`Regex.Replace`

This VB.NET function performs complex text substitutions. It can use a delegate function for replacements—this is a MatchEvaluator type.

In simpler cases, it is often sufficient to use a String replacement in VB.NET programs. And for performance, string methods are better.

Example

Let us begin with this program. After declaring the input string, we invoke the Regex.Replace function and pass 3 arguments to it.

Info The arguments to Replace() are input string reference, the pattern to match, and the replacement for matching sequences.

Finally The program demonstrates that the function call performed the task correctly.

Imports System.Text.RegularExpressions

Module Module1
    Sub Main()
        ' Input string.
        Dim input As String = "abc def axe"
        ' Use Regex.Replace with string arguments.
        Dim output As String = Regex.Replace(input, "a..", "CHANGED")
        ' Print.
        Console.WriteLine(input)
        Console.WriteLine(output)
    End Sub
End Moduleabc def axe
CHANGED def CHANGED

`MatchEvaluator`

Regex.Replace function has a more powerful version. We must create a MatchEvaluator instance, which is a delegate pointer to a function.

Start Please look at the UpperFirst function. This invokes the Regex.Replace shared method, and passes 3 arguments.

Note The second argument is the pattern of character sequences we want to replace. The third argument is a MatchEvaluator instance.

Tip To create the MatchEvaluator, use the New operator and the AddressOf operator with a function name.

Tip 2 The UpperEvaluator method describes the implementation for the MatchEvaluator. It uppercases only the first character of its input.

Imports System.Text.RegularExpressions

Module Module1

    Sub Main()
        ' Write the uppercased forms.
        Console.WriteLine(UpperFirst("bird"))
        Console.WriteLine(UpperFirst("bird dog"))
        Console.WriteLine(UpperFirst("frog"))
    End Sub

    Function UpperFirst(ByRef value As String) As String
        ' Invoke the Regex.Replace function.
        Return Regex.Replace(value, _
                             "\b[a-z]\w+", _
                             New MatchEvaluator(AddressOf UpperEvaluator))
    End Function

    Function UpperEvaluator(ByVal match As Match) As String
        ' Get string from match.
        Dim v As String = match.ToString()
        ' Uppercase only first letter.
        Return Char.ToUpper(v(0)) + v.Substring(1)
    End Function

End ModuleBird
Bird Dog
Frog

Notes, performance

We can store the MatchEvaluator instance itself as a field upon the enclosing type. Then, you can simply access this field on each call to Regex.Replace.

And Not only this, but you could cache a Regex object as a field and call Replace on that.

Notes, usage

MatchEvaluator requires an understanding of how to construct an instance by assigning it to the address of an appropriate implementation.

Note You hook up the MatchEvaluator delegate type to an implementation. This must have the correct Match and String types.

Note 2 With MatchEvaluator, we create powerful mechanisms to programmatically mutate matching text patterns.

Regex.Replace is powerful. Using patterns and special characters, you can add power to your replacements, without adding imperative logic that is hard to maintain.