C# and Regular Expressions

The Classes of System.Text.RegularExpressions

The following table lists the classes that are contained in the System.Text.RegularExpressions namespace. The properties and methods of several of the classes listed are described in detail later in this chapter, together with examples demonstrating how to use them.

Class	Description
Capture	Represents the text captured by a single set of parentheses
	surrounding a subexpression
CaptureCollection	Represents a collection of Capture objects
Group	Represents the result of a single capturing group of paired
	parentheses
GroupCollection	Represents a collection of Group objects
Match	Represents the result of a single regular expression match
MatchCollection	Represents a collection of Match objects
Regex	The class that contains the regular expression pattern
RegexCompilationInfo	Provides information that the compiler uses to compile a
	regular expression into an assembly

The Regex class is the most important class in the System.Text.RegularExpressions namespace.

The Regex Class

The Regex object can be instantiated to make its properties and methods accessible to programmatic manipulation. In addition, three of the methods of the Regex class are available as static methods. The use of these shared methods is described and demonstrated later.

The Regex object has two public properties, described briefly in the following table.

Property	Description
Options	Contains information about the options passed to the Regex
	object.
RightToLeft	Returns a Boolean value indicating whether or not right-to-
	left processing of matching is operative. A value of True
	indicates right-to-left matching.

If you are used to creating regular expression objects in JScript or VBScript, where the regular expression object is the RegExp object, be careful to spell the .NET Regex object correctly in your code.

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Chapter 22

The Options Property of the Regex Class

The Options property contains a value each bit of which corresponds to the options set. The individual bits of the value each contain a 0 or 1, which corresponds to whether a particular option is or is not chosen. When the default value, equivalent to RegexOptions.None, is passed in, for example, a Match() method, the value of the Options property is 0. The available options are described later in the description of the RegexOptions class.

The Regex Class’s RightToLeft Property

The .NET Framework allows right-to-left matching to be used. When attempting matching in English and other languages where words and lines are written left to right, regular expression matching also takes place in a left-to-right direction. In languages such as Arabic and Hebrew, writing and reading may take place in a right-to-left direction. The RightToLeft property of the Regex class supports, or is intended to support, matching from right to left.

However, not all of the matching process is reversed. Lookahead still looks at character sequences to the right of the current matching position, and lookbehind still looks at character sequences to the left of the matching position. In practice, matching using the RightToLeft property has proved to be less reliable than I had hoped and, therefore, is not demonstrated here.

Regex Class Methods

The following table summarizes the methods of the Regex object. Some of the concepts and techniques of the Regex object are not found in standard regular expressions. Therefore, some of the concepts summarized in the following table may usefully be clarified for you when you read following sections that further describe the methods and/or demonstrate how they are used.

Method	Description
CompileToAssembly	Compiles a regular expression to an assembly (the default
	behavior of a regular expression is not to compile to an
	assembly).
Equals	Determines whether two objects are equal.
Escape	Escapes a set of metacharacters, replacing the metacharacter
	with the corresponding escaped character.
GetGroupNames	Returns an array of capturing group names.
GetGroupNumbers	Returns an array of capturing group numbers.
GetHashCode	Inherited from the Object class.
GetType	Gets the type of the current instance.
GroupNameFromNumber	Gets a group name that corresponds to the group number
	supplied as an argument.
GroupNumberFromName	Gets a group number that corresponds to the group name
	supplied as an argument.

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	Method	Description
	IsMatch	Returns a Boolean value that indicates whether the regular
		expression pattern is matched in the string, which is the
		argument to the IsMatch() method.
	Match	Returns zero or one Match object, depending on whether
		the string supplied to the method as its argument contains a
		match.
	Matches	Returns a MatchCollection object containing zero or more
		Match objects, which contain all matches (or none) in the
		string that is the argument to the Matches() method.
	Replace	Replaces all occurrences of a regular expression pattern
		with a specified character sequence.
	Split	Splits an input string into an array of strings. The split occurs
		at a position indicated by a regular expression pattern.
	ToString	Returns a string containing the regular expression passed
		into the Regex object in its constructor.
	Unescape	Unescapes any escaped characters in the input string.

The CompileToAssembly() Method

The Regex class’s CompileToAssembly() method takes two arguments: the RegexCompilationInfo object (which is a member of the System.Text.RegularExpressions namespace and contains the information necessary to specify how compilation is to be carried out) and the name of the assembly to be created.

When the CompileToAssembly() method is used, the startup time can be expected to increase but with the benefit of faster running.

The GetGroupNames() Method

The GetGroupNames() method retrieves the names of any named groups associated with a Match object. The GetGroupNames() method takes no argument.

The GetGroupNumbers() Method

The GetGroupNumbers() method retrieves the numbers of any numbered groups associated with a Match object. There is always at least one group, which matches the entire regular expression pattern. If paired parentheses are included in the regular expression pattern, there may be additional numbered groups. The GetGroupNumbers() method takes no argument.

GroupNumberFromName() and GroupNameFromNumber() Methods

The GroupNumberFromName() method retrieves a group number given a group name as its argument. The group’s name is supplied as a string argument. The GroupNameFromNumber() method retrieves a group name, if one exists, for a group number supplied as the method’s argument. The group’s number is supplied to the method as an int argument.

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Chapter 22

The IsMatch() Method

The Regex object’s IsMatch() method takes a single string argument and tests whether the regular expression pattern is matched in that string argument. It returns a bool value. Optionally, the IsMatch() method takes a second argument, an int value, which specifies the position in the string argument at which the attempt at matching is to begin.

Try It Out

The IsMatch() Method

1.Open Visual Studio 2003, create a new application from a console application template, and name the new project IsMatchDemo.

2.Add the following statement after the using System; statement:

using System.Text.RegularExpressions;

3.Edit the content of the Main() method as follows:

Console.WriteLine(@”This will find a match for the regular expression ‘[A-Z]\d’.”);

Console.WriteLine(“Enter a test string now.”);

Regex myRegex = new Regex(@”[A-Z]\d”, RegexOptions.IgnoreCase); string inputString;

inputString = Console.ReadLine();

Match myMatch = myRegex.Match(inputString);

string outputString = “The following option(s) are set: “; Console.WriteLine(outputString + myRegex.Options.ToString()); Console.WriteLine(“You entered the string: ‘“ + inputString + “‘.”); if (myRegex.IsMatch(inputString))

Console.WriteLine(“The match ‘“ + myMatch.ToString() + “‘ was found in the string you entered.”);

else

Console.WriteLine(“No match was found.”); Console.ReadLine();

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

5.Enter the test string J88 at the command-line prompt; press Return; and inspect the displayed information, as shown in Figure 22-3.

Figure 22-3

The IsMatch() method can also be statically overloaded so that you can use it without having to instantiate a Regex object.

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