Deserialize an email header into key-value pairs

I've created a function that will deserialize an email header into a list of key-value pairs. I've run numerous tests using MS Office Outlook 2010 and MS Office 14.0 Object Library, all of which were successful.

However, I'd still like to know if this function could be optimized/improved.

Note

• I didn't use the Dictionary(Of TKey, TValue) class because a key can appear multiple times.

• The reason as to why I wrote this RegEx pattern is because a value can also contain a new line and/or :.

Public Shared Function Deserialize(header As String) As List(Of KeyValuePair(Of String, String))

Const pattern As String = "(\n|^)[a-z|A-Z|-]*:\s"

If (header Is Nothing) Then Throw New ArgumentNullException("header")

Dim result As New List(Of KeyValuePair(Of String, String))
Dim matches As MatchCollection = Regex.Matches(header, pattern)

If ((Not matches Is Nothing) AndAlso (matches.Count > 0)) Then

Dim endIndex As Integer = header.Length
Dim startIndex As Integer = Nothing
Dim item As Match = Nothing
Dim key As String = Nothing
Dim value As String = Nothing

For index As Integer = (matches.Count - 1) To 0 Step -1
item = matches.Item(index)
startIndex = (item.Index + item.Length)
key = item.Value.Trim().Replace(":", "")
value = header.Substring(startIndex, (endIndex - startIndex))
result.Insert(0, New KeyValuePair(Of String, String)(key, value))
endIndex = item.Index
Next

End If

Return result

End Function


There are a couple of things that I find odd about this code. This is the first thing that jumps out at me.

Dim startIndex As Integer = Nothing
Dim item As Match = Nothing
Dim key As String = Nothing

I see no purpose in initializing variable to Nothing. Simply declaring them would be cleaner in my opinion.

Dim startIndex As Integer
Dim item as Match
Dim key As String

I also find your For loop a bit strange. Is there a reason you're stepping backwards? If there is, I don't see it and you should comment on why in the code.

For index As Integer = (matches.Count - 1) To 0 Step -1

Personally, I would rewrite this to loop forward like this.

For index As Integer = 0 To matches.Count - 1
    '...
Next index

You could also use a comment here.

Const pattern As String = "(\n|^)[a-z|A-Z|-]*:\s"

Regular Expression patterns are rarely obvious. Tell Mr. Maintainer in English what you're matching on.

StackExchange Code Review Q#63357, answer score: 4