C# Cookbook

8.6. Augmenting the Basic String Replacement Function

Problem

You need to replace character patterns within the target string with a new string. However, in this case, each replacement operation has a unique set of conditions that must be satisfied in order to allow the replacement to occur. Consider, for example, that you receive a string in the form of XML (or possibly HTML). You wish to modify an attribute of a specific XML tag to a particular number, but only if that number is within a specified range (or possibly outside of a particular range).

Solution

Use the overloaded instance Replace method that accepts a MatchEvaluator delegate along with its other parameters. The MatchEvaluator delegate, which is a callback method that overrides the default behavior of the Replace method, is shown here:

using System; using System.Text.RegularExpressions; public static string MatchHandler(Match theMatch) { // Handle Top property of the Property tag if (theMatch.Value.StartsWith("<Property")) { long topPropertyValue = 0; // Obtain the numeric value of the Top property Match topPropertyMatch = Regex.Match(theMatch.Value, "Top=\"([-]*\\d*)"); if (topPropertyMatch.Success) { if (topPropertyMatch.Groups[1].Value.Trim( ).Equals("")) { // If blank, set to zero return (theMatch.Value.Replace("Top=\"\"", "Top=\"0\"")); } else if (topPropertyMatch.Groups[1].Value.Trim( ).Equals("-")) { // If only a negative sign (syntax error), set to zero return (theMatch.Value.Replace("Top=\"-\"", "Top=\"0\"")); ...