using System;
using System.Text.RegularExpressions;
public class Example
{
public static void Main()
{
string pattern = @"(?:\G(?!^)|^(?=.*string3)).*?\Kstring1";
string substitution = @"string2";
string input = @"This is line string1
This is line two with string1 and string3 string1
This is line two with string1 string1 and string3
This is line three with string3
This is line string3 with string1 and string1";
RegexOptions options = RegexOptions.Multiline;
Regex regex = new Regex(pattern, options);
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