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Regular Expression Compilation and Reuse
 Compiled Regular Expressions
 The Regular Expressions Cache
 Scanning of Valid Email Format
 See also
 Source/Referencee

Regular Expression Compilation and Reuse

You can optimize the performance of applications that make extensive use of regular expressions by understanding how the regular expression engine compiles expressions and by understanding how regular expressions are cached. This topic discusses both compilation and caching.

Compiled Regular Expressions

By default, the regular expression engine compiles a regular expression to a sequence of internal instructions (these are high-level codes that are different from Microsoft intermediate language, or MSIL). When the engine executes a regular expression, it interprets the internal codes.

If a Regex object is constructed with the RegexOptions.Compiled option, it compiles the regular expression to explicit MSIL code instead of high-level regular expression internal instructions. This allows .NET's just-in-time (JIT) compiler to convert the expression to native machine code for higher performance.

However, generated MSIL cannot be unloaded. The only way to unload code is to unload an entire application domain (that is, to unload all of your application's code.). Effectively, once a regular expression is compiled with the RegexOptions.Compiled option, never releases the resources used by the compiled expression, even if the regular expression was created by a Regex object that is itself released to garbage collection.

You must be careful to limit the number of different regular expressions you compile with the RegexOptions.Compiled option to avoid consuming too many resources. If an application must use a large or unbounded number of regular expressions, each expression should be interpreted, not compiled. However, if a small number of regular expressions are used repeatedly, they should be compiled with RegexOptions.Compiled for better performance. An alternative is to use precompiled regular expressions. You can compile all of your expressions into a reusable DLL by using the CompileToAssembly method. This avoids the need to compile at runtime while still benefiting from the speed of compiled regular expressions.

The Regular Expressions Cache

To improve performance, the regular expression engine maintains an application-wide cache of compiled regular expressions. The cache stores regular expression patterns that are used only in static method calls. (Regular expression patterns supplied to instance methods are not cached.) This avoids the need to reparse an expression into high-level byte code each time it is used.

The maximum number of cached regular expressions is determined by the value of the static (Shared in Visual Basic) Regex.CacheSize property. By default, the regular expression engine caches up to 15 compiled regular expressions. If the number of compiled regular expressions exceeds the cache size, the least recently used regular expression is discarded and the new regular expression is cached.

Your application can take advantage of precompiled regular expressions in one of the following two ways:

By using a static method of the Regex object to define the regular expression. If you are using a regular expression pattern that has already been defined in another static method call, the regular expression engine will retrieve it from the cache. If not, the engine will compile the regular expression and add it to the cache.
By reusing an existing Regex object as long as its regular expression pattern is needed.

Because of the overhead of object instantiation and regular expression compilation, creating and rapidly destroying numerous Regex objects is a very expensive process. For applications that use a large number of different regular expressions, you can optimize performance by using calls to static Regex methods and possibly by increasing the size of the regular expression cache.

Scanning of Valid Email Format

Examples of Scanning of Valid Email Format.

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 = "dahhhvid.johgnes@prghjjosehware.com"
               Dim xmatchstr As String = ""
               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 Scanning of of Valid Email Format</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/compilation-and-reuse-in-regular-expressions

©sideway

ID: 190800007 Last Updated: 8/7/2019 Revision: 0 Ref:

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