This is not supposed to be a tutorial on Regular Expressions, just how to make use of them in Excel using VBA.
This is based on the fact that I have a sheet with large strings in column B and I want to extract relevant keys to column C.
I have highlighted the salient pieces of code in red.
Sub RegEx()
    Dim RegEx As Object
    Dim strTest As String
    Dim valid As Boolean
    Dim Matches As Object
    Dim i As Integer

    Set RegEx = CreateObject("VBScript.RegExp")
    'What I happen to be looking for    
    RegEx.Pattern = "MT\d{6}V\d"
    For i = 2 To 115
        Range("B" & i).Activate
        strTest = ActiveCell.Text
        valid = RegEx.test(strTest)
        If valid = True Then
            Set Matches = RegEx.Execute(strTest)
            Range("C" & i).Value = CStr(Matches(0))
            Range("C" & i).Value = "#N/A#"
        End If
    Set RegEx = Nothing
End Sub