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.NET regular expressions

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Regular expressions provide a powerful, flexible, and efficient method for processing text. The extensive pattern-matching notation of regular expressions enables you to quickly parse large amounts of text to:

  • Find specific character patterns.
  • Validate text to ensure that it matches a predefined pattern (such as an email address).
  • Extract, edit, replace, or delete text substrings.
  • Add extracted strings to a collection in order to generate a report.

For many applications that deal with strings or that parse large blocks of text, regular expressions are an indispensable tool.

How regular expressions work

The centerpiece of text processing with regular expressions is the regular expression engine, which is represented by the System.Text.RegularExpressions.Regex object in .NET. At a minimum, processing text using regular expressions requires that the regular expression engine be provided with the following two items of information:

  • The regular expression pattern to identify in the text.

    In .NET, regular expression patterns are defined by a special syntax or language, which is compatible with Perl 5 regular expressions and adds some additional features such as right-to-left matching. For more information, see Regular Expression Language - Quick Reference.

  • The text to parse for the regular expression pattern.

The methods of the Regex class let you perform the following operations:

For an overview of the regular expression object model, see The Regular Expression Object Model.

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

Regular expression examples

The String class includes string search and replacement methods that you can use when you want to locate literal strings in a larger string. Regular expressions are most useful either when you want to locate one of several substrings in a larger string, or when you want to identify patterns in a string, as the following examples illustrate.

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions, causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

Tip

The System.Web.RegularExpressions namespace contains a number of regular expression objects that implement predefined regular expression patterns for parsing strings from HTML, XML, and ASP.NET documents. For example, the TagRegex class identifies start tags in a string, and the CommentRegex class identifies ASP.NET comments in a string.

Example 1: Replace substrings

Assume that a mailing list contains names that sometimes include a title (Mr., Mrs., Miss, or Ms.) along with a first and last name. Suppose you don't want to include the titles when you generate envelope labels from the list. In that case, you can use a regular expression to remove the titles, as the following example illustrates:

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern = "(Mr\\.? |Mrs\\.? |Miss |Ms\\.? )";
      string[] names = { "Mr. Henry Hunt", "Ms. Sara Samuels",
                         "Abraham Adams", "Ms. Nicole Norris" };
      foreach (string name in names)
         Console.WriteLine(Regex.Replace(name, pattern, String.Empty));
   }
}
// The example displays the following output:
//    Henry Hunt
//    Sara Samuels
//    Abraham Adams
//    Nicole Norris

Imports System.Text.RegularExpressions

Module Example
    Public Sub Main()
        Dim pattern As String = "(Mr\.? |Mrs\.? |Miss |Ms\.? )"
        Dim names() As String = {"Mr. Henry Hunt", "Ms. Sara Samuels", _
                                  "Abraham Adams", "Ms. Nicole Norris"}
        For Each name As String In names
            Console.WriteLine(Regex.Replace(name, pattern, String.Empty))
        Next
    End Sub
End Module
' The example displays the following output:
'    Henry Hunt
'    Sara Samuels
'    Abraham Adams
'    Nicole Norris

The regular expression pattern (Mr\.? |Mrs\.? |Miss |Ms\.? ) matches any occurrence of "Mr ", "Mr. ", "Mrs ", "Mrs. ", "Miss ", "Ms ", or "Ms. ". The call to the Regex.Replace method replaces the matched string with String.Empty; in other words, it removes it from the original string.

Example 2: Identify duplicated words

Accidentally duplicating words is a common error that writers make. Use a regular expression to identify duplicated words, as the following example shows:

using System;
using System.Text.RegularExpressions;

public class Class1
{
   public static void Main()
   {
      string pattern = @"\b(\w+?)\s\1\b";
      string input = "This this is a nice day. What about this? This tastes good. I saw a a dog.";
      foreach (Match match in Regex.Matches(input, pattern, RegexOptions.IgnoreCase))
         Console.WriteLine("{0} (duplicates '{1}') at position {2}",
                           match.Value, match.Groups[1].Value, match.Index);
   }
}
// The example displays the following output:
//       This this (duplicates 'This') at position 0
//       a a (duplicates 'a') at position 66

Imports System.Text.RegularExpressions

Module modMain
    Public Sub Main()
        Dim pattern As String = "\b(\w+?)\s\1\b"
        Dim input As String = "This this is a nice day. What about this? This tastes good. I saw a a dog."
        For Each match As Match In Regex.Matches(input, pattern, RegexOptions.IgnoreCase)
            Console.WriteLine("{0} (duplicates '{1}') at position {2}", _
                              match.Value, match.Groups(1).Value, match.Index)
        Next
    End Sub
End Module
' The example displays the following output:
'       This this (duplicates 'This') at position 0
'       a a (duplicates 'a') at position 66

The regular expression pattern \b(\w+?)\s\1\b can be interpreted as follows:

Pattern Interpretation
\b Start at a word boundary.
(\w+?) Match one or more word characters, but as few characters as possible. Together, they form a group that can be referred to as \1.
\s Match a white-space character.
\1 Match the substring that's equal to the group named \1.
\b Match a word boundary.

The Regex.Matches method is called with regular expression options set to RegexOptions.IgnoreCase. Therefore, the match operation is case-insensitive, and the example identifies the substring "This this" as a duplication.

The input string includes the substring "this? This". However, because of the intervening punctuation mark, it isn't identified as a duplication.

Example 3: Dynamically build a culture-sensitive regular expression

The following example illustrates the power of regular expressions combined with the flexibility offered by .NET's globalization features. It uses the NumberFormatInfo object to determine the format of currency values in the system's current culture. It then uses that information to dynamically construct a regular expression that extracts currency values from the text. For each match, it extracts the subgroup that contains the numeric string only, converts it to a Decimal value, and calculates a running total.

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      // Define text to be parsed.
      string input = "Office expenses on 2/13/2008:\n" +
                     "Paper (500 sheets)                      $3.95\n" +
                     "Pencils (box of 10)                     $1.00\n" +
                     "Pens (box of 10)                        $4.49\n" +
                     "Erasers                                 $2.19\n" +
                     "Ink jet printer                        $69.95\n\n" +
                     "Total Expenses                        $ 81.58\n";

      // Get current culture's NumberFormatInfo object.
      NumberFormatInfo nfi = CultureInfo.CurrentCulture.NumberFormat;
      // Assign needed property values to variables.
      string currencySymbol = nfi.CurrencySymbol;
      bool symbolPrecedesIfPositive = nfi.CurrencyPositivePattern % 2 == 0;
      string groupSeparator = nfi.CurrencyGroupSeparator;
      string decimalSeparator = nfi.CurrencyDecimalSeparator;

      // Form regular expression pattern.
      string pattern = Regex.Escape( symbolPrecedesIfPositive ? currencySymbol : "") +
                       @"\s*[-+]?" + "([0-9]{0,3}(" + groupSeparator + "[0-9]{3})*(" +
                       Regex.Escape(decimalSeparator) + "[0-9]+)?)" +
                       (! symbolPrecedesIfPositive ? currencySymbol : "");
      Console.WriteLine( "The regular expression pattern is:");
      Console.WriteLine("   " + pattern);

      // Get text that matches regular expression pattern.
      MatchCollection matches = Regex.Matches(input, pattern,
                                              RegexOptions.IgnorePatternWhitespace);
      Console.WriteLine("Found {0} matches.", matches.Count);

      // Get numeric string, convert it to a value, and add it to List object.
      List<decimal> expenses = new List<Decimal>();

      foreach (Match match in matches)
         expenses.Add(Decimal.Parse(match.Groups[1].Value));

      // Determine whether total is present and if present, whether it is correct.
      decimal total = 0;
      foreach (decimal value in expenses)
         total += value;

      if (total / 2 == expenses[expenses.Count - 1])
         Console.WriteLine("The expenses total {0:C2}.", expenses[expenses.Count - 1]);
      else
         Console.WriteLine("The expenses total {0:C2}.", total);
   }
}
// The example displays the following output:
//       The regular expression pattern is:
//          \$\s*[-+]?([0-9]{0,3}(,[0-9]{3})*(\.[0-9]+)?)
//       Found 6 matches.
//       The expenses total $81.58.

Imports System.Collections.Generic
Imports System.Globalization
Imports System.Text.RegularExpressions

Public Module Example
    Public Sub Main()
        ' Define text to be parsed.
        Dim input As String = "Office expenses on 2/13/2008:" + vbCrLf + _
                              "Paper (500 sheets)                      $3.95" + vbCrLf + _
                              "Pencils (box of 10)                     $1.00" + vbCrLf + _
                              "Pens (box of 10)                        $4.49" + vbCrLf + _
                              "Erasers                                 $2.19" + vbCrLf + _
                              "Ink jet printer                        $69.95" + vbCrLf + vbCrLf + _
                              "Total Expenses                        $ 81.58" + vbCrLf
        ' Get current culture's NumberFormatInfo object.
        Dim nfi As NumberFormatInfo = CultureInfo.CurrentCulture.NumberFormat
        ' Assign needed property values to variables.
        Dim currencySymbol As String = nfi.CurrencySymbol
        Dim symbolPrecedesIfPositive As Boolean = CBool(nfi.CurrencyPositivePattern Mod 2 = 0)
        Dim groupSeparator As String = nfi.CurrencyGroupSeparator
        Dim decimalSeparator As String = nfi.CurrencyDecimalSeparator

        ' Form regular expression pattern.
        Dim pattern As String = Regex.Escape(CStr(IIf(symbolPrecedesIfPositive, currencySymbol, ""))) + _
                                "\s*[-+]?" + "([0-9]{0,3}(" + groupSeparator + "[0-9]{3})*(" + _
                                Regex.Escape(decimalSeparator) + "[0-9]+)?)" + _
                                CStr(IIf(Not symbolPrecedesIfPositive, currencySymbol, ""))
        Console.WriteLine("The regular expression pattern is: ")
        Console.WriteLine("   " + pattern)

        ' Get text that matches regular expression pattern.
        Dim matches As MatchCollection = Regex.Matches(input, pattern, RegexOptions.IgnorePatternWhitespace)
        Console.WriteLine("Found {0} matches. ", matches.Count)

        ' Get numeric string, convert it to a value, and add it to List object.
        Dim expenses As New List(Of Decimal)

        For Each match As Match In matches
            expenses.Add(Decimal.Parse(match.Groups.Item(1).Value))
        Next

        ' Determine whether total is present and if present, whether it is correct.
        Dim total As Decimal
        For Each value As Decimal In expenses
            total += value
        Next

        If total / 2 = expenses(expenses.Count - 1) Then
            Console.WriteLine("The expenses total {0:C2}.", expenses(expenses.Count - 1))
        Else
            Console.WriteLine("The expenses total {0:C2}.", total)
        End If
    End Sub
End Module
' The example displays the following output:
'       The regular expression pattern is:
'          \$\s*[-+]?([0-9]{0,3}(,[0-9]{3})*(\.[0-9]+)?)
'       Found 6 matches.
'       The expenses total $81.58.

On a computer whose current culture is English - United States (en-US), the example dynamically builds the regular expression \$\s*[-+]?([0-9]{0,3}(,[0-9]{3})*(\.[0-9]+)?). This regular expression pattern can be interpreted as follows:

Pattern Interpretation
\$ Look for a single occurrence of the dollar symbol ($) in the input string. The regular expression pattern string includes a backslash to indicate that the dollar symbol is to be interpreted literally rather than as a regular expression anchor. The $ symbol alone would indicate that the regular expression engine should try to begin its match at the end of a string. To ensure that the current culture's currency symbol isn't misinterpreted as a regular expression symbol, the example calls the Regex.Escape method to escape the character.
\s* Look for zero or more occurrences of a white-space character.
[-+]? Look for zero or one occurrence of either a positive or negative sign.
([0-9]{0,3}(,[0-9]{3})*(\.[0-9]+)?) The outer parentheses define this expression as a capturing group or a subexpression. If a match is found, information about this part of the matching string can be retrieved from the second Group object in the GroupCollection object returned by the Match.Groups property. The first element in the collection represents the entire match.
[0-9]{0,3} Look for zero to three occurrences of the decimal digits 0 through 9.
(,[0-9]{3})* Look for zero or more occurrences of a group separator followed by three decimal digits.
\. Look for a single occurrence of the decimal separator.
[0-9]+ Look for one or more decimal digits.
(\.[0-9]+)? Look for zero or one occurrence of the decimal separator followed by at least one decimal digit.

If each subpattern is found in the input string, the match succeeds, and a Match object that contains information about the match is added to the MatchCollection object.

Title Description
Regular Expression Language - Quick Reference Provides information on the set of characters, operators, and constructs that you can use to define regular expressions.
The Regular Expression Object Model Provides information and code examples that illustrate how to use the regular expression classes.
Details of Regular Expression Behavior Provides information about the capabilities and behavior of .NET regular expressions.
Use regular expressions in Visual Studio

Reference