Recipe 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\""));
            }
            else
            {
                // We have a valid number
                // Convert the matched string to a numeric value
                topPropertyValue = long.Parse(
                                   topPropertyMatch.Groups[1].Value,
                                   System.Globalization.NumberStyles.Any);

                // If the Top property is out of the specified 
                //    range, set it to zero
                if (topPropertyValue < 0 || topPropertyValue > 5000)
                {
                    return (theMatch.Value.Replace("Top=\"" + 
                                                   topPropertyValue + 
                                                   "\"", "Top=\"0\""));
                }
            }
        }
    }

    return (theMatch.Value);
}

The callback method for the Replace method is shown here:

public static void ComplexReplace(string matchPattern, string source)
{
    MatchEvaluator replaceCallback = new MatchEvaluator(MatchHandler);
    Regex RE = new Regex(matchPattern, RegexOptions.Multiline);
    string newString = RE.Replace(source, replaceCallback);

    Console.WriteLine("Replaced String = " + newString); 
}

To use this callback method with the static Replace method, modify the previous ComplexReplace method as follows:

public void ComplexReplace(string matchPattern, string source)
{
    MatchEvaluator replaceCallback = new MatchEvaluator(MatchHandler);
    string newString = Regex.Replace(source, matchPattern, 
                                     replaceCallback);

    Console.WriteLine("Replaced String = " + newString); 
}

where source is the original string to run the replace operation against, and matchPattern is the regular expression pattern to match in the source string.

If the ComplexReplace method is called from the following code:

public static void TestComplexReplace( )
{
    string matchPattern = "<.*>";
    string source = @"<?xml version=""1.0\"" encoding=\""UTF-8\""?>
   <Window ID=""Main"">
       <Control ID=""TextBox"">
           <Property Top=""-100"" Left=""0"" Text=""BLANK""/>
       </Control>
       <Control ID=""Label"">
      <Property Top=""99990"" Left=""0"" Caption=""Enter Name Here""/>
       </Control>
   </Window>";

    ComplexReplace(matchPattern, source);
}

only the Top attributes of the Property tags are changed from their original values to 0.

The result of this replace action will change only the Top property value of a Property tag to zero if it is less than zero or greater than 5000. Any other tag that contains a Top property will remain unchanged. The following two lines of the source string will be changed from:

<Property Top="100" Left="0" Text="BLANK"/>
<Property Top="99999" Left="0" Caption="Enter Name Here"/>

to:

<Property Top="100" Left="0" Text="BLANK"/>
<Property Top="0" Left="0" Caption="Enter Name Here"/>

Discussion

The MatchEvaluator delegate, which is automatically invoked when it is supplied as a parameter to the Regexp class's Replace method, allows for custom replacement of each string that conforms to the regular expression pattern.

If the current Match object is operating on a Property tag whose Top property is out of the specified range, the code within the MatchHandler callback method returns a new modified string. Otherwise, the currently matched string is returned unchanged. This ability allows you to override the default Replace functionality by replacing only that part of the source string that meets certain criteria. The code within this callback method gives you some idea of what can be accomplished using this replacement technique.

To make use of this callback method, we need a way to call it from the ComplexReplace method. First, a variable of type System.Text.RegularExpressions.MatchEvaluator is created. This variable (replaceCallback) is the delegate that is used to call the MatchHandler method:

MatchEvaluator replaceCallback = new MatchEvaluator(MatchHandler);

Finally, the Replace method is called with the reference to the MatchEvaluator delegate passed in as a parameter:

string newString = RE.Replace(source, replaceCallback);

See Also

See the ".NET Framework Regular Expressions" topic in the MSDN documentation.