using System;
using System.Text.RegularExpressions;
public class Example
{
public static void Main()
{
string pattern = @"([a-z]+)\*\1(?=[^#]*#[^#]*(?:[^#]*#[^#]*#[^#]*)*$)";
string substitution = @"$1^2";
string input = @"As a 1st step, you can rewrite #21x*x - 1y = 1z + 1#. But what about the 2nd #3x + 1y = 1z#?";
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