", start) - end);
        
        Console.WriteLine(result);
        
    }
}

Unicode String Replace At Issue
Lets examine string "🎶🔥é-"
🎶🔥é- is length of 6, but there are ONLY 4 characters! Why not len=4?
🎶🔥 are double byte UNICODE characters (> \u10000) of width or len 2 each
🎶🔥é- below will replace space after lasting character '-' (position 4) with a sub using most common techniques seen online

🎶🔥X- using ReplaceAtCharArray
in 1,295 ticks.
🎶🔥Y- using ReplaceAtLinq
in 25,679 ticks.
🎶🔥Z- using ReplaceAtStringManipulation
in 1,133 ticks.
🎶🔥A- using ReplaceAtSubstring
in 1,579 ticks.
🎶🔥W- using SubsituteStringStringBuilder
in 1,387 ticks.
🎶🔥é-4using UnicodeReplaceAt
in 4,633 ticks.

UnicodeReplaceAt replaces properly at position 4 in zero based index string

0🔥é- using UnicodeReplaceAt(0, '0') in 45 ticks.
🎶1é- using UnicodeReplaceAt(1, '1') in 14 ticks.
🎶🔥2- using UnicodeReplaceAt(2, '2') in 14 ticks.
🎶🔥é3 using UnicodeReplaceAt(3, '3') in 36 ticks.
🎶🔥é-4using UnicodeReplaceAt(4, '4') in 11 ticks.
🎶🔥é- using UnicodeReplaceAt(5, '5') - this is beyond end of string, so return orginal string in 11 ticks.

FAST testing.

🎶🔥😊-ā̈bç using UnicodeReplaceAtFast(5, '😊') - cool but still O(100) more :( in 7,253 ticks.
in 1 ticks.
😊 using UnicodeReplaceAtFast(1, '😊') - bounds check - 1 char in 22 ticks.
in 0 ticks.
ā using UnicodeReplaceAtFast(0, '😊') - bounds check - wrong index in 12 ticks.

FASTEST testing.

🎶🔥é-😊bç using UnicodeReplaceAtFastest(5, '😊') - cool but still O(100) more than string.Replace :( in 2,424 ticks.
in 1 ticks.
😊 using UnicodeReplaceAtFastest(1, '😊') - bounds check - 1 char in 23 ticks.
in 0 ticks.
ā using UnicodeReplaceAtFastest(0, '😊') - bounds check - 0 index in 12 ticks.
in 0 ticks.
😊 using UnicodeReplaceAtFastest(5, '😊') - bounds check - after end index in 12 ticks.
in 0 ticks.
🎶🔥é-ā̈😊ç using UnicodeReplaceAtFastest(6, '😊') - bounds check - 1 before end index in 17 ticks.
in 0 ticks.
🎶🔥é-ā̈b😊 using UnicodeReplaceAtFastest(7, '😊') - bounds check - end index in 13 ticks.
in 0 ticks.
🎶🔥é-ā̈bç using UnicodeReplaceAtFastest(8, '😊') - bounds check - after end index in 27 ticks.

String.Replace works, but replaces all characters, not at specific location as above functions

🎶+é- using String.Replace('🔥', '+') in 16 ticks.

Cached Result
Last Run:	10:43:31 pm
Compile:	0.126s
Execute:	0.016s
Memory:	40kb
CPU:	0.031s

View IL Code