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Regex Constructor (String)

Updated: July 2009

Initializes and compiles a new instance of the Regex class for the specified regular expression.

Namespace:  System.Text.RegularExpressions
Assembly:  System (in System.dll)

public Regex(
	string pattern


Type: System.String

The regular expression pattern to match.


A regular expression parsing error has occurred.


pattern is null.

The pattern parameter consists of various regular expression language elements that symbolically describe the string to match. For more information about regular expressions, see the .NET Framework Regular Expressions and Regular Expression Language - Quick Reference topics.

A Regex object is immutable, which means that it can be used only for the match parameters defined when it is created. It can be used any number of times without being recompiled, however.

This constructor instantiates a regular expression object that performs a case-sensitive match of any alphabetic characters defined in pattern. For a case-insensitive match, use the Regex.Regex(String, RegexOptions) constructor.

The following example illustrates how to use this constructor to instantiate a regular expression that matches any word that begins with the letters "a" or "t".

string pattern = @"\b[at]\w+";
string text = "The threaded application ate up the thread pool as it executed.";
MatchCollection matches;

Regex defaultRegex = new Regex(pattern);
// Get matches of pattern in text
matches = defaultRegex.Matches(text);
Console.WriteLine("Parsing '{0}'", text);
// Iterate matches    
for (int ctr = 1; ctr <= matches.Count; ctr++)
   Console.WriteLine("{0}. {1}", ctr, matches[ctr-1].Value);
// This example displays the following output to the console: 
//       Parsing 'The threaded application ate up the thread pool as it executed.' 
//       0. threaded 
//       1. application 
//       2. ate 
//       3. the 
//       4. thread 
//       5. as      

Note that, because comparisons are case-sensitive by default, the regular expression pattern fails to match the word "The" at the beginning of the text.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0




July 2009

Included additional exception information.

Content bug fix.