Replacing 2x vbCrLf at once - vb.net

Question

I have a string in which I'm trying to replace all VbCr / VbLf with VbCrLf. This is in an attempt to scrape some HTML.
My code looks like this:
leHTML = leHTML.Replace(vbLf, vbCrLf)
leHTML = leHTML.Replace(vbCr, vbCrLf)
However in many cases I'm then left with 2x vbCrLf of which I only want 1.
leHTML = leHTML.Replace(vbCrLf & vbCrLf, vbCrLf)
The line above doesn't seem to be doing anything. How can I replace 2x vbCrLf with 1x vbCrLf? Is there a better way of going about "normalizing" Line Feeds and Carriage Returns?

Answer 1

You should not replace a correct vbCrLf in the first place. Instead replace only those characters where replacement is necessary. A handy tool for this task is a regular expression.
There are two cases that you want to get rid off:
vbCr with no following vbLf
the Regex for this is (vbCr)(?!vbLf)
vbLf with no preceeding vbCr
the Regex for this is (?<!vbCr)(vbLf)
Putting this together, we get the following regex:
Dim regex = New Regex("((" & vbCr & ")(?!" & vbLf & ")|(?<!" & vbCr & ")(" & vbLf & "))")
Throw this on your input and you're done:
leHTML = regex.Replace(leHTML, vbCrLf)
Here is a simple test program (vbCr and vbLf have been replaced by cr and lf respectively, so there is a visible output):
Dim str = "crlf cr cr lf crlf lf"
Dim regex = New Regex("((cr)(?!lf)|(?<!cr)(lf))")
str = regex.Replace(str, "crlf")
Console.WriteLine(str)
The result is:
crlf crlf crlf crlf crlf crlf

Answer 2

You're going to have to work a little harder at this. Instead of blindly replacing characters, you need to see what is there first, then determine what you are replacing. For example (this is NOT the complete code):
if leHTML.contains(vbcr) andalso leHTML.contains(vblf) then
leHTML = leHTML.Replace(vbCr & vbLf, vbCrLf)
elseif leHTML.contains(vbcr) then
leHTML = leHTML.Replace(vbCr, vbCrLf)
elseif leHTML.contains(vblf) then
leHTML = leHTML.Replace(vbLf, vbCrLf)
else
...
end

Answer 3

Probably this is a good pattern to use a Regex replace expression.
For example
Dim pattern = "(\r|\n)"
Dim search = "The" & vbCr & "Test string" & vbCr & _
"used as an" & vbLf & "Example" & vbCrLf & "."
Dim m = Regex.Replace(search, pattern, vbCrLf)
Console.WriteLine(m)
The first line prepare the pattern to search for using the C# syntax for vbCr=\r and vbLf=\n enclosing the two characters in an optional group (find a vbCr or a vbLf).
Then the replace method search one or the other char and replace it with the double vbCrLf character sequence.
But now we have a problem, the single vbCrLf present in the test string has been doubled, so you need another replace to remove the double sequence with just one vbCrLf
pattern = "\r\n\r\n"
m = Regex.Replace(search, pattern, vbCrLf)
Console.WriteLine(m)