Excel macro - retrieve the text between two characters or delimiters

Eg text :   origin_code: string[5]; -> To extract string between '[' and ']'. i.e. 5 here.

Private Sub Datatypebytes_Click()
row_number = 0
Dim firstDelPos As Integer
Dim secondDelPos As Integer
Dim stringBwDels As String
 
    If InStr(items, "string[") Then
        strValue = Sheets("Sheet1").Range("A" & row_number)
        firstDelPos = InStrRev(strValue, "[")
        secondDelPos = InStrRev(strValue, "]")
        stringBwDels = Mid(strValue, firstDelPos + 1, secondDelPos - (firstDelPos + 1))
        Sheets("Sheet1").Range("B" & row_number) = stringBwDels
    End If