Regex Constructors

Reference

Definition

Namespace:: System.Text.RegularExpressions

Assembly:: System.Text.RegularExpressions.dll

Assembly:: System.dll

Assembly:: netstandard.dll

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Initializes a new instance of the Regex class.

Overloads

Regex()	Initializes a new instance of the Regex class.
Regex(String)	Initializes a new instance of the Regex class for the specified regular expression.
Regex(SerializationInfo, StreamingContext)	Obsolete. Initializes a new instance of the Regex class by using serialized data.
Regex(String, RegexOptions)	Initializes a new instance of the Regex class for the specified regular expression, with options that modify the pattern.
Regex(String, RegexOptions, TimeSpan)	Initializes a new instance of the Regex class for the specified regular expression, with options that modify the pattern and a value that specifies how long a pattern matching method should attempt a match before it times out.

Regex()

Source:: Regex.cs

Source:: Regex.cs

Source:: Regex.cs

Initializes a new instance of the Regex class.

protected:
 Regex();

protected Regex ();

Protected Sub New ()

Remarks

Note that this constructor is protected; it can only be called by classes derived from the Regex class.

Applies to

Regex(String)

Source:: Regex.cs

Source:: Regex.cs

Source:: Regex.cs

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

public:
 Regex(System::String ^ pattern);

public Regex (string pattern);

new System.Text.RegularExpressions.Regex : string -> System.Text.RegularExpressions.Regex

Public Sub New (pattern As String)

Parameters

pattern: String

The regular expression pattern to match.

Exceptions

ArgumentException

A regular expression parsing error occurred.

ArgumentNullException

pattern is null.

Examples

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".

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      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 = 0; ctr < matches.Count; ctr++)
         Console.WriteLine("{0}. {1}", ctr, matches[ctr].Value);
   }
}
// The example displays the following output:
//       Parsing 'The threaded application ate up the thread pool as it executed.'
//       0. threaded
//       1. application
//       2. ate
//       3. the
//       4. thread
//       5. as

Imports System.Text.RegularExpressions

Module Example
   Public Sub Main()
      Dim pattern As String = "\b[at]\w+"
      Dim text As String = "The threaded application ate up the thread pool as it executed."
      Dim matches As MatchCollection

      Dim defaultRegex As New Regex(pattern)
      ' Get matches of pattern in text
      matches = defaultRegex.Matches(text)
      Console.WriteLine("Parsing '{0}'", text)
      ' Iterate matches
      For ctr As Integer = 0 to matches.Count - 1
         Console.WriteLine("{0}. {1}", ctr, matches(ctr).Value)
      Next
   End Sub
End Module
' The example displays the following output:
'       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 the regular expression pattern cannot match the word "The" at the beginning of the text, because comparisons are case-sensitive by default. For an example of case-insensitive comparison, see the Regex(String, RegexOptions) constructor.

Remarks

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a time-out value to prevent malicious users from causing a denial-of-service attack. A time-out value specifies how long a pattern-matching method should try to find a match before it times out.

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

Calling the Regex(String) constructor is equivalent to calling the Regex(String, RegexOptions) constructor with a value of None for the options argument.

A Regex object is immutable, which means that it can be used only for the match pattern you define when you create it. However, it can be used any number of times without being recompiled.

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

Notes to Callers

This constructor creates a Regex object that uses the default time-out value of the application domain in which it is created. If a time-out value has not been defined for the application domain, the Regex object uses the value InfiniteMatchTimeout, which prevents the operation from timing out. The recommended constructor for creating a Regex object is Regex(String, RegexOptions, TimeSpan), which lets you set the time-out interval.

Applies to

Regex(SerializationInfo, StreamingContext)

Source:: Regex.cs

Source:: Regex.cs

Source:: Regex.cs

Caution

This API supports obsolete formatter-based serialization. It should not be called or extended by application code.

Initializes a new instance of the Regex class by using serialized data.

protected:
 Regex(System::Runtime::Serialization::SerializationInfo ^ info, System::Runtime::Serialization::StreamingContext context);

protected Regex (System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context);

[System.Obsolete("This API supports obsolete formatter-based serialization. It should not be called or extended by application code.", DiagnosticId="SYSLIB0051", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
protected Regex (System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context);

new System.Text.RegularExpressions.Regex : System.Runtime.Serialization.SerializationInfo * System.Runtime.Serialization.StreamingContext -> System.Text.RegularExpressions.Regex

[<System.Obsolete("This API supports obsolete formatter-based serialization. It should not be called or extended by application code.", DiagnosticId="SYSLIB0051", UrlFormat="https://aka.ms/dotnet-warnings/{0}")>]
new System.Text.RegularExpressions.Regex : System.Runtime.Serialization.SerializationInfo * System.Runtime.Serialization.StreamingContext -> System.Text.RegularExpressions.Regex

Protected Sub New (info As SerializationInfo, context As StreamingContext)

Parameters

info: SerializationInfo

The object that contains a serialized pattern and RegexOptions information.

context: StreamingContext

The destination for this serialization. (This parameter is not used; specify null.)

Attributes: ObsoleteAttribute

Exceptions

ArgumentException

A regular expression parsing error occurred.

ArgumentNullException

The pattern that info contains is null.

ArgumentOutOfRangeException

info contains an invalid RegexOptions flag.

Applies to

Regex(String, RegexOptions)

Source:: Regex.cs

Source:: Regex.cs

Source:: Regex.cs

Initializes a new instance of the Regex class for the specified regular expression, with options that modify the pattern.

public:
 Regex(System::String ^ pattern, System::Text::RegularExpressions::RegexOptions options);

public Regex (string pattern, System.Text.RegularExpressions.RegexOptions options);

new System.Text.RegularExpressions.Regex : string * System.Text.RegularExpressions.RegexOptions -> System.Text.RegularExpressions.Regex

Public Sub New (pattern As String, options As RegexOptions)

Parameters

pattern: String

The regular expression pattern to match.

options: RegexOptions

A bitwise combination of the enumeration values that modify the regular expression.

Exceptions

ArgumentException

A regular expression parsing error occurred.

ArgumentNullException

pattern is null.

ArgumentOutOfRangeException

options contains an invalid flag.

Examples

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".

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern = @"\b[at]\w+";
      RegexOptions options = RegexOptions.IgnoreCase | RegexOptions.Compiled;
      string text = "The threaded application ate up the thread pool as it executed.";
      MatchCollection matches;

      Regex optionRegex = new Regex(pattern, options);
      Console.WriteLine("Parsing '{0}' with options {1}:", text, options.ToString());
      // Get matches of pattern in text
      matches = optionRegex.Matches(text);
      // Iterate matches
      for (int ctr = 0; ctr < matches.Count; ctr++)
         Console.WriteLine("{0}. {1}", ctr, matches[ctr].Value);
   }
}
// The example displays the following output:
//    Parsing 'The threaded application ate up the thread pool as it executed.'
//        with options IgnoreCase, Compiled:
//    0. The
//    1. threaded
//    2. application
//    3. ate
//    4. the
//    5. thread
//    6. as

Imports System.Text.RegularExpressions

Module Example
   Public Sub Main()
      Dim pattern As String = "\b[at]\w+"
      Dim options As RegexOptions = RegexOptions.IgnoreCase Or RegexOptions.Compiled
      Dim text As String = "The threaded application ate up the thread pool as it executed."
      Dim matches As MatchCollection

      Dim optionRegex As New Regex(pattern, options)
      Console.WriteLine("Parsing '{0}' with options {1}:", text, options.ToString())
      ' Get matches of pattern in text
      matches = optionRegex.Matches(text)
      ' Iterate matches   
      For ctr As Integer = 0 to matches.Count - 1
         Console.WriteLine("{0}. {1}", ctr, matches(ctr).Value)
      Next
   End Sub
End Module
' The example displays the following output:
'    Parsing 'The threaded application ate up the thread pool as it executed.'
'       with options IgnoreCase, Compiled:
'    0. The
'    1. threaded
'    2. application
'    3. ate
'    4. the
'    5. thread
'    6. as

Note that the match collection includes the word "The" that begins the text because the options parameter has defined case-insensitive comparisons.

Remarks

Warning

A Regex object is immutable, which means that it can be used only for the match parameters you define when you create it. However, it can be used any number of times without being recompiled.

Notes to Callers

Applies to

Regex(String, RegexOptions, TimeSpan)

Source:: Regex.cs

Source:: Regex.cs

Source:: Regex.cs

Initializes a new instance of the Regex class for the specified regular expression, with options that modify the pattern and a value that specifies how long a pattern matching method should attempt a match before it times out.

public:
 Regex(System::String ^ pattern, System::Text::RegularExpressions::RegexOptions options, TimeSpan matchTimeout);

public Regex (string pattern, System.Text.RegularExpressions.RegexOptions options, TimeSpan matchTimeout);

new System.Text.RegularExpressions.Regex : string * System.Text.RegularExpressions.RegexOptions * TimeSpan -> System.Text.RegularExpressions.Regex

Public Sub New (pattern As String, options As RegexOptions, matchTimeout As TimeSpan)

Parameters

pattern: String

The regular expression pattern to match.

options: RegexOptions

A bitwise combination of the enumeration values that modify the regular expression.

matchTimeout: TimeSpan

A time-out interval, or InfiniteMatchTimeout to indicate that the method should not time out.

Exceptions

ArgumentException

A regular expression parsing error occurred.

ArgumentNullException

pattern is null.

ArgumentOutOfRangeException

options is not a valid RegexOptions value.

-or-

matchTimeout is negative, zero, or greater than approximately 24 days.

Examples

The following example calls the Regex(String, RegexOptions, TimeSpan) constructor to instantiate a Regex object with a time-out value of one second. The regular expression pattern (a+)+$, which matches one or more sequences of one or more "a" characters at the end of a line, is subject to excessive backtracking. If a RegexMatchTimeoutException is thrown, the example increases the time-out value up to the maximum value of three seconds. Otherwise, it abandons the attempt to match the pattern.

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Security;
using System.Text.RegularExpressions;
using System.Threading; 

public class Example
{
   const int MaxTimeoutInSeconds = 3;

   public static void Main()
   {
      string pattern = @"(a+)+$";    // DO NOT REUSE THIS PATTERN.
      Regex rgx = new Regex(pattern, RegexOptions.IgnoreCase, TimeSpan.FromSeconds(1));       
      Stopwatch sw = null;
      
      string[] inputs= { "aa", "aaaa>", 
                         "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
                         "aaaaaaaaaaaaaaaaaaaaaa>",
                         "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>" };
                                 
      foreach (var inputValue in inputs) {
         Console.WriteLine("Processing {0}", inputValue);
         bool timedOut = false;
         do { 
            try {
               sw = Stopwatch.StartNew();
               // Display the result.
               if (rgx.IsMatch(inputValue)) {
                  sw.Stop();
                  Console.WriteLine(@"Valid: '{0}' ({1:ss\.fffffff} seconds)", 
                                    inputValue, sw.Elapsed); 
               }
               else {
                  sw.Stop();
                  Console.WriteLine(@"'{0}' is not a valid string. ({1:ss\.fffff} seconds)", 
                                    inputValue, sw.Elapsed);
               }
            }
            catch (RegexMatchTimeoutException e) {   
               sw.Stop();
               // Display the elapsed time until the exception.
               Console.WriteLine(@"Timeout with '{0}' after {1:ss\.fffff}", 
                                 inputValue, sw.Elapsed);
               Thread.Sleep(1500);       // Pause for 1.5 seconds.

               // Increase the timeout interval and retry.
               TimeSpan timeout = e.MatchTimeout.Add(TimeSpan.FromSeconds(1));
               if (timeout.TotalSeconds > MaxTimeoutInSeconds) {
                  Console.WriteLine("Maximum timeout interval of {0} seconds exceeded.",
                                    MaxTimeoutInSeconds);
                  timedOut = false;
               }
               else {               
                  Console.WriteLine("Changing the timeout interval to {0}", 
                                    timeout); 
                  rgx = new Regex(pattern, RegexOptions.IgnoreCase, timeout);
                  timedOut = true;
               }
            }
         } while (timedOut);
         Console.WriteLine();
      }   
   }
}
// The example displays output like the following :
//    Processing aa
//    Valid: 'aa' (00.0000779 seconds)
//    
//    Processing aaaa>
//    'aaaa>' is not a valid string. (00.00005 seconds)
//    
//    Processing aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
//    Valid: 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' (00.0000043 seconds)
//    
//    Processing aaaaaaaaaaaaaaaaaaaaaa>
//    Timeout with 'aaaaaaaaaaaaaaaaaaaaaa>' after 01.00469
//    Changing the timeout interval to 00:00:02
//    Timeout with 'aaaaaaaaaaaaaaaaaaaaaa>' after 02.01202
//    Changing the timeout interval to 00:00:03
//    Timeout with 'aaaaaaaaaaaaaaaaaaaaaa>' after 03.01043
//    Maximum timeout interval of 3 seconds exceeded.
//    
//    Processing aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>
//    Timeout with 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>' after 03.01018
//    Maximum timeout interval of 3 seconds exceeded.

Imports System.ComponentModel
Imports System.Diagnostics
Imports System.Security
Imports System.Text.RegularExpressions
Imports System.Threading 

Module Example
   Const MaxTimeoutInSeconds As Integer = 3
   
   Public Sub Main()
      Dim pattern As String = "(a+)+$"    ' DO NOT REUSE THIS PATTERN.
      Dim rgx As New Regex(pattern, RegexOptions.IgnoreCase, TimeSpan.FromSeconds(1))       
      Dim sw As Stopwatch = Nothing
      
      Dim inputs() As String = { "aa", "aaaa>", 
                                 "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
                                 "aaaaaaaaaaaaaaaaaaaaaa>",
                                 "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>" }
                                 
      For Each inputValue In inputs
         Console.WriteLine("Processing {0}", inputValue)
         Dim timedOut As Boolean = False
         Do 
            Try
               sw = Stopwatch.StartNew()
               ' Display the result.
               If rgx.IsMatch(inputValue) Then
                  sw.Stop()
                  Console.WriteLine("Valid: '{0}' ({1:ss\.fffffff} seconds)", 
                                    inputValue, sw.Elapsed) 
               Else
                  sw.Stop()
                  Console.WriteLine("'{0}' is not a valid string. ({1:ss\.fffff} seconds)", 
                                    inputValue, sw.Elapsed)
               End If
            Catch e As RegexMatchTimeoutException   
               sw.Stop()
               ' Display the elapsed time until the exception.
               Console.WriteLine("Timeout with '{0}' after {1:ss\.fffff}", 
                                 inputValue, sw.Elapsed)
               Thread.Sleep(1500)       ' Pause for 1.5 seconds.

               ' Increase the timeout interval and retry.
               Dim timeout As TimeSpan = e.MatchTimeout.Add(TimeSpan.FromSeconds(1))
               If timeout.TotalSeconds > MaxTimeoutInSeconds Then
                  Console.WriteLine("Maximum timeout interval of {0} seconds exceeded.",
                                    MaxTimeoutInSeconds)
                  timedOut = False
               Else                
                  Console.WriteLine("Changing the timeout interval to {0}", 
                                    timeout) 
                  rgx = New Regex(pattern, RegexOptions.IgnoreCase, timeout)
                  timedOut = True
               End If
            End Try
         Loop While timedOut
         Console.WriteLine()
      Next   
   End Sub 
End Module
' The example displays output like the following:
'    Processing aa
'    Valid: 'aa' (00.0000779 seconds)
'    
'    Processing aaaa>
'    'aaaa>' is not a valid string. (00.00005 seconds)
'    
'    Processing aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
'    Valid: 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' (00.0000043 seconds)
'    
'    Processing aaaaaaaaaaaaaaaaaaaaaa>
'    Timeout with 'aaaaaaaaaaaaaaaaaaaaaa>' after 01.00469
'    Changing the timeout interval to 00:00:02
'    Timeout with 'aaaaaaaaaaaaaaaaaaaaaa>' after 02.01202
'    Changing the timeout interval to 00:00:03
'    Timeout with 'aaaaaaaaaaaaaaaaaaaaaa>' after 03.01043
'    Maximum timeout interval of 3 seconds exceeded.
'    
'    Processing aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>
'    Timeout with 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>' after 03.01018
'    Maximum timeout interval of 3 seconds exceeded.

Remarks

A Regex object is immutable, which means that it can be used only for the match pattern that you define when you create it. However, it can be used any number of times without being recompiled.

The matchTimeout parameter specifies how long a pattern-matching method should try to find a match before it times out. If no match is found in that time interval, the pattern-matching method throws a RegexMatchTimeoutException exception. matchTimeout overrides any default time-out value defined for the application domain in which the Regex object is created. The instance pattern-matching methods that observe the matchTimeout time-out interval include the following:

Setting a time-out interval prevents regular expressions that rely on excessive backtracking from appearing to stop responding when they process input that contains near matches. For more information, see Best Practices for Regular Expressions and Backtracking. To set a reasonable time-out interval, consider the following factors:

The length and complexity of the regular expression pattern. Longer and more complex regular expressions require more time than shorter and simpler ones.
The expected machine load. Processing takes more time on systems that have high CPU and memory utilization.

Notes to Callers

We recommend that you set the matchTimeout parameter to an appropriate value, such as two seconds. If you disable time-outs by specifying InfiniteMatchTimeout, the regular expression engine offers slightly better performance. However, you should disable time-outs only under the following conditions:

When the input processed by a regular expression is derived from a known and trusted source or consists of static text. This excludes text that has been dynamically input by users.
When the regular expression pattern has been thoroughly tested to ensure that it efficiently handles matches, non-matches, and near matches.
When the regular expression pattern contains no language elements that are known to cause excessive backtracking when processing a near match.

Applies to

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Regex Constructors

Definition

Overloads

Regex()

Remarks

Applies to

Regex(String)

Parameters

Exceptions

Examples

Remarks

Notes to Callers

See also

Applies to

Regex(SerializationInfo, StreamingContext)

Parameters

Exceptions

Applies to

Regex(String, RegexOptions)

Parameters

Exceptions

Examples

Remarks

Notes to Callers

See also

Applies to

Regex(String, RegexOptions, TimeSpan)

Parameters

Exceptions

Examples

Remarks

Notes to Callers

See also

Applies to

Additional resources