how to read all strings that are between 2 other strings - vb.net

Question

i have managed to find a string between 2 specified strings,
the only issue now is that it will only find one and then stop.
how am i possible to make it grab all the strings in a textbox?
the textbox is multiline and i have put a litle config in it.
now i want that the listbox will add all the strings that are between my 2 specified strings.
textbox3.text containts "<"
and textbox 4.text contains ">"
Public Function GetClosedText(ByVal source As String, ByVal opener As String, ByVal closer As String) As String
Dim intStart As Integer = InStr(source, opener)
If intStart > 0 Then
Dim intStop As Integer = InStr(intStart + Len(opener), source, closer)
If intStop > 0 Then
Try
Dim value As String = source.Substring(intStart + Len(opener) - 1, intStop - intStart - Len(opener))
Return value
Catch ex As Exception
Return ""
End Try
End If
End If
Return ""
End Function
usage:
ListBox1.Items.Add(GetClosedText(TextBox1.Text, TextBox3.Text, TextBox4.Text))

Answer 1

The easiest way (least lines of code) to do this would be to use a regular expression. For instance, to find all of that strings enclosed in pointy brackets, you could use this regular expression:
\<(?<value>.*?)\>
Here's what that all means:
\< - Find a string which starts with a < character. Since < has a special meaning in RegEx, it must be escaped (i.e. preceded with a backslash)
(?<value>xxx) - This creates a named group so that we can later access this portion of the matched string by the name "value". Everything contained in the name group (i.e. where xxx is), is considered part of that group.
.*? - This means find any number of any characters, up to, but not including whatever comes next. The . is a wildcard which means any character. The * means any number of times. The ? makes it non-greedy so it stops matching as soon as if finds whatever comes next (the closing >).
\> - Specifies that matching strings must end with a > character. Since > has a special meaning in RegEx, it must also be escaped.
You could use that RegEx expression to find all the matches, like this:
Dim items As New List(Of String)()
For Each i As Match In Regex.Matches(source, "\<(?<value>.*?)\>")
items.Add(i.Groups("value").Value)
Next
The trick to making it work in your scenario is that you need to dynamically specify the opening and closing characters. You can do that by concatenating them to the RegEx, like this:
Regex.Matches(source, opener & "(?<value>.*?)" & closer)
But the problem is, that will only work if source and closer are not special RegEx characters. In your example, they are < and >, which are special characters, so they need to be escaped. The safe way to do that is to use the Regex.Escape method, which only escapes the string if it needs to be:
Private Function GetClosedText(source As String, opener As String, closer As String) As String()
Dim items As New List(Of String)()
For Each i As Match In Regex.Matches(source, Regex.Escape(opener) & "(?<value>.*?)" & Regex.Escape(closer))
items.Add(i.Groups("value").Value)
Next
Return items.ToArray()
End Function
Notice that in the above example, rather than finding a single item and returning it, I changed the GetClosedText function to return an array of strings. So now, you can call it like this:
ListBox1.Items.AddRange(GetClosedText(TextBox1.Text, TextBox3.Text, TextBox4.Text))

Answer 2

I ssume you want to loop all openers and closers:
' always use meaningful variable/control names instead of
If TextBox3.Lines.Length <> TextBox4.Lines.Length Then
MessageBox.Show("Please provide the same count of openers and closers!")
Return
End If
Dim longText = TextBox1.Text
For i As Int32 = 0 To TextBox3.Lines.Length - 1
Dim opener = TextBox3.Lines(i)
Dim closer = TextBox4.Lines(i)
listBox1.Items.Add(GetClosedText(longText, opener , closer))
Next
However, you should use .NET methods as shown here:
Public Function GetClosedText(ByVal source As String, ByVal opener As String, ByVal closer As String) As String
Dim indexOfOpener = source.IndexOf(opener)
Dim result As String = ""
If indexOfOpener >= 0 Then ' default is -1 and indices start with 0
indexOfOpener += opener.Length ' now look behind the opener
Dim indexOfCloser = source.IndexOf(closer, indexOfOpener)
If indexOfCloser >= 0 Then
result = source.Substring(indexOfOpener, indexOfCloser - indexOfOpener)
Else
result = source.Substring(indexOfOpener) ' takes the rest behind the opener
End If
End If
Return result
End Function