.NET regular expressions

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:

• Determine whether the regular expression pattern occurs in the input text by calling the Regex.IsMatch method. For an example that uses the IsMatch method for validating text, see How to: Verify that Strings Are in Valid Email Format.

• Retrieve one or all occurrences of text that matches the regular expression pattern by calling the Regex.Match or Regex.Matches method. The former method returns a System.Text.RegularExpressions.Match object that provides information about the matching text. The latter returns a MatchCollection object that contains one System.Text.RegularExpressions.Match object for each match found in the parsed text.

• Replace text that matches the regular expression pattern by calling the Regex.Replace method. For examples that use the Replace method to change date formats and remove invalid characters from a string, see How to: Strip Invalid Characters from a String and Example: Changing Date Formats.

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:

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

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.

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:

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:

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.

Related articles

Reference

• System.Text.RegularExpressions
• System.Text.RegularExpressions.Regex
• Regular Expressions - Quick Reference (download in Word format)
• Regular Expressions - Quick Reference (download in PDF format)