RegEx Slows Down If Text Not Read From RichTextBox?

I found a strange issue with the combination of RichTextBox and RegEx.If I load a text file into a RichTextBox and use

Dim matches As MatchCollection = Regex.Matches(RichTextBox1.Text, "^H(.+ ){10}
", RegexOptions.Multiline Or RegexOptions.IgnoreCase)

then the RegEx will run blazing fast.OTOH if I load the text from a file straight into a string and feed this string with RegEx.Matches using

Dim FullText As String = New StreamReader("c:TempTextFile.txt", System.Text.Encoding.UTF8, True).ReadToEnd
Dim matches As MatchCollection = Regex.Matches(FullText, "^H(.+ ){10}
", RegexOptions.Multiline Or RegexOptions.IgnoreCase)

then the regex will run for ages.Now obviously the RichTextBox does something to the text. I found that loading the text into the RichTextBox will eliminate the first 4 bytes of the text (is that the byte order mark?). However, trimming those 4 characters with SubString won't solve the problem. RichTextBox does some other processing that I'm unaware of.I'm using RegEx both on client and server side (client can load text files into a RichTextBox to test and setup various RegEx's and the server will process text files according to these previously setup RegEx's). Needless to say that a regex will run blazing fast on the text file loaded into a RichTextBox, while the very same RegEx ran through the very same text file will seem to lock up the server side service.My question: what kind of text processing does the RichTextBox do and how can I mimic the behavior in a server-side process?