Regular Expressions Cookbook

3.16. Replace Matches with Replacements Generated in Code

Problem

You want to replace all matches of a regular expression with a new string that you build up in procedural code. You want to be able to replace each match with a different string, based on the text that was actually matched.

For example, suppose you want to replace all numbers in a string with the number multiplied by two.

Solution

C#

You can use the static call when you process only a small number of strings with the same regular expression:

string resultString = Regex.Replace(subjectString, @"\d+",
                      new MatchEvaluator(ComputeReplacement));

Construct a Regex object if you want to use the same regular expression with a large number of strings:

Regex regexObj = new Regex(@"\d+");
string resultString = regexObj.Replace(subjectString,
                      new MatchEvaluator(ComputeReplacement));

Both code snippets call the function ComputeReplacement. You should add this method to the class in which you’re implementing this solution:

public String ComputeReplacement(Match matchResult) {
    int twiceasmuch = int.Parse(matchResult.Value) * 2;
    return twiceasmuch.ToString();
}

VB.NET

You can use the static call when you process only a small number of strings with the same regular expression:

Dim MyMatchEvaluator As New MatchEvaluator(AddressOf ComputeReplacement)
Dim ResultString = Regex.Replace(SubjectString, "\d+", MyMatchEvaluator)

Construct a Regex object if you want to use the same regular expression with a large number of strings:

Dim RegexObj As New Regex("\d+") ...