Chapter 22

Metacharacters Supported in Visual C# .NET

Visual C#.NET has a very complete and extensive regular expressions implementation, which exceeds in functionality many of the tools you saw in earlier chapters of this book.

Much of the regular expression support in Visual C# .NET can reasonably be termed standard. However, as with many Microsoft technologies, the standard syntax and techniques have been extended or modified in places.

The following table summarizes many of the metacharacters supported in Visual C# .NET.

542

543

Chapter 22

Using Named Groups

One of the features supported in the .NET Framework but not supported in many other regular expression implementations is the notion of named groups.

The syntax is (<nameOfGroup>pattern). Naming a group of characters can make understanding and maintenance of code easier than using numbered groups. For example, examine the following pattern:

${lastName}, ${firstName}

The purpose of this pattern in a replacement string is more easily understood than the purpose of the same replacement operation expressed as numbered, rather than named, groups:

${1}, ${2}

The following example reverses first name and last name using named groups.

1.Create a new project in Visual Studio 2003 using the Console Application template, and name the project NamedGroupsDemo.

2.In the code editor, add the following line after any default using statements:

using System.Text.RegularExpressions;

3.Enter the following code between the curly braces of the Main() method:

Console.WriteLine(@”This will find a match for the regular expression ‘^(?<firstName>\w+)\s+(?<lastName>\w+)$’.”); Console.WriteLine(“Enter a test string consisting of a first name then a last name.”);

string inputString;

inputString = Console.ReadLine();

string outputString = Regex.Replace(inputString, @”^(?<firstName>\w+)\s+(?<lastName>\w+)$”, “${lastName}, ${firstName}”); Console.WriteLine(“You entered the string: ‘“ + inputString +

“‘.”);

Console.WriteLine(“The replaced string is ‘“ + outputString + “‘.”);

Console.ReadLine();

4.Save the code, and press F5 to run it.

5.At the command line, enter the test string John Smith, and inspect the displayed result, as shown in Figure 22-15.

Figure 22-15

544

C# and Regular Expressions

How It Works

The content of the Main() method is explained here.

First, the pattern to be matched against is displayed, and the user is invited to enter a first name and last name. The pattern to be matched contains two named groups, represented respectively by

(?<firstName>\w+) and (?<lastName>\w+):

Console.WriteLine(@”This will find a match for the regular expression ‘^(?<firstName>\w+)\s+(?<lastName>\w+)$’.”);

Console.WriteLine(“Enter a test string consisting of a first name then a last name.”);

The inputString variable is declared; then the Console.ReadLine() method is used to capture the string entered by the user. That string value is assigned to the inputString variable:

string inputString;

inputString = Console.ReadLine();

The Regex class’s Replace() method is used statically, with three arguments. The first argument specifies the string in which replacement is to take place — in this case, the string specified by the inputString variable. The pattern to be used to match is specified by the second argument — in this case, the pattern ^(?<firstName>\w+)\s+(?<lastName>\w+)$. The third argument, which is formally a string value, uses the notation ${namedGroup} to represent each named group.

The ${firstName} group, not surprisingly, contains the alphabetic character sequence entered first, and the ${lastName} group contains the alphabetic character sequence entered second:

string outputString = Regex.Replace(inputString,

@”^(?<firstName>\w+)\s+(?<lastName>\w+)$”, “${lastName}, ${firstName}”);

The user is shown the string that was entered and the string produced when the Replace() method was applied:

Console.WriteLine(“You entered the string: ‘“ + inputString + “‘.”);

Console.WriteLine(“The replaced string is ‘“ + outputString + “‘.”);

Console.ReadLine();

Using Back References

Back references are supported in C# .NET. A typical use for back references is finding doubled words and removing them. The following example shows this.

1.Create a new project in Visual Studio 2003 using the Console Application template, and name the project BackReferenceDemo.

2.Add a using System.Text.RegularExpressions; statement.

545

Chapter 22

3.In the code editor, add the following code between the paired braces of the Main() method:

Console.WriteLine(“This example will find a doubled word.”); Console.WriteLine(“Using a backreference and the Replace() method the doubled word will be removed.”);

Console.WriteLine(“Enter a test string containing a doubled word.”);

string inputString;

inputString = Console.ReadLine();

string outputString = Regex.Replace(inputString, @”(\w+)\s+(\1)”, “${1}”);

Console.WriteLine(“You entered the string: ‘“ + inputString + “‘.”);

Console.WriteLine(“The replaced string is ‘“ + outputString + “‘.”);

Console.ReadLine();

4.Save the code, and press F5 to run it.

5.Enter the test string Paris in the the Spring (note the doubled the in the test string); press Return; and inspect the displayed information, as shown in Figure 22-16.

Figure 22-16

6.Press Return to close the application. In Visual Studio, press F5 to run the code again.

7.Enter the test string Hello Hello, press Return, and inspect the displayed information. Again, the doubled word is identified and replaced with a single occurrence of the same word.

How It Works

The Main() method code begins by displaying information to the user about the use of back references and invites the user to enter a string containing a doubled word:

Console.WriteLine(“This example will find a doubled word.”); Console.WriteLine(“Using a backreference and the Replace() method the doubled word will be removed.”);

Console.WriteLine(“Enter a test string containing a doubled word.”);

The inputString variable is declared. And the string that the user entered is assigned to the inputString variable:

string inputString;

inputString = Console.ReadLine();

The Regex class’s Replace() method is used statically and is applied to the inputString variable, and the result is assigned to the outputString variable.

546