Regex, file

Suppose we have a file, like a server log file, and want to read in each line and match a part of the line. The Regex.Match method, and StreamReader, can be used.

With a While-loop, we can enumerate each line in the file. Then we extract matching parts with the Groups property on the Match. This returns a string value.

An example

Here we have a log file, contained in log.txt, and we access this in the StreamReader when we create it. We create a Regex that matches a certain URL pattern.

Imports System.IO
Imports System.Text.RegularExpressions

Module Module1

    Sub Main()

        ' Create Regex.
        Dim regex As Regex = New Regex("\s/Content/([a-zA-Z0-9\-]+?)\.aspx")

        ' Loop over the lines in this text file.
        Using reader As StreamReader = New StreamReader("C:\programs\log.txt")

            While (True)
                Dim line As String = reader.ReadLine()

                ' End if Nothing.
                If line = Nothing Then
                    Return
                End If

                ' Try to match.
                Dim match As Match = regex.Match(line)

                If match.Success Then
                    ' Get first group and print it.
                    Dim value As String = match.Groups(1).Value
                    Console.WriteLine(line)
                    Console.WriteLine("... " + value)
                End If
            End While
        End Using

    End Sub

End Module
2008-10-16 23:56:44 W3SVC2915713 GET /Content/String.aspx - 80 66.249
2008-10-16 23:59:50 W3SVC2915713 GET /Content/Trim-String-Regex.aspx - 80 66.249
2008-10-16 23:56:44 W3SVC2915713 GET /Content/String.aspx - 80 66.249
... String
2008-10-16 23:59:50 W3SVC2915713 GET /Content/Trim-String-Regex.aspx - 80 66.249
... Trim-String-Regex

Notes, results

See how the part after the "Content" part of the URLs is matched. A specific part of the file names is extracted with Regex.

Regex is often used with StreamReader to match each line in a file. Parts of each line can be extracted and stored in the remainder of the VB.NET program.