Regular Expressions Cookbook

3.17. Replace All Matches Within the Matches of Another Regex

Problem

You want to replace all the matches of a particular regular expression, but only within certain sections of the subject string. Another regular expression matches each of the sections in the string.

Say you have an HTML file in which various passages are marked as bold with <b> tags. Between each pair of bold tags, you want to replace all matches of the regular expression ‹before› with the replacement text ‹after›. For example, when processing the string before <b>first before</b> before <b>before before</b>, you want to end up with: before <b>first after</b> before <b>after after</b>.

Solution

C#

Regex outerRegex = new Regex("<b>.*?</b>", RegexOptions.Singleline);
Regex innerRegex = new Regex("before");
string resultString = outerRegex.Replace(subjectString,
                      new MatchEvaluator(ComputeReplacement));

public String ComputeReplacement(Match matchResult) {
    // Run the inner search-and-replace on each match of the outer regex
    return innerRegex.Replace(matchResult.Value, "after");
}

VB.NET

Dim OuterRegex As New Regex("<b>.*?</b>", RegexOptions.Singleline) Dim InnerRegex As New Regex("before") Dim MyMatchEvaluator As New MatchEvaluator(AddressOf ComputeReplacement) Dim ResultString = OuterRegex.Replace(SubjectString, MyMatchEvaluator) Public Function ComputeReplacement(ByVal MatchResult As Match) As String 'Run the inner search-and-replace on each match of the outer regex Return InnerRegex.Replace(MatchResult.Value, "after"); ...