VB.NET - Regex.Match Examples

Regex. For testing and manipulating text, the VB.NET Regex class is useful. With Regex, we use a text-processing language. This language easily handles string data.

With the Match function, we search strings. And with Replace, we change those we find. And often RegexOptions are used to change how these functions are evaluated.

Match example. This program uses Regex. Please notice the System.Text.RegularExpressions namespace—this is important to get started with regular expressions.

Imports

Step 1 We create a Regex object. The Regex pattern "\w+" matches one or more word characters together.

Step 2 We invoke the Match Function on the Regex instance. This returns a Match (which we test next).

Step 3 We test if the match is successful. If it is, we print (with Console.WriteLine) its value—the string "do."

Imports System.Text.RegularExpressions

Module Module1
    Sub Main()
        ' Step 1: create Regex.
        Dim regex As Regex = New Regex("\w+")

        ' Step 2: call Match on Regex.
        Dim match As Match = regex.Match(" do?")

        ' Step 3: test the Success bool.
        ' ... If we have Success, write the Value.
        If match.Success Then
            Console.WriteLine("RESULT: [{0}]", match.Value)
        End If
    End Sub
End Module
RESULT: [do]

IgnoreCase. Next, we use different syntax, and an option, for Match. We call the Regex.Match shared Function—no Regex object is needed. We then specify an option, RegexOptions.IgnoreCase.

Info This enum value, a constant, specifies that lower and uppercase letters are equal.

RegexOptions.IgnoreCase

Imports System.Text.RegularExpressions

Module Module1

    Sub Main()
        ' Match ignoring case of letters.
        Dim match As Match = Regex.Match("I like that cat",
                                         "C.T",
                                         RegexOptions.IgnoreCase)
        If match.Success Then
            ' Write value.
            Console.WriteLine(match.Value)
        End If
    End Sub

End Module
cat

Groups. This example uses Match and Groups. We specify the case of letters is unimportant with RegexOptions.IgnoreCase. And finally we test for Success on the Match object received.

Info When we execute this program, we see the target text was successfully extracted from the input.

Important We use the value 1 to get the first group from the Match. With Regex, indexing starts at 1 not 0.

Imports System.Text.RegularExpressions

Module Module1

    Sub Main()
        ' The input string.
        Dim value As String = "/content/alternate-1.aspx"

        ' Invoke the Match method.
        Dim m As Match = Regex.Match(value, _
                                     "content/([A-Za-z0-9\-]+)\.aspx$", _
                                     RegexOptions.IgnoreCase)

        ' If successful, write the group.
        If (m.Success) Then
            Dim key As String = m.Groups(1).Value
            Console.WriteLine(key)
        End If
    End Sub

End Module
alternate-1

Shared. A Regex object requires time to be created. We can instead share Regex objects, with the shared keyword. A shared Regex object is faster than shared Regex Functions.

Shared

Detail Storing a Regex as a field in a module or class often results in a speed boost, when Match is called more than once.

Detail The Match function is an instance function on a Regex object. This program has the same result as the previous program.

Imports System.Text.RegularExpressions

Module Module1

    ''' <summary>
    ''' Member field regular expression.
    ''' </summary>
    Private _reg As Regex = New Regex("content/([A-Za-z0-9\-]+)\.aspx$", _
                                      RegexOptions.IgnoreCase)

    Sub Main()
        ' The input string.
        Dim value As String = "/content/alternate-1.aspx"

        ' Invoke the Match method.
        ' ... Use the regex field.
        Dim m As Match = _reg.Match(value)

        ' If successful, write the group.
        If (m.Success) Then
            Dim key As String = m.Groups(1).Value
            Console.WriteLine(key)
        End If
    End Sub

End Module
alternate-1

Match, NextMatch. The Match() Function returns the first match only. But we can call NextMatch() on that returned Match object. This is a match that is found in the text, further on.

Tip NextMatch can be called in a loop. This results in behavior similar to the Matches method (which may be easier to use).

For

Imports System.Text.RegularExpressions

Module Module1

    Sub Main()
        ' Get first match.
        Dim match As Match = Regex.Match("4 and 5", "\d")

        If match.Success Then
            Console.WriteLine(match.Value)
        End If

        ' Get next match.
        match = match.NextMatch()

        If match.Success Then
            Console.WriteLine(match.Value)
        End If
    End Sub

End Module4
5

IsMatch. This returns true if a String matches the regular expression. We get a Boolean that tells us whether a pattern matches. If no other results are needed, IsMatch is useful.

Here This program introduces the IsValid Boolean function, which computes the result of the Regex.IsMatch function on its parameter.

Note The regular expression pattern indicates any string of lowercase ASCII letters, uppercase ASCII letters, or digits.

Imports System.Text.RegularExpressions

Module Module1
    Function IsValid(ByRef value As String) As Boolean
        Return Regex.IsMatch(value, "^[a-zA-Z0-9]*$")
    End Function

    Sub Main()
        Console.WriteLine(IsValid("dotnetperls0123"))
        Console.WriteLine(IsValid("DotNetPerls"))
        Console.WriteLine(IsValid(":-)"))
    End Sub
End ModuleTrue
True
False

Start and ends. Matching the start and end of a String is commonly-needed. We use the metacharacters "^" and "$" to match the starts and ends of a string.

Note IsMatch() evaluates these metacharacters in the same way that Match (or Matches) can—the result is different for each function.

Imports System.Text.RegularExpressions

Module Module1

    Sub Main()
        Dim value As String = "XXYY"

        ' Match the start with a "^" char.
        If Regex.IsMatch(value, "^XX") Then
            Console.WriteLine("ISMATCH START")
        End If

        ' Match the end with a "$" char.
        If Regex.IsMatch(value, "YY$") Then
            Console.WriteLine("ISMATCH END")
        End If
    End Sub

End ModuleISMATCH START
ISMATCH END

Benchmark, Compiled Regex. To optimize Regex performance in VB.NET, we can use the RegexOptions.Compiled enum and store the Regex in a field. Here we test Compiled Regexes.

Version 1 This code uses a Regex field in the Module, and calls IsMatch on the field instance.

Version 2 Uses Regex.IsMatch directly with no stored Regex instance. This code does the same thing as version 1.

Result The optimizations applied (a field, and RegexOptions.Compiled) makes regular expression testing much faster.

Imports System.Text.RegularExpressions

Module Module1
    Dim _regex As Regex = New Regex("X.+0", RegexOptions.Compiled)

    Sub Version1()
        ' Use compiled regular expression stored as field.
        If _regex.IsMatch("X12340") = False Then
            Throw New Exception
        End If
    End Sub

    Sub Version2()
        ' Do not use compiled Regex.
        If Regex.IsMatch("X12340", "X.+0") = False Then
            Throw New Exception
        End If
    End Sub

    Sub Main()
        Dim m As Integer = 100000
        Dim s1 As Stopwatch = Stopwatch.StartNew

        ' Version 1: use RegexOptions.Compiled.
        For i As Integer = 0 To m - 1
            Version1()
        Next
        s1.Stop()

        Dim s2 As Stopwatch = Stopwatch.StartNew

        ' Version 2: do not compile the Regex.
        For i As Integer = 0 To m - 1
            Version2()
        Next
        s2.Stop()

        Dim u As Integer = 1000000
        Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
        Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
    End Sub
End Module131.78 ns    IsMatch, RegexOptions.Compiled
484.66 ns    IsMatch

In some programs, a Regex is the easiest way to process text. At its core, the Regex type exposes a text-processing language—one built upon finite deterministic automata.