using System;
using System.Text.RegularExpressions;
public class Example
{
public static void Main()
{
string pattern = @"(?<=\d{6})(\d+)(?=\d{4})";
string substitution = @"X";
string input = @"4548812049400004
5229782641993099
345678000000007
6362970000457013";
Regex regex = new Regex(pattern);
string result = regex.Replace(input, substitution);
}
}
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for C#, please visit: https://msdn.microsoft.com/en-us/library/system.text.regularexpressions.regex(v=vs.110).aspx