Between, before, after

A string contains the syntax of a simple language. It has characters between, before and after other strings.

With special functions, we can extract these substrings. Functions like Between, Before and After get substrings based on the surrounding text. This helps parse a simple language.

Input and output

When parsing strings, we may need to extract a variable name in some way. Using Between() should yield the output shown here.

Input = "DEFINE:A=TWO"

Between("DEFINE:", "=TWO") = "A"

An example program

This module has implementations for the Between, Before and After string functions. These functions take substrings based on adjacent substrings.

Module Module1

    Function Between(value As String, a As String,
                     b As String) As String
        ' Get positions for both string arguments.
        Dim posA As Integer = value.IndexOf(a)
        Dim posB As Integer = value.LastIndexOf(b)
        If posA = -1 Then
            Return ""
        End If
        If posB = -1 Then
            Return ""
        End If

        Dim adjustedPosA As Integer = posA + a.Length
        If adjustedPosA >= posB Then
            Return ""
        End If

        ' Get the substring between the two positions.
        Return value.Substring(adjustedPosA, posB - adjustedPosA)
    End Function

    Function Before(value As String, a As String) As String
        ' Get index of argument and return substring up to that point.
        Dim posA As Integer = value.IndexOf(a)
        If posA = -1 Then
            Return ""
        End If
        Return value.Substring(0, posA)
    End Function

    Function After(value As String, a As String) As String
        ' Get index of argument and return substring after its position.
        Dim posA As Integer = value.LastIndexOf(a)
        If posA = -1 Then
            Return ""
        End If
        Dim adjustedPosA As Integer = posA + a.Length
        If adjustedPosA >= value.Length Then
            Return ""
        End If
        Return value.Substring(adjustedPosA)
    End Function

    Sub Main()
        Dim test As String = "DEFINE:A=TWO"
        ' Test the Between Function.
        Console.WriteLine(Between(test, "DEFINE:", "="))
        Console.WriteLine(Between(test, ":", "="))

        ' Test the Before Function.
        Console.WriteLine(Before(test, ":"))
        Console.WriteLine(Before(test, "="))

        ' Test the After Function.
        Console.WriteLine(After(test, ":"))
        Console.WriteLine(After(test, "DEFINE:"))
        Console.WriteLine(After(test, "="))
    End Sub

End Module
A
A
DEFINE
DEFINE:A
A=TWO
A=TWO
TWO

In Main, we parse a simple example string. We use a simple language syntax that could be used to define variables. We parse this string with Between, Before and After.

For good performance, avoiding substrings and IndexOf calls is beneficial. If in your code the above functions cause extra work, adding more custom methods is worth consideration.

Sometimes it is helpful to create functions that extract parts of strings. Substring() could be used directly. But this could become complicated and prone to errors.