Regular Expression Anchors

Anchors, or atomic zero-width assertions, specify a position in the string where a match must occur. When using an anchor in a search expression, the regular expression engine does not advance through the string or consume characters; it looks for a match in the specified position only.

a symbol is used to represent an anchor. e.g. ^, $.
an escaped character is used to represent an anchor, e.g. \A, \Z, \z, \G, \b, and \B.

.NET Anchors

Start of String or Line: ^

By default, the ^ anchor is used to specify that the following pattern must begin at the first character position of the string. If the ^ anchor is used with the RegexOptions.Multiline option, the match must occur at the beginning of each line.

End of String or Line: $

By default, the $ anchor is used to specify that the preceding pattern must occur at the end of the input string, or before \n at the end of the input string. If the $ anchor is used with the RegexOptions.Multiline option, the match can also occur at the end of a line. Note that $ matches \n but does not match \r\n (the combination of carriage return and newline characters, or CR/LF). To match the CR/LF character combination, include \r?$ in the regular expression pattern.

Start of String Only: \A

The escaped \A anchor is used specify that the following pattern must occur at the beginning of the input string. It is identical to the ^ anchor, except that \A ignores the RegexOptions.Multiline option. Therefore, it can only match the start of the first line in a multiline input string.

End of String or Before Ending Newline: \Z

The escaped \Z anchor is used to specify that the preceding must occur at the end of the input string, or before \n at the end of the input string. It is identical to the $ anchor, except that \Z ignores the RegexOptions.Multiline option. Therefore, in a multiline string, it can only match the end of the last line, or the last line before \n.

Note that \Z matches \n but does not match \r\n (the CR/LF character combination). To match CR/LF, include \r?\Z in the regular expression pattern.

End of String Only: \z

The escaped \z anchor is used to specify that the preceding pattern must occur at the end of the input string. Like the $ language element, \z ignores the RegexOptions.Multiline option. Unlike the \Z language element, \z does not match a \n character at the end of a string. Therefore, it can only match the last line of the input string.

Contiguous Matches: \G

The escaped \G anchor is used to specify that the following pattern must occur at the point where the previous match ended. When you use this anchor with the Regex.Matches or Match.NextMatch method, it ensures that all matches are contiguous.

Word Boundary: \b

The escaped \b anchor is used to specify that the match must occur on a boundary between a word character (the \w language element) and a non-word character (the \W language element). Word characters consist of alphanumeric characters and underscores; a non-word character is any character that is not alphanumeric or an underscore. (For more information, see Character Classes.) The match may also occur on a word boundary at the beginning or end of the string.

The \b anchor is frequently used to ensure that a subexpression matches an entire word instead of just the beginning or end of a word.

Non-Word Boundary: \B

The escaped \B anchor is used to specify that the match must not occur on a word boundary. It is the opposite of the \b anchor.

Examples

Examples of Anchors

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 = "01234 67 9"&Chr(10)&"01 345"&Chr(13)&"67 9012 456"&Chr(13)&Chr(10)&"7890"&Chr(13)&Chr(10)
               Dim xmatchstr As String = ""
               Dim xoption As RegexOptions = RegexOptions.Multiline
               xmatchstr = xmatchstr & "Given string: " & """01234 67 9""&amp;Chr(10)&amp;""01 345""&amp;Chr(13)&amp;""67 9012 456""&amp;Chr(13)&amp;Chr(10)&amp;""7890""&amp;Chr(13)&amp;Chr(10)" & "<br />"
               xmatchstr = xmatchstr & showresult(xstring,"^(\w+)",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"^(\w+)",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)$",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)$",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r?$",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r?$",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r+$",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r+$",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"\A(\w+)",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\A(\w+)",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r*\Z",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r*\Z",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)[\r\n]*\Z",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)[\r\n]*\Z",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r*\z",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)\r*\z",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)[\r\n]*\z",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"(\w+)[\r\n]*\z",RegexOptions.Multiline)
               xmatchstr = xmatchstr & showresult(xstring,"\G(\w+ )",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\G(\w+ )*",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\G(\w+ )+",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\b\w+",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\w+\b",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\b\w+\b",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\b\w",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\w\b",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\b\w\b",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\B\w+",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\w+\B",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\B\w+\B",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\B\w",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\w\B",RegexOptions.None)
               xmatchstr = xmatchstr & showresult(xstring,"\B\w\B",RegexOptions.None)
               lbl01.Text = xmatchstr
           End Sub
           Function showresult(xstring,xpattern,xoption)
               Dim xmatches As MatchCollection
               Dim xmatchstr As String = ""
               Dim xint As Integer
               xmatchstr = xmatchstr & "<br />Result of Regex.Matches(string,""" & xpattern & """," & xoption & "): <br />"
               xmatches = Regex.Matches(xstring,xpattern,xoption)
               xmatchstr = xmatchstr & "->Result of MatchCollection.Count: """
               xmatchstr = xmatchstr & xmatches.Count & """<br />"
               For xint = 0 to xmatches.Count - 1
                   xmatchstr = xmatchstr & "->->Result of MatchCollection("& xint & ").Value, Index, Length: """
                   xmatchstr = xmatchstr & xmatches(xint).Value & ", " & xmatches(xint).Index & ", " & xmatches(xint).Length & """<br />"
               Next
               Return xmatchstr
           End Function
       </script>
    </head>
    <body>
       <% Response.Write ("<h1>This is a Sample Page of Anchors</h1>") %>
       <p>
           <%-- Set on Page_Load --%>
           <asp:Label id="lbl01" runat="server" />
       </p>
    </body>
</html>

HTML Web Page Embedded Output:

Source/Reference

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/anchors-in-regular-expressions

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

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