Content

Regular Expression 
    Alternation Constructs
 .NET Alternation Constructs
  Pattern Matching with |
  Conditional Matching with an Expression
  Conditional Matching Based on a Valid Captured Group
 Examples
 See also
 Source/Referencee

Regular Expression Alternation Constructs

Alternation constructs modify a regular expression to enable either/or or conditional matching.

.NET Alternation Constructs

.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 |

The vertical bar (|) character can be used 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.

Conditional Matching with an Expression

The conditional matching with an expression to match one of two patterns depending on whether it can match an initial pattern. The conditional matching with an expression is of the form, (?( expression ) yes | no ). The parameter expression is the initial pattern to match. The parameter yes is the pattern to match if expression is matched. The parameter no is the optional pattern to match if parameter expression is not matched. 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 (?(?= expression ) yes | no ). The parameter, (?= expression ) is a zero-width assertion construct. Because the rgular expression engine interprets expression as an anchor (a zero-width assertion), expression must either be a zero-width assertion or a subexpression that is also contained in yes. Otherwise, the yes pattern cannot be matched.

If expressionis a named or numbered capturing group, the alternation construct is interpreted as a capture test; In other words, the regular expression engine does not attempt to match the captured substring, but instead tests for the presence or absence of the group.

Conditional Matching Based on a Valid Captured Group

The conditional matching based on a valid captured group attempts to match one of two patterns depending on whether it has matched a specified capturing group. The conditional matching based on a valid captured group is of the form (?( name ) yes | no ) or (?( number ) yes | no ).

the parameter name is the name and the parameter number is the number of a capturing group. The parameter yes is the expression to match if name or number has a match, and the parameter no is the optional expression to match if it does not.

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. 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.

Examples

Examples of Alternation Constructs

ASP.NET Code Input:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
    <head>
       <title>Sample Page</title>
       <meta http-equiv="Content-Type" content="text/html;charset=utf-8">
       <script runat="server">
           Sub Page_Load()
               Dim xstring As String = "zabcd fg ijaab"
               Dim xmatchstr As String = ""
               xmatchstr = xmatchstr & "Given string: """ & xstring & """<br />"
               xmatchstr = xmatchstr & showresult(xstring,"(\w)(a|b)b")
               xmatchstr = xmatchstr & showresult(xstring,"(\w)(?(a)a|b)b")
               xmatchstr = xmatchstr & showresult(xstring,"(\w)*(?(ab)abc|bc).*")
               xmatchstr = xmatchstr & showresult(xstring,"(\w)*(?(bc)bc|abc).*")
               xmatchstr = xmatchstr & showresult(xstring,"(?<zero>\w)*(?(1)bc|abc).*")
               xmatchstr = xmatchstr & showresult(xstring,"(?<zero>\w)*(?(zero)bc|abc).*")
               lbl01.Text = xmatchstr
           End Sub
           Function showresult(xstring,xpattern)
               Dim xmatch As Match
               Dim xcaptures As CaptureCollection
               Dim ycaptures As CaptureCollection
               Dim xgroups As GroupCollection
               Dim xmatchstr As String = ""
               Dim xint As Integer
               Dim yint As Integer
               Dim zint As Integer
               xmatchstr = xmatchstr & "<br />Result of Regex.Match(string,""" & Replace(xpattern,"<","&lt;") & """): """
               xmatch = Regex.Match(xstring,xpattern)
               xmatchstr = xmatchstr & xmatch.value & """<br />"
               xcaptures = xmatch.Captures
               xmatchstr = xmatchstr & "->Result of CaptureCollection.Count: """
               xmatchstr = xmatchstr & xcaptures.Count & """<br />"
               For xint = 0 to xcaptures.Count - 1
                   xmatchstr = xmatchstr & "->->Result of CaptureCollection("& xint & ").Value, Index, Length: """
                   xmatchstr = xmatchstr & xcaptures(xint).Value & ", " & xcaptures(xint).Index & ", " & xcaptures(xint).Length & """<br />"
                   xgroups = xmatch.Groups
                   xmatchstr = xmatchstr & "->Result of GroupCollection.Count: """
                   xmatchstr = xmatchstr & xgroups.Count & """<br />"
                   For yint = 0 to xgroups.Count - 1
                       xmatchstr = xmatchstr & "->->Result of GroupCollection("& yint & ").Value, Index, Length: """
                       xmatchstr = xmatchstr & xgroups(yint).Value & ", " & xgroups(yint).Index & ", " & xgroups(yint).Length & """<br />"
                       ycaptures = xgroups(yint).Captures
                       xmatchstr = xmatchstr & "->->->Result of CaptureCollection.Count: """
                       xmatchstr = xmatchstr & ycaptures.Count & """<br />"
                       For zint = 0 to ycaptures.Count - 1
                           xmatchstr = xmatchstr & "->->->->Result of CaptureCollection("& zint & ").Value, Index, Length: """
                           xmatchstr = xmatchstr & ycaptures(zint).Value & ", " & ycaptures(zint).Index & ", " & ycaptures(zint).Length & """<br />"
                       Next
                   Next
               Next
               Return xmatchstr
           End Function
       </script>
    </head>
    <body>
       <% Response.Write ("<h1>This is a Sample Page of Alternation Constructs</h1>") %>
       <p>
           <%-- Set on Page_Load --%>
           <asp:Label id="lbl01" runat="server" />
       </p>
    </body>
</html>

HTML Web Page Embedded Output:

Source/Referencee

https://docs.microsoft.com/en-us/dotnet/standard/base-types/regular-expression-language-quick-reference
https://docs.microsoft.com/en-us/dotnet/standard/base-types/alternation-constructs-in-regular-expressions

©sideway

ID: 190700031 Last Updated: 7/31/2019 Revision: 0 Ref:

References

Active Server Pages, , http://msdn.microsoft.com/en-us/library/aa286483.aspx
ASP Overview, , http://msdn.microsoft.com/en-us/library/ms524929%28v=vs.90%29.aspx
ASP Best Practices, , http://technet.microsoft.com/en-us/library/cc939157.aspx
ASP Built-in Objects, , http://msdn.microsoft.com/en-us/library/ie/ms524716(v=vs.90).aspx
Response Object, , http://msdn.microsoft.com/en-us/library/ms525405(v=vs.90).aspx
Request Object, , http://msdn.microsoft.com/en-us/library/ms524948(v=vs.90).aspx
Server Object (IIS), , http://msdn.microsoft.com/en-us/library/ms525541(v=vs.90).aspx
Application Object (IIS), , http://msdn.microsoft.com/en-us/library/ms525360(v=vs.90).aspx
Session Object (IIS), , http://msdn.microsoft.com/en-us/library/ms524319(8v=vs.90).aspx
ASPError Object, , http://msdn.microsoft.com/en-us/library/ms524942(v=vs.90).aspx
ObjectContext Object (IIS), , http://msdn.microsoft.com/en-us/library/ms525667(v=vs.90).aspx
Debugging Global.asa Files, , http://msdn.microsoft.com/en-us/library/aa291249(v=vs.71).aspx
How to: Debug Global.asa files, , http://msdn.microsoft.com/en-us/library/ms241868(v=vs.80).aspx
Calling COM Components from ASP Pages, , http://msdn.microsoft.com/en-us/library/ms524620(v=VS.90).aspx
IIS ASP Scripting Reference, , http://msdn.microsoft.com/en-us/library/ms524664(v=vs.90).aspx
ASP Keywords, , http://msdn.microsoft.com/en-us/library/ms524672(v=vs.90).aspx
Creating Simple ASP Pages, , http://msdn.microsoft.com/en-us/library/ms524741(v=vs.90).aspx
Including Files in ASP Applications, , http://msdn.microsoft.com/en-us/library/ms524876(v=vs.90).aspx
ASP Overview, , http://msdn.microsoft.com/en-us/library/ms524929(v=vs.90).aspx
FileSystemObject Object, , http://msdn.microsoft.com/en-us/library/z9ty6h50(v=vs.84).aspx
http://msdn.microsoft.com/en-us/library/windows/desktop/ms675944(v=vs.85).aspx, , ADO Object Model
ADO Fundamentals, , http://msdn.microsoft.com/en-us/library/windows/desktop/ms680928(v=vs.85).aspx