What is the C# Regex equivalent to Java's appendReplacement and appendTail

Answer 1

I'm not familiar with the Java regex classes, but this is my C# interpretation of what I think your code does:

private const string tempUserBlock = "%%%COMPRESS~USER{0}~{1}%%%"; 
string html = "some html"; 
int p = 0; 
var userBlock = new List<string>(); 

MatchCollection matcher = preservePattern.Matches(html); 
StringBuilder sb = new StringBuilder(); 
int last = 0;
foreach (Match m in matcher)
{
    string match = m.Groups[0].Value; 
    if(match.Trim().Length > 0) { 
        userBlock.Add(match); 
        sb.Append(html.Substring(last, m.Index - last));
        sb.Append(m.Result(string.Format(tempUserBlock, p, index++)));
    }
    last = m.Index + m.Length;
}
sb.Append(html.Substring(last);
html = sb.ToString(); 

Answer 2

There's no need to reproduce Java's appendReplacement/appendTail functionality; .NET has something better: MatchEvaluator. Check it out:

string holder = "Element {0} = {1}";
string s0 = "111 222 XYZ";
ArrayList arr = new ArrayList();

string s1 = Regex.Replace(s0, @"\d+",
  m => string.Format(holder, arr.Add(m.Value), m.Value)
);

Console.WriteLine(s1);
foreach (string s in arr)
{
  Console.WriteLine(s);
}

output:

Element 0 = 111 Element 1 = 222 XYZ
111
222

There are several ways to implement the MatchEvaluator, all thoroughly discussed in this article. This one (lambda expressions) is by far the coolest.