Regex Class
 

Represents an immutable regular expression.

To browse the .NET Framework source code for this type, see the Reference Source.

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


Syntax
[SerializableAttribute]
public class Regex : ISerializable
[SerializableAttribute]
public ref class Regex : ISerializable
[<SerializableAttribute>]
type Regex = 
    class
        interface ISerializable
    end
<SerializableAttribute>
Public Class Regex
	Implements ISerializable
Constructors
NameDescription
System_CAPS_protmethod Regex

Initializes a new instance of the Regex class.

System_CAPS_protmethod Regex

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

System_CAPS_pubmethod Regex

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

System_CAPS_pubmethod Regex

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

System_CAPS_pubmethod Regex

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.

Properties
NameDescription
System_CAPS_pubproperty System_CAPS_static

Gets or sets the maximum number of entries in the current static cache of compiled regular expressions.

System_CAPS_pubproperty

Gets the time-out interval of the current instance.

System_CAPS_pubproperty

Gets the options that were passed into the Regex constructor.

System_CAPS_pubproperty

Gets a value that indicates whether the regular expression searches from right to left.

Methods
NameDescription
System_CAPS_pubmethod System_CAPS_static CompileToAssembly

Compiles one or more specified Regex objects to a named assembly.

System_CAPS_pubmethod System_CAPS_static CompileToAssembly

Compiles one or more specified Regex objects to a named assembly with the specified attributes.

System_CAPS_pubmethod System_CAPS_static CompileToAssembly

Compiles one or more specified Regex objects and a specified resource file to a named assembly with the specified attributes.

System_CAPS_pubmethod Equals

Determines whether the specified object is equal to the current object.(Inherited from Object.)

System_CAPS_pubmethod System_CAPS_static Escape

Escapes 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.

System_CAPS_protmethod Finalize

Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)

System_CAPS_pubmethod GetGroupNames

Returns an array of capturing group names for the regular expression.

System_CAPS_pubmethod GetGroupNumbers

Returns an array of capturing group numbers that correspond to group names in an array.

System_CAPS_pubmethod GetHashCode

Serves as the default hash function. (Inherited from Object.)

System_CAPS_pubmethod GetType

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_pubmethod GroupNameFromNumber

Gets the group name that corresponds to the specified group number.

System_CAPS_pubmethod GroupNumberFromName

Returns the group number that corresponds to the specified group name.

System_CAPS_protmethod InitializeReferences

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_pubmethod IsMatch

Indicates whether the regular expression specified in the Regex constructor finds a match in a specified input string.

System_CAPS_pubmethod IsMatch

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.

System_CAPS_pubmethod System_CAPS_static IsMatch

Indicates whether the specified regular expression finds a match in the specified input string.

System_CAPS_pubmethod System_CAPS_static IsMatch

Indicates whether the specified regular expression finds a match in the specified input string, using the specified matching options.

System_CAPS_pubmethod System_CAPS_static IsMatch

Indicates whether the specified regular expression finds a match in the specified input string, using the specified matching options and time-out interval.

System_CAPS_pubmethod Match

Searches the specified input string for the first occurrence of the regular expression specified in the Regex constructor.

System_CAPS_pubmethod Match

Searches the input string for the first occurrence of a regular expression, beginning at the specified starting position in the string.

System_CAPS_pubmethod Match

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.

System_CAPS_pubmethod System_CAPS_static Match

Searches the specified input string for the first occurrence of the specified regular expression.

System_CAPS_pubmethod System_CAPS_static Match

Searches the input string for the first occurrence of the specified regular expression, using the specified matching options.

System_CAPS_pubmethod System_CAPS_static Match

Searches the input string for the first occurrence of the specified regular expression, using the specified matching options and time-out interval.

System_CAPS_pubmethod Matches

Searches the specified input string for all occurrences of a regular expression.

System_CAPS_pubmethod Matches

Searches the specified input string for all occurrences of a regular expression, beginning at the specified starting position in the string.

System_CAPS_pubmethod System_CAPS_static Matches

Searches the specified input string for all occurrences of a specified regular expression.

System_CAPS_pubmethod System_CAPS_static Matches

Searches the specified input string for all occurrences of a specified regular expression, using the specified matching options.

System_CAPS_pubmethod System_CAPS_static Matches

Searches the specified input string for all occurrences of a specified regular expression, using the specified matching options and time-out interval.

System_CAPS_protmethod MemberwiseClone

Creates a shallow copy of the current Object.(Inherited from Object.)

System_CAPS_pubmethod Replace

In a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate.

System_CAPS_pubmethod Replace

In 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.

System_CAPS_pubmethod Replace

In 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.

System_CAPS_pubmethod Replace

In a specified input string, replaces all strings that match a regular expression pattern with a specified replacement string.

System_CAPS_pubmethod Replace

In a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.

System_CAPS_pubmethod Replace

In a specified input substring, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.

System_CAPS_pubmethod System_CAPS_static Replace

In a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate.

System_CAPS_pubmethod System_CAPS_static Replace

In 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.

System_CAPS_pubmethod System_CAPS_static Replace

In a specified input string, replaces all substrings that match a specified regular expression with a string returned by a MatchEvaluator delegate. Additional parameters specify options that modify the matching operation and a time-out interval if no match is found.

System_CAPS_pubmethod System_CAPS_static Replace

In a specified input string, replaces all strings that match a specified regular expression with a specified replacement string.

System_CAPS_pubmethod System_CAPS_static Replace

In a specified input string, replaces all strings that match a specified regular expression with a specified replacement string. Specified options modify the matching operation.

System_CAPS_pubmethod System_CAPS_static Replace

In a specified input string, replaces all strings that match a specified regular expression with a specified replacement string. Additional parameters specify options that modify the matching operation and a time-out interval if no match is found.

System_CAPS_pubmethod Split

Splits an input string into an array of substrings at the positions defined by a regular expression pattern specified in the Regex constructor.

System_CAPS_pubmethod Split

Splits an input string a specified maximum number of times into an array of substrings, at the positions defined by a regular expression specified in the Regex constructor.

System_CAPS_pubmethod Split

Splits an input string a specified maximum number of times into an array of substrings, 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.

System_CAPS_pubmethod System_CAPS_static Split

Splits an input string into an array of substrings at the positions defined by a regular expression pattern.

System_CAPS_pubmethod System_CAPS_static Split

Splits an input string into an array of substrings at the positions defined by a specified regular expression pattern. Specified options modify the matching operation.

System_CAPS_pubmethod System_CAPS_static Split

Splits an input string into an array of substrings at the positions defined by a specified regular expression pattern. Additional parameters specify options that modify the matching operation and a time-out interval if no match is found.

System_CAPS_pubmethod ToString

Returns the regular expression pattern that was passed into the Regex constructor.(Overrides ObjectToString.)

System_CAPS_pubmethod System_CAPS_static Unescape

Converts any escaped characters in the input string.

System_CAPS_protmethod UseOptionC

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protmethod UseOptionR

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protmethod System_CAPS_static ValidateMatchTimeout

This API supports the product infrastructure and is not intended to be used directly from your code. Checks whether a time-out interval is within an acceptable range.

Fields
NameDescription
System_CAPS_protfield capnames

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protfield caps

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protfield capsize

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protfield capslist

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protfield factory

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_pubfield System_CAPS_static InfiniteMatchTimeout

Specifies that a pattern-matching operation should not time out.

System_CAPS_protfield internalMatchTimeout

This API supports the product infrastructure and is not intended to be used directly from your code. The maximum amount of time that can elapse in a pattern-matching operation before the operation times out.

System_CAPS_protfield pattern

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

System_CAPS_protfield roptions

This API supports the product infrastructure and is not intended to be used directly from your code. Used by a Regex object generated by the CompileToAssembly method.

Explicit Interface Implementations
NameDescription
System_CAPS_pubinterface System_CAPS_privmethod ISerializableGetObjectData

This API supports the product infrastructure and is not intended to be used directly from your code. Populates a SerializationInfo object with the data necessary to deserialize the current Regex object.

Remarks
System_CAPS_noteNote

To view the .NET Framework source code for this type, see the Reference Source. You can browse through the source code online, download the reference for offline viewing, and step through the sources (including patches and updates) during debugging; see instructions.

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; and to add the extracted strings to a collection to generate a report.

System_CAPS_noteNote

If your primary interest is to validate a string by determining whether it conforms to a particular pattern, you can use the System.ConfigurationRegexStringValidator 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 call a method that performs some operation, such as replacing text that matches the regular expression pattern, or identifying a pattern match.

System_CAPS_noteNote

For some common regular expression patterns, see Regular Expression Examples. There are also a number of online libraries of regular expression patterns, such as the one at Regular-Expressions.info.

For more information about using the Regex class, see the following sections in this topic:

  • Regex vs. String Methods

  • Static vs. Instance Methods

  • Performing Regular Expression Operations

  • Defining a Time-Out Value

For more information about the regular expression language, see Regular Expression Language - Quick Reference or download and print one of these brochures:

       Quick Reference in Word (.docx) format
       Quick Reference in PDF (.pdf) format

Regex vs. String Methods

The SystemString class includes several search and comparison methods that you can use to perform pattern matching with text. For example, the StringContains, StringEndsWith, and StringStartsWith methods determine whether a string instance contains a specified substring; and the StringIndexOf, StringIndexOfAny, StringLastIndexOf, and StringLastIndexOfAny methods return the starting position of a specified substring in a string. Use the methods of the SystemString 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.

Back to Remarks

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.

System_CAPS_importantImportant

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, caching 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 property.

  • When you instantiate 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 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();   
    Dim sr As New StreamReader(filename)
    Dim input As String
    Dim pattern As String = "\b(\w+)\s\1\b"
    Do While sr.Peek() >= 0
       input = sr.ReadLine()
       Dim rgx As New Regex(pattern, RegexOptions.IgnoreCase)
       Dim matches As MatchCollection = rgx.Matches(input)
       If matches.Count > 0 Then
          Console.WriteLine("{0} ({1} matches):", input, matches.Count)
          For Each match As Match In matches
             Console.WriteLine("   " + match.Value)
          Next   
       End If
    Loop
    sr.Close()   

    To prevent recompilation, you 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();   
    Dim sr As New StreamReader(filename)
    Dim input As String
    Dim pattern As String = "\b(\w+)\s\1\b"
    Dim rgx As New Regex(pattern, RegexOptions.IgnoreCase)
    Do While sr.Peek() >= 0
       input = sr.ReadLine()
       Dim matches As MatchCollection = rgx.Matches(input)
       If matches.Count > 0 Then
          Console.WriteLine("{0} ({1} matches):", input, matches.Count)
          For Each match As Match In matches
             Console.WriteLine("   " + match.Value)
          Next   
       End If
    Loop
    sr.Close()   

Back to Remarks

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 MatchNextMatch method.

  • Retrieval of all matches. You call the Matches method to retrieve a System.Text.RegularExpressionsMatchCollection 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.

Back to Remarks

Defining a Time-Out Value

The .NET Framework supports a full-featured regular expression language that provides substantial power and flexibility in pattern matching. However, the power and flexibility come at a cost: the risk of poor performance. Regular expressions that perform poorly are surprisingly easy to create. In some cases, regular expression operations that rely on excessive backtracking can appear to stop responding when they process text that nearly matches the regular expression pattern. For more information about the .NET Framework regular expression engine, see Details of Regular Expression Behavior. For more information about excessive backtracking, see Backtracking in Regular Expressions.

Starting with the .NET Framework 4.5, you can define a time-out interval for regular expression matches. If the regular expression engine cannot identify a match within this time interval, the matching operation throws a RegexMatchTimeoutException exception. In most cases, this prevents the regular expression engine from wasting processing power by trying to match text that nearly matches the regular expression pattern. It also could indicate, however, that the timeout interval has been set too low, or that the current machine load has caused an overall degradation in performance.

How you handle the exception depends on the cause of the exception. If the exception occurs because the time-out interval is set too low or because of excessive machine load, you can increase the time-out interval and retry the matching operation. If the exception occurs because the regular expression relies on excessive backtracking, you can assume that a match does not exist, and, optionally, you can log information that will help you modify the regular expression pattern.

You can set a time-out interval by calling the Regex constructor when you instantiate a regular expression object. For static methods, you can set a time-out interval by calling an overload of a matching method that has a matchTimeout parameter. If you do not set a time-out value explicitly, the default time-out value is determined as follows:

  • By using the application-wide time-out value, if one exists. This can be any time-out value that applies to the application domain in which the Regex object is instantiated or the static method call is made. You can set the application-wide time-out value by calling the AppDomainSetData method to assign the string representation of a TimeSpan value to the "REGEX_DEFAULT_MATCH_TIMEOUT" property.

  • By using the value InfiniteMatchTimeout, if no application-wide time-out value has been set.

System_CAPS_importantImportant

We recommend that you set a time-out value in all regular expression pattern-matching operations. For more information, see Best Practices for Regular Expressions in the .NET Framework.

Back to Remarks

Examples

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
Imports System
Imports System.Text.RegularExpressions

Public Module Test

    Public Sub Main()
        ' Define a regular expression for repeated words.
        Dim rx As New Regex("\b(?<word>\w+)\s+(\k<word>)\b", _
               RegexOptions.Compiled Or RegexOptions.IgnoreCase)

        ' Define a test string.        
        Dim text As String = "The the quick brown fox  fox jumped over the lazy dog dog."

        ' Find matches.
        Dim matches As MatchCollection = rx.Matches(text)

        ' Report the number of matches found.
        Console.WriteLine("{0} matches found in:", matches.Count)
        Console.WriteLine("   {0}", text)

        ' Report on each match.
        For Each match As Match In matches
            Dim groups As GroupCollection = match.Groups
            Console.WriteLine("'{0}' repeated at positions {1} and {2}", _ 
                              groups.Item("word").Value, _
                              groups.Item(0).Index, _
                              groups.Item(1).Index)
        Next
    End Sub
End Module
' 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
#using <System.dll>

using namespace System;
using namespace System::Text::RegularExpressions;
int main()
{
   // Define a regular expression for repeated words.
   Regex^ rx = gcnew Regex( "\\b(?<word>\\w+)\\s+(\\k<word>)\\b",static_cast<RegexOptions>(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.", matches->Count );

   // Report on each match.
   for each (Match^ match in matches)
   {
      String^ word = match->Groups["word"]->Value;
      int index = match->Index;
      Console::WriteLine("{0} repeated at position {1}", word, index);   
   }
}

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 , , , , and 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 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.
Imports System.Globalization
Imports System.Text.RegularExpressions

Public Module Example
   Public Sub Main()
      ' Get the current NumberFormatInfo object to build the regular 
      ' expression pattern dynamically.
      Dim nfi As NumberFormatInfo = NumberFormatInfo.CurrentInfo

      ' Define the regular expression pattern.
      Dim pattern As String 
      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}$"

      Dim rgx As New Regex(pattern)

      ' Define some test strings.
      Dim tests() As String = {"-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.
      For Each test As String In tests
         If rgx.IsMatch(test) Then
            Console.WriteLine("{0} is a currency value.", test)
         Else
            Console.WriteLine("{0} is not a currency value.", test)
         End If
      Next
   End Sub
End Module
' 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.

Version Information
Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
Thread Safety

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 in Regular Expressions.

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