How can I replace a string without the replace function? - vb.net

Question

I have a long string of random letters and I need to remove a couple of the front letters a few at a time. By using the replace function, if I replace a piece of string that then repeats later on, it removes the piece of string entirely from the long string instead of just the beginning.
Is there a way to remove a piece of string without using the replace function? The code below might clear up some of the confusion.
Dim protein As String
protein = "GLSDGEWQQVLNVWGKVEADIAGHGQEVLIRLFTGHPETLEKFDKFKHLKTEAEMKASEDLKKHGTVVLTALGGILKKKEGHHEAELKPLAQSHATKHKIPIKYLEFISDAIIHVLHSKHRPGDFGADAQGAMTKALELFRNDIAAKYKELGFQG"
Dim IndexPosition
For Each index In protein
If index = "K" Or index = "R" Then
IndexPosition = InStr(protein, index)
Dim NextPosition = IndexPosition + 1
Dim NextLetter = Mid(protein, NextPosition, 0)
If NextLetter <> "P" Then
Dim PortionToCutOut = Mid(protein, 1, IndexPosition)
protein = Replace(protein, PortionToCutOut, "")
Console.WriteLine(PortionToCutOut)
End If
End If
Next index

Answer 1

Regex might be a simpler way to solve this:
Regex.Replace(protein, "^(.*?)[KR][^P]", "$1")
It means "from the start of the string, for zero or more captured characters up to the first occurrence of K or R followed by anything other than P, replace it with (the captured string)"
GLSDGEWQQVLNVWGKVEADIAGHGQEVLIRLFTGHPETL
^^^^^^^^^^^^^^^^^
captured string||
xx
Everything underlined with ^^^ is replaced by everything apart from the xx bit
It makes a single replacement, because that's what I interpreted you required when you said:
By using the replace function, if I replace a piece of string that then repeats later on, it removes the piece of string entirely from the long string instead of just the beginning
However if you do want to replace all occurrences of "K OR R followed by not P" it gets simpler:
Regex.Replace(protein, "[KR][^P]", "")
This is "K or R followed by anything other than P", replace with "nothing"

Answer 2

There are several issues with your code. The first issue that is likely to throw an exception is that you're modifying a collection in a For/Each loop.
The second issue that is less severe in immediate impact, but just as important in my opinion is that you're using almost exclusively legacy Visual Basic methods.
InStr should be replaced with IndexOf: https://learn.microsoft.com/en-us/dotnet/api/system.string.indexof
Mid should be replaced with Substring: https://learn.microsoft.com/en-us/dotnet/api/system.string.substring
The third issue is that you're not using the short-circuit operator OrElse in your conditional statement. Or will evaluate the right-hand side of your condition regardles of if the left-hand side is true whereas OrElse won't bother to evaluate the right-hand side if the left-hand side is true.
In terms of wanting to remove a piece of the String without using Replace, well you'd use Substring as well.
Consider this example:
Dim protein = "GLSDGEWQQVLNVWGKVEADIAGHGQEVLIRLFTGHPETLEKFDKFKHLKTEAEMKASEDLKKHGTVVLTALGGILKKKEGHHEAELKPLAQSHATKHKIPIKYLEFISDAIIHVLHSKHRPGDFGADAQGAMTKALELFRNDIAAKYKELGFQG"
Dim counter = 0
Do While counter < protein.Length - 2
counter += 1
Dim currentLetter = protein(counter)
Dim nextLetter = protein(counter + 1)
If (currentLetter = "K"c OrElse currentLetter = "R"c) AndAlso nextLetter <> "P"c Then
protein = protein.Substring(0, counter) & protein.Substring(counter + 1)
End If
Loop
Example: https://dotnetfiddle.net/vrhRdO