C# Online Compiler | .NET Fiddle

<no name> by Anonymous

using System;
using System.Linq;
using System.Text.RegularExpressions;
					
public class Program
{
	public static void Main()
	{
		Console.WriteLine(f(1)); //returns 2.
		Console.WriteLine(f(665)); //returns 667.
		Console.WriteLine(f(665999999)); //returns 667000000 without having looped a million times.
		//(Following examples added since the question was first posed.)
		Console.WriteLine(f(666666666)); //also returns 667000000.
		Console.WriteLine(f(66600)); // returns 66700.
	}
	
	public static int f(int b)
	{
		b++;
		var z = new Regex("666[\\d]*");
		var q = z.Replace(b+"", "667", 1);
		return Int32.Parse(q.PadRight((b+"").Length, '0'));	
	}
}

 
using System;
using System.Linq;
using System.Text.RegularExpressions;
                    
public class Program
{
    public static void Main()
    {
        Console.WriteLine(f(1)); //returns 2.
        Console.WriteLine(f(665)); //returns 667.
        Console.WriteLine(f(665999999)); //returns 667000000 without having looped a million times.
        //(Following examples added since the question was first posed.)
        Console.WriteLine(f(666666666)); //also returns 667000000.
        Console.WriteLine(f(66600)); // returns 66700.
    }
    
    public static int f(int b)
    {
        b++;
        var z = new Regex("666[\\d]*");
        var q = z.Replace(b+"", "667", 1);
        return Int32.Parse(q.PadRight((b+"").Length, '0')); 
    }
}

Environment version: .NET 9.0.0 (9.0.0)

Testing original XML:
Simple read:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Read, <Name> optional:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Read all names:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Nested Read:
  documentClass
  SAPNr.
  GT-Invoice
done.

Testing XML without whitespace:
Simple read:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Read, <Name> optional:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Read all names:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Nested Read:
  documentClass
  SAPNr.
  GT-Invoice
done.

Testing XML with whitespace:
Simple read:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Read, <Name> optional:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Read all names:
  documentClass
  SAPNr.
  GT-Invoice
  Should Not See Me For Nested Read
Nested Read:
  documentClass
  SAPNr.
  GT-Invoice
done.
Tests passed.

Last Run:	2:07:11 pm
Compile:	0.049s
Execute:	0.08s
Memory:	9.82Mb
CPU:	0.129s

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