How does MatchEvaluator in Regex.Replace work?

Question

This is the input string 23x * y34x2. I want to insert " * " (star surrounded by whitespaces) after every number followed by letter, and after every letter followed by number. So my output string would look like this: 23 * x * y * 34 * x * 2.

This is the regex that does the job: @"\d(?=[a-z])|[a-z](?=\d)". This is the function that I wrote that inserts the " * ".

Regex reg = new Regex(@"\d(?=[a-z])|[a-z](?=\d)");    MatchCollection matchC; matchC = reg.Matches(input); int ii = 1; foreach (Match element in matchC)//foreach match I will find the index of that match {     input = input.Insert(element.Index + ii, " * ");//since I' am inserting " * " ( 3 characters )     ii += 3;                                        //I must increment index by 3 } return input; //return modified input 

My question how to do same job using .net MatchEvaluator? I am new to regex and don't understand good replacing with MatchEvaluator. This is the code that I tried to wrote:

{     Regex reg = new Regex(@"\d(?=[a-z])|[a-z](?=\d)");     MatchEvaluator matchEval = new MatchEvaluator(ReplaceStar);     input = reg.Replace(input, matchEval);     return input; } public string ReplaceStar( Match match ) {     //return What?? } 

Answer 1

A MatchEvaluator is a delegate that takes a Match object and returns a string that should be replaced instead of the match. You can also refer to groups from the match. You can rewrite your code as follows:

string input = "23x * y34x2"; Regex reg = new Regex(@"\d(?=[a-z])|[a-z](?=\d)"); string result = reg.Replace(input, delegate(Match m) {     return m.Value + " * "; }); 

To give an example of how this works, the first time the delegate is called, Match parameter will be a match on the string "3". The delegate in this case is defined to return the match itself as a string concatenated with " * ". So the first "3" is replaced with "3 * ".

The process continues in this way, with delegate being called once for each match in the original string.