Alternation Constructs in Regular Expressions

Alternation constructs modify a regular expression to enable either/or or conditional matching. .NET supports three alternation constructs:

• Pattern matching with |
• Conditional matching with (?(expression)yes|no)
• Conditional matching based on a valid captured group

Pattern Matching with |

You can use the vertical bar (|) character to match any one of a series of patterns, where the | character separates each pattern.

Like the positive character class, the | character can be used to match any one of a number of single characters. The following example uses both a positive character class and either/or pattern matching with the | character to locate occurrences of the words "gray" or "grey" in a string. In this case, the | character produces a regular expression that is more verbose.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      // Regular expression using character class.
      string pattern1 = @"\bgr[ae]y\b";
      // Regular expression using either/or.
      string pattern2 = @"\bgr(a|e)y\b";

      string input = "The gray wolf blended in among the grey rocks.";
      foreach (Match match in Regex.Matches(input, pattern1))
         Console.WriteLine("'{0}' found at position {1}",
                           match.Value, match.Index);
      Console.WriteLine();
      foreach (Match match in Regex.Matches(input, pattern2))
         Console.WriteLine("'{0}' found at position {1}",
                           match.Value, match.Index);
   }
}
// The example displays the following output:
//       'gray' found at position 4
//       'grey' found at position 35
//
//       'gray' found at position 4
//       'grey' found at position 35

Imports System.Text.RegularExpressions

Module Example
    Public Sub Main()
        ' Regular expression using character class.
        Dim pattern1 As String = "\bgr[ae]y\b"
        ' Regular expression using either/or.
        Dim pattern2 As String = "\bgr(a|e)y\b"

        Dim input As String = "The gray wolf blended in among the grey rocks."
        For Each match As Match In Regex.Matches(input, pattern1)
            Console.WriteLine("'{0}' found at position {1}", _
                              match.Value, match.Index)
        Next
        Console.WriteLine()
        For Each match As Match In Regex.Matches(input, pattern2)
            Console.WriteLine("'{0}' found at position {1}", _
                              match.Value, match.Index)
        Next
    End Sub
End Module
' The example displays the following output:
'       'gray' found at position 4
'       'grey' found at position 35
'       
'       'gray' found at position 4
'       'grey' found at position 35           

The regular expression that uses the | character, \bgr(a|e)y\b, is interpreted as shown in the following table:

The | character can also be used to perform an either/or match with multiple characters or subexpressions, which can include any combination of character literals and regular expression language elements. (The character class does not provide this functionality.) The following example uses the | character to extract either a U.S. Social Security Number (SSN), which is a 9-digit number with the format ddd-dd-dddd, or a U.S. Employer Identification Number (EIN), which is a 9-digit number with the format dd-ddddddd.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern = @"\b(\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b";
      string input = "01-9999999 020-333333 777-88-9999";
      Console.WriteLine("Matches for {0}:", pattern);
      foreach (Match match in Regex.Matches(input, pattern))
         Console.WriteLine("   {0} at position {1}", match.Value, match.Index);
   }
}
// The example displays the following output:
//       Matches for \b(\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b:
//          01-9999999 at position 0
//          777-88-9999 at position 22

Imports System.Text.RegularExpressions

Module Example
    Public Sub Main()
        Dim pattern As String = "\b(\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b"
        Dim input As String = "01-9999999 020-333333 777-88-9999"
        Console.WriteLine("Matches for {0}:", pattern)
        For Each match As Match In Regex.Matches(input, pattern)
            Console.WriteLine("   {0} at position {1}", match.Value, match.Index)
        Next
    End Sub
End Module
' The example displays the following output:
'       Matches for \b(\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b:
'          01-9999999 at position 0
'          777-88-9999 at position 22

The regular expression \b(\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b is interpreted as shown in the following table:

Conditional matching with an expression

This language element attempts to match one of two patterns depending on whether it can match an initial pattern. Its syntax is:

(?( expression ) yes )

or

(?( expression ) yes | no )

where expression is the initial pattern to match, yes is the pattern to match if expression is matched, and no is the optional pattern to match if expression is not matched (if a no pattern is not provided, it's equivalent to an empty no). The regular expression engine treats expression as a zero-width assertion; that is, the regular expression engine does not advance in the input stream after it evaluates expression. Therefore, this construct is equivalent to the following:

(?(?= expression ) yes | no )

where (?=expression) is a zero-width assertion construct. (For more information, see Grouping Constructs.) Because the regular expression engine interprets expression as an anchor (a zero-width assertion), expression must either be a zero-width assertion (for more information, see Anchors) or a subexpression that is also contained in yes. Otherwise, the yes pattern cannot be matched.

The following example is a variation of the example that appears in the Either/Or Pattern Matching with | section. It uses conditional matching to determine whether the first three characters after a word boundary are two digits followed by a hyphen. If they are, it attempts to match a U.S. Employer Identification Number (EIN). If not, it attempts to match a U.S. Social Security Number (SSN).

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern = @"\b(?(\d{2}-)\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b";
      string input = "01-9999999 020-333333 777-88-9999";
      Console.WriteLine("Matches for {0}:", pattern);
      foreach (Match match in Regex.Matches(input, pattern))
         Console.WriteLine("   {0} at position {1}", match.Value, match.Index);
   }
}
// The example displays the following output:
//       Matches for \b(\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b:
//          01-9999999 at position 0
//          777-88-9999 at position 22

Imports System.Text.RegularExpressions

Module Example
    Public Sub Main()
        Dim pattern As String = "\b(?(\d{2}-)\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b"
        Dim input As String = "01-9999999 020-333333 777-88-9999"
        Console.WriteLine("Matches for {0}:", pattern)
        For Each match As Match In Regex.Matches(input, pattern)
            Console.WriteLine("   {0} at position {1}", match.Value, match.Index)
        Next
    End Sub
End Module
' The example displays the following output:
'       Matches for \b(?(\d{2}-)\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b:
'          01-9999999 at position 0
'          777-88-9999 at position 22

The regular expression pattern \b(?(\d{2}-)\d{2}-\d{7}|\d{3}-\d{2}-\d{4})\b is interpreted as shown in the following table:

Conditional matching based on a valid captured group

This language element attempts to match one of two patterns depending on whether it has matched a specified capturing group. Its syntax is:

(?( name ) yes )

or

(?( name ) yes | no )

or

(?( number ) yes )

or

(?( number ) yes | no )

where name is the name and number is the number of a capturing group, yes is the expression to match if name or number has a match, and no is the optional expression to match if it does not (if a no pattern is not provided, it's equivalent to an empty no).

If name does not correspond to the name of a capturing group that is used in the regular expression pattern, the alternation construct is interpreted as an expression test, as explained in the previous section. Typically, this means that expression evaluates to false. If number does not correspond to a numbered capturing group that is used in the regular expression pattern, the regular expression engine throws an ArgumentException.

The following example is a variation of the example that appears in the Either/Or Pattern Matching with | section. It uses a capturing group named n2 that consists of two digits followed by a hyphen. The alternation construct tests whether this capturing group has been matched in the input string. If it has, the alternation construct attempts to match the last seven digits of a nine-digit U.S. Employer Identification Number (EIN). If it has not, it attempts to match a nine-digit U.S. Social Security Number (SSN).

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern = @"\b(?<n2>\d{2}-)?(?(n2)\d{7}|\d{3}-\d{2}-\d{4})\b";
      string input = "01-9999999 020-333333 777-88-9999";
      Console.WriteLine("Matches for {0}:", pattern);
      foreach (Match match in Regex.Matches(input, pattern))
         Console.WriteLine("   {0} at position {1}", match.Value, match.Index);
   }
}
// The example displays the following output:
//       Matches for \b(?<n2>\d{2}-)?(?(n2)\d{7}|\d{3}-\d{2}-\d{4})\b:
//          01-9999999 at position 0
//          777-88-9999 at position 22

Imports System.Text.RegularExpressions

Module Example
    Public Sub Main()
        Dim pattern As String = "\b(?<n2>\d{2}-)?(?(n2)\d{7}|\d{3}-\d{2}-\d{4})\b"
        Dim input As String = "01-9999999 020-333333 777-88-9999"
        Console.WriteLine("Matches for {0}:", pattern)
        For Each match As Match In Regex.Matches(input, pattern)
            Console.WriteLine("   {0} at position {1}", match.Value, match.Index)
        Next
    End Sub
End Module

The regular expression pattern \b(?<n2>\d{2}-)?(?(n2)\d{7}|\d{3}-\d{2}-\d{4})\b is interpreted as shown in the following table:

A variation of this example that uses a numbered group instead of a named group is shown in the following example. Its regular expression pattern is \b(\d{2}-)?(?(1)\d{7}|\d{3}-\d{2}-\d{4})\b.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern = @"\b(\d{2}-)?(?(1)\d{7}|\d{3}-\d{2}-\d{4})\b";
      string input = "01-9999999 020-333333 777-88-9999";
      Console.WriteLine("Matches for {0}:", pattern);
      foreach (Match match in Regex.Matches(input, pattern))
         Console.WriteLine("   {0} at position {1}", match.Value, match.Index);
   }
}
// The example display the following output:
//       Matches for \b(\d{2}-)?(?(1)\d{7}|\d{3}-\d{2}-\d{4})\b:
//          01-9999999 at position 0
//          777-88-9999 at position 22

Imports System.Text.RegularExpressions

Module Example
    Public Sub Main()
        Dim pattern As String = "\b(\d{2}-)?(?(1)\d{7}|\d{3}-\d{2}-\d{4})\b"
        Dim input As String = "01-9999999 020-333333 777-88-9999"
        Console.WriteLine("Matches for {0}:", pattern)
        For Each match As Match In Regex.Matches(input, pattern)
            Console.WriteLine("   {0} at position {1}", match.Value, match.Index)
        Next
    End Sub
End Module
' The example displays the following output:
'       Matches for \b(\d{2}-)?(?(1)\d{7}|\d{3}-\d{2}-\d{4})\b:
'          01-9999999 at position 0
'          777-88-9999 at position 22

See also

• Regular Expression Language - Quick Reference