dontoo dontoo - 1 month ago 6
C# Question

How does MatchEvaluator in Regex.Replace work?

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 input 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

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.

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