Using a MatchEvaluator with Regex.Replace

by Brian Davis

The regular expression engine in .NET is obviously a powerful alternative to traditional String manipulation methods when dealing with complex parsing or validation.  The power of the Regex.Replace method allows the developer to perform replacements based on patterns rather than literal text.  Beyond this, though, .NET Regex offers an even more powerful tool - the MatchEvaluator.

A MatchEvaluator is simply a pointer to a function that takes a Match as its only parameter and returns a String.  When calling Regex.Replace, you can then pass the MatchEvaluator instead of a replacement string, and .NET will call your MatchEvaluator and then use the returned Strnig as the result of the Replace.  This offers the ability to perform complex parsing withing a call to Replace, including the ability to reference other methods and classes.  One common example is an expression evaluator.  You may want to replace the String "3*2" with "6" in an expression.  The following code shows how to do this simple example using a MatchEvaluator:

Public Function EvaluateString(ByVal s As String) As String       Dim MatchEval As New MatchEvaluator(AddressOf RegexReplaceEvaluator)       Dim Pattern As String = "(?<param1>\d+)(?<sign>\+|-|/|\*)(?<param2>\d+)"       Return Regex.Replace(s, Pattern, MatchEval)    End Function    Public Function RegexReplaceEvaluator(ByVal m As Match) As String       Select Case m.Groups("sign").Value          Case "+"             Return (CInt(m.Groups("param1").Value) + _                     CInt(m.Groups("param2").Value)).ToString          Case "-"             Return (CInt(m.Groups("param1").Value) - _                     CInt(m.Groups("param2").Value)).ToString          Case "/"             Return (CInt(m.Groups("param1").Value) / _                     CInt(m.Groups("param2").Value)).ToString          Case "*"             Return (CInt(m.Groups("param1").Value) * _                     CInt(m.Groups("param2").Value)).ToString          Case Else             Return "ERROR"       End Select    End Function

This is a simplified example, but it shows how a MatchEvaluator can be used to perform operations that would be much more complex with standard parsing methods.  For those who want to realize the full power of the MatchEvaluator, check out Chapter 12 of Francesco Balena's book, Programming Visual Basic .NET, in which he offers code that will evaluate complex expressions with nested levels of parentheses, logarithmic functions, and trigonometric functions.

Note also that you do not need to declare a MatchEvaluator object.  The Regex.Replace method will also take a delegate within the call itself like this:

Return Regex.Replace(s, Pattern, AddressOf RegexReplaceEvaluator)