Regex Class

Represents an immutable regular expression.

System.Object
  System.Text.RegularExpressions.Regex
    More...

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

[SerializableAttribute]
public class Regex : ISerializable

The Regex type exposes the following members.

  NameDescription
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryRegex()Initializes a new instance of the Regex class.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRegex(String)Initializes and compiles a new instance of the Regex class for the specified regular expression.
Protected methodRegex(SerializationInfo, StreamingContext)Initializes a new instance of the Regex class by using serialized data.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryRegex(String, RegexOptions)Initializes and compiles a new instance of the Regex class for the specified regular expression, with options that modify the pattern.
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  NameDescription
Public propertyStatic memberCacheSizeGets or sets the maximum number of entries in the current static cache of compiled regular expressions.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryOptionsReturns the options passed into the Regex constructor.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryRightToLeftGets a value indicating whether the regular expression searches from right to left.
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  NameDescription
Public methodStatic memberCompileToAssembly(RegexCompilationInfo[], AssemblyName)Compiles one or more specified Regex objects to a named assembly.
Public methodStatic memberCompileToAssembly(RegexCompilationInfo[], AssemblyName, CustomAttributeBuilder[])Compiles one or more specified Regex objects to a named assembly with the specified attributes.
Public methodStatic memberCompileToAssembly(RegexCompilationInfo[], AssemblyName, CustomAttributeBuilder[], String)Compiles one or more specified Regex objects and a specified resource file to a named assembly with the specified attributes.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryEscapeEscapes a minimal set of characters (\, *, +, ?, |, {, [, (,), ^, $,., #, and white space) by replacing them with their escape codes. This instructs the regular expression engine to interpret these characters literally rather than as metacharacters.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetGroupNamesReturns an array of capturing group names for the regular expression.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetGroupNumbersReturns an array of capturing group numbers that correspond to group names in an array.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGroupNameFromNumberGets the group name that corresponds to the specified group number.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGroupNumberFromNameReturns the group number that corresponds to the specified group name.
Protected methodInitializeReferencesInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryIsMatch(String)Indicates whether the regular expression specified in the Regex constructor finds a match in a specified input string.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryIsMatch(String, Int32)Indicates whether the regular expression specified in the Regex constructor finds a match in the specified input string, beginning at the specified starting position in the string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryIsMatch(String, String)Indicates whether the specified regular expression finds a match in the specified input string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryIsMatch(String, String, RegexOptions)Indicates whether the specified regular expression finds a match in the specified input string, using the specified matching options.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryMatch(String)Searches the specified input string for the first occurrence of the regular expression specified in the Regex constructor.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryMatch(String, Int32)Searches the input string for the first occurrence of a regular expression, beginning at the specified starting position in the string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryMatch(String, String)Searches the specified input string for the first occurrence of the specified regular expression.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryMatch(String, Int32, Int32)Searches the input string for the first occurrence of a regular expression, beginning at the specified starting position and searching only the specified number of characters.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryMatch(String, String, RegexOptions)Searches the input string for the first occurrence of the specified regular expression, using the specified matching options.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryMatches(String)Searches the specified input string for all occurrences of a regular expression.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryMatches(String, Int32)Searches the specified input string for all occurrences of a regular expression, beginning at the specified starting position in the string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryMatches(String, String)Searches the specified input string for all occurrences of a specified regular expression.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryMatches(String, String, RegexOptions)Searches the specified input string for all occurrences of a specified regular expression, using the specified matching options.
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String)Within a specified input string, replaces all strings that match a regular expression pattern with a specified replacement string.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, MatchEvaluator)Within a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String, Int32)Within a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String, String)Within a specified input string, replaces all strings that match a specified regular expression with a specified replacement string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String, MatchEvaluator)Within a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, MatchEvaluator, Int32)Within a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a string returned by a MatchEvaluator delegate.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String, Int32, Int32)Within a specified input substring, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String, String, RegexOptions)Within a specified input string, replaces all strings that match a specified regular expression with a specified replacement string. Specified options modify the matching operation.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, String, MatchEvaluator, RegexOptions)Within a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate. Specified options modify the matching operation.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryReplace(String, MatchEvaluator, Int32, Int32)Within a specified input substring, replaces a specified maximum number of strings that match a regular expression pattern with a string returned by a MatchEvaluator delegate.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySplit(String)Splits the specified input string at the positions defined by a regular expression pattern specified in the Regex constructor.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySplit(String, Int32)Splits the specified input string a specified maximum number of times at the positions defined by a regular expression specified in the Regex constructor.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibrarySplit(String, String)Splits the input string at the positions defined by a regular expression pattern.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySplit(String, Int32, Int32)Splits the specified input string a specified maximum number of times at the positions defined by a regular expression specified in the Regex constructor. The search for the regular expression pattern starts at a specified character position in the input string.
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibrarySplit(String, String, RegexOptions)Splits the input string at the positions defined by a specified regular expression pattern. Specified options modify the matching operation.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryToStringReturns the regular expression pattern that was passed into the Regex constructor. (Overrides Object.ToString().)
Public methodStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryUnescapeConverts any escaped characters in the input string.
Protected methodSupported by the XNA FrameworkUseOptionCInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected methodSupported by the XNA FrameworkUseOptionRInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
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  NameDescription
Protected fieldSupported by the XNA FrameworkcapnamesInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected fieldSupported by the XNA FrameworkcapsInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected fieldSupported by the XNA FrameworkcapsizeInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected fieldSupported by the XNA FrameworkcapslistInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected fieldfactoryInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected fieldSupported by the XNA FrameworkpatternInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
Protected fieldSupported by the XNA FrameworkroptionsInfrastructure. Used by a Regex object generated by the CompileToAssembly method.
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  NameDescription
Explicit interface implemetationPrivate methodISerializable.GetObjectDataInfrastructure. Populates a SerializationInfo object with the data necessary to deserialize the current Regex object.
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The Regex class represents the .NET Framework's regular expression engine. It can be used to quickly parse large amounts of text to find specific character patterns; to extract, edit, replace, or delete text substrings; or to add the extracted strings to a collection to generate a report.

NoteNote

If your primary interest is to validate a string by determining whether it conforms to a particular pattern, you can use the System.Configuration.RegexStringValidator class.

To use regular expressions, you define the pattern that you want to identify in a text stream by using the syntax documented in Regular Expression Language - Quick Reference. Next, you can optionally instantiate a Regex object. Finally, you perform some operation, such as replacing text that matches the regular expression pattern, or identifying a pattern match.

Regex vs. String Methods

The System.String class includes several search and comparison methods that you can use to perform pattern matching with text. For example, the String.Contains, String.EndsWith, and String.StartsWith methods determine whether a string instance contains a specified substring; and the String.IndexOf, String.IndexOfAny, String.LastIndexOf, and String.LastIndexOfAny methods return the starting position of a specified substring in a string. Use the methods of the System.String class when you are searching for a specific string. Use the Regex class when you are searching for a specific pattern in a string. For more information and examples, see .NET Framework Regular Expressions.

Static vs. Instance Methods

After you define a regular expression pattern, you can provide it to the regular expression engine in either of two ways.

  • By instantiating a Regex object that represents the regular expression. To do this, you pass the regular expression pattern to a Regex constructor. A Regex object is immutable; when you instantiate a Regex object with a regular expression, that object's regular expression cannot be changed.

  • By supplying both the regular expression and the text to search to a static (Shared in Visual Basic) Regex method. This enables you to use a regular expression without explicitly creating a Regex object.

All Regex pattern identification methods include both static and instance overloads.

The regular expression engine must compile a particular pattern before the pattern can be used. Because Regex objects are immutable, this is a one-time procedure that occurs when a Regex class constructor or a static method is called. To eliminate the need to repeatedly compile a single regular expression, the regular expression engine caches the compiled regular expressions used in static method calls. As a result, regular expression pattern-matching methods offer comparable performance for static and instance methods.

Important noteImportant

In the .NET Framework versions 1.0 and 1.1, all compiled regular expressions, whether they were used in instance or static method calls, were cached. Starting with the .NET Framework 2.0, only regular expressions used in static method calls are cached.

However, the system of caching implemented by the regular expression engine can adversely affect performance in the following two cases:

  • When you use static method calls with a large number of regular expressions. By default, the regular expression engine caches the 15 most recently used static regular expressions. If your application uses more than 15 static regular expressions, some regular expressions must be recompiled. To prevent this recompilation, you can increase the Regex.CacheSize property to an appropriate value.

  • When your application instantiates new Regex objects with regular expressions that have previously been compiled. For example, the following code defines a regular expression to locate duplicated words in individual lines of a text stream. Although the example uses a single regular expression, it instantiates a new Regex object to process each line of text. This results in the recompilation of the regular expression with each iteration of the loop.

    
    StreamReader sr = new StreamReader(filename);
    string input;
    string pattern = @"\b(\w+)\s\1\b";
    while (sr.Peek() >= 0)
    {
       input = sr.ReadLine();
       Regex rgx = new Regex(pattern, RegexOptions.IgnoreCase);
       MatchCollection matches = rgx.Matches(input);
       if (matches.Count > 0)
       {
          Console.WriteLine("{0} ({1} matches):", input, matches.Count);
          foreach (Match match in matches)
             Console.WriteLine("   " + match.Value);
       }
    }
    sr.Close();   
    
    
    

    To prevent recompilation, the application should instantiate a single Regex object that is accessible to all code that requires it, as shown in the following rewritten example.

    
    StreamReader sr = new StreamReader(filename);
    string input;
    string pattern = @"\b(\w+)\s\1\b";
    Regex rgx = new Regex(pattern, RegexOptions.IgnoreCase);
    
    while (sr.Peek() >= 0)
    {
       input = sr.ReadLine();
       MatchCollection matches = rgx.Matches(input);
       if (matches.Count > 0)
       {
          Console.WriteLine("{0} ({1} matches):", input, matches.Count);
          foreach (Match match in matches)
             Console.WriteLine("   " + match.Value);
       }
    }
    sr.Close();   
    
    
    

Performing Regular Expression Operations

Whether you decide to instantiate a Regex object and call its methods or call static methods, the Regex class offers the following pattern-matching functionality:

  • Validation of a match. You call the IsMatch method to determine whether a match is present.

  • Retrieval of a single match. You call the Match method to retrieve a Match object that represents the first match in a string or in part of a string. Subsequent matches can be retrieved by calling the Match.NextMatch method.

  • Retrieval of all matches. You call the Matches method to retrieve a System.Text.RegularExpressions.MatchCollection object that represents all the matches found in a string or in part of a string.

  • Replacement of matched text. You call the Replace method to replace matched text. The replacement text can also be defined by a regular expression. In addition, some of the Replace methods include a MatchEvaluator parameter that enables you to programmatically define the replacement text.

  • Creation of a string array that is formed from parts of an input string. You call the Split method to split an input string at positions that are defined by the regular expression.

In addition to its pattern-matching methods, the Regex class includes several special-purpose methods:

  • The Escape method escapes any characters that may be interpreted as regular expression operators in a regular expression or input string.

  • The Unescape method removes these escape characters.

  • The CompileToAssembly method creates an assembly that contains predefined regular expressions. The .NET Framework contains examples of these special-purpose assemblies in the System.Web.RegularExpressions namespace.

The following example uses a regular expression to check for repeated occurrences of words in a string. The regular expression \b(?<word>\w+)\s+(\k<word>)\b can be interpreted as shown in the following table.

Pattern

Description

\b

Start the match at a word boundary.

(?<word>\w+)

Match one or more word characters up to a word boundary. Name this captured group word.

\s+

Match one or more white-space characters.

(\k<word>)

Match the captured group that is named word.

\b

Match a word boundary.


using System;
using System.Text.RegularExpressions;

public class Test
{

    public static void Main ()
    {

        // Define a regular expression for repeated words.
        Regex rx = new Regex(@"\b(?<word>\w+)\s+(\k<word>)\b",
          RegexOptions.Compiled | RegexOptions.IgnoreCase);

        // Define a test string.        
        string text = "The the quick brown fox  fox jumped over the lazy dog dog.";

        // Find matches.
        MatchCollection matches = rx.Matches(text);

        // Report the number of matches found.
        Console.WriteLine("{0} matches found in:\n   {1}", 
                          matches.Count, 
                          text);

        // Report on each match.
        foreach (Match match in matches)
        {
            GroupCollection groups = match.Groups;
            Console.WriteLine("'{0}' repeated at positions {1} and {2}",  
                              groups["word"].Value, 
                              groups[0].Index, 
                              groups[1].Index);
        }

    }
	
}
// The example produces the following output to the console:
//       3 matches found in:
//          The the quick brown fox  fox jumped over the lazy dog dog.
//       'The' repeated at positions 0 and 4
//       'fox' repeated at positions 20 and 25
//       'dog' repeated at positions 50 and 54


The following example illustrates the use of a regular expression to check whether a string either represents a currency value or has the correct format to represent a currency value. In this case, the regular expression is built dynamically from the NumberFormatInfo.CurrencyDecimalSeparator, CurrencyDecimalDigits, NumberFormatInfo.CurrencySymbol, NumberFormatInfo.NegativeSign, and NumberFormatInfo.PositiveSign properties for the user's current culture. If the system's current culture is en-US, the resulting regular expression is ^\s*[\+-]?\s?\$?\s?(\d*\.?\d{2}?){1}$. This regular expression can be interpreted as shown in the following table.

Pattern

Description

^

Start at the beginning of the string.

\s*

Match zero or more white-space characters.

[\+-]?

Match zero or one occurrence of either the positive sign or the negative sign.

\s?

Match zero or one white-space character.

\$?

Match zero or one occurrence of the dollar sign.

\s?

Match zero or one white-space character.

\d*

Match zero or more decimal digits.

\.?

Match zero or one decimal point symbol.

\d{2}?

Match two decimal digits zero or one time.

(\d*\.?\d{2}?){1}

Match the pattern of integral and fractional digits separated by a decimal point symbol at least one time.

$

Match the end of the string.

In this case, the regular expression assumes that a valid currency string does not contain group separator symbols, and that it has either no fractional digits or the number of fractional digits defined by the current culture's CurrencyDecimalDigits property.


using System;
using System.Globalization;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      // Get the current NumberFormatInfo object to build the regular 
      // expression pattern dynamically.
      NumberFormatInfo nfi = NumberFormatInfo.CurrentInfo;

      // Define the regular expression pattern.
      string pattern; 
      pattern = @"^\s*[";
      // Get the positive and negative sign symbols.
      pattern += Regex.Escape(nfi.PositiveSign + nfi.NegativeSign) + @"]?\s?";
      // Get the currency symbol.
      pattern += Regex.Escape(nfi.CurrencySymbol) + @"?\s?";
      // Add integral digits to the pattern.
      pattern += @"(\d*";
      // Add the decimal separator.
      pattern += Regex.Escape(nfi.CurrencyDecimalSeparator) + "?";
      // Add the fractional digits.
      pattern += @"\d{";
      // Determine the number of fractional digits in currency values.
      pattern += nfi.CurrencyDecimalDigits.ToString() + "}?){1}$";

      Regex rgx = new Regex(pattern);

      // Define some test strings.
      string[] tests = { "-42", "19.99", "0.001", "100 USD", 
                         ".34", "0.34", "1,052.21", "$10.62", 
                         "+1.43", "-$0.23" };

      // Check each test string against the regular expression.
      foreach (string test in tests)
      {
         if (rgx.IsMatch(test))
            Console.WriteLine("{0} is a currency value.", test);
         else
            Console.WriteLine("{0} is not a currency value.", test);
      }
   }
}
// The example displays the following output:
//       -42 is a currency value.
//       19.99 is a currency value.
//       0.001 is not a currency value.
//       100 USD is not a currency value.
//       .34 is a currency value.
//       0.34 is a currency value.
//       1,052.21 is not a currency value.
//       $10.62 is a currency value.
//       +1.43 is a currency value.
//       -$0.23 is a currency value.


Because the regular expression in this example is built dynamically, we do not know at design time whether the current culture's currency symbol, decimal sign, or positive and negative signs might be misinterpreted by the regular expression engine as regular expression language operators. To prevent any misinterpretation, the example passes each dynamically generated string to the Escape method.

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

The Regex class is immutable (read-only) and thread safe. Regex objects can be created on any thread and shared between threads. For more information, see Thread Safety.

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