This sub does what you want.
The difference is that the index begins with 0 not with 1 like in your example.
To get 'is', you should use ANS = SplitGetWord("This is a test string.", " ", 1)
B4X:
Sub SplitGetWord(txt As String, SplitCharacter As String, Index As Int) As String
Private words() As String
words = Regex.Split(SplitCharacter, txt)
Return words(Index)
End Sub