I have a slightly different approach now which is going wrong because
the MatchCollection has 8 values (correct) which are blank (incorrect)

Can anyone see what i'm blatantly missing?

Ta in advance

Dim strText As String = "This is my long string with a <a
href=""http://localhost/test.aspx"">Test Link using quotes</a>,<BR>a
<a href = 'http://localhost/test.aspx'>Test Link using apostrophes</a>
<BR> and one with a <A
HREF=""http://localhost/test.aspx?test"">querystring</a>."

public Function Highlight(strText As String)
   
    Dim objRegEx As New RegEx("<[^<>]*>", RegexOptions.IgnoreCase)

       'Find All tags and place in collection
    Dim colM As MatchCollection = objRegEx.Matches(strText)

       'Replace all tags with $!$
    Dim strReplaced As String = objRegEx.Replace(strText,"$!$")
   
       'Create new objRegEx looking for test
    objRegEx = New RegEx("(test)", RegexOptions.IgnoreCase)
   
       'Replace All with highlight
    strText = objRegEx.Replace(strReplaced, _
                "<span style='background-color:#FFFF66;'>$1</span>")

       'Loop through all items in the collection replacing $!$
    For i As Integer = 0 to colM.Count -1
        Dim m As Match = colM.Item(i)
        strText = strText.Replace("$!$",m.ToString()) & vbcrlf
    Next i

       'Return
    Return strText
   
End Function

Ignore my last message, I was being stupid and forgot what Replace does