how to extract certain text from string - vb.net

Question

How do I filter/extract strings?
I have converted a PDF file into String using itextsharp and I have the text displayed into a Richtextbox1.
However there are too many irrelevant text that I don't need in the Richtextbox.
Is there a way I can display the text I want based on keywords, the entire length of the text.
Example of text that is displayed in textrichbox1 after conversation of PDF to text:
**774**
**Bos00232940
Bos00320491
Das1234
Das3216**
RAGE*
So the keywords would be "Bos", "Das", "774". and the new text that would be displayed in the richtextbox1 is shown below, instead of the entire text above.
*Bos00232940
Bos00320491
Das1234
Das3216
774*
Here is what I have so far. But it doesn't work it still displays the entire PDF in the richtextbox.
Public Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim pdffilename As String
pdffilename = TextBox1.Text
Dim filepath = "c:\temp\" & TextBox1.Text & ".pdf"
Dim thetext As String
thetext = GetTextFromPDF(filepath)
Dim lines() As String = System.Text.RegularExpressions.Regex.Split(thetext, Environment.NewLine)
Dim keywords As New List(Of String)
keywords.Add("Bos")
keywords.Add("Das")
keywords.Add("774")
Dim newTextLines As New List(Of String)
For Each line As String In lines
For Each keyw As String In thetext
If line.Contains(keyw) Then
newTextLines.Add(line)
Exit For
End If
Next
Next
RichTextBox1.Text = String.Join(Environment.NewLine, newTextLines.ToArray)
End Sub
SOLUTION
Thanks everyone for your help. Below is the code that worked and did exactly what I wanted it to do.
Public Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim pdffilename As String
pdffilename = TextBox1.Text
Dim filepath = "c:\temp\" & TextBox1.Text & ".pdf"
Dim thetext As String
thetext = GetTextFromPDF(filepath)
Dim re As New Regex("[\t ](?<w>((774)|(Bos)|(Das))[a-z0-9]*)[\t ]", RegexOptions.ExplicitCapture Or RegexOptions.IgnoreCase Or RegexOptions.Compiled)
Dim Lines() As String = {thetext}
Dim words As New List(Of String)
For Each s As String In Lines
Dim mc As MatchCollection = re.Matches(s)
For Each m As Match In mc
words.Add(m.Groups("w").Value)
Next
Next
RichTextBox1.Text = String.Join(Environment.NewLine, words.ToArray)
End Sub

Answer 1

For Each Word As String In thetext.Split(" ")
For Each key As String In keywords
If Word.StartsWith(key) Then
newTextLines.Add(Word)
Continue For
End If
Next
Next
or using LINQ:
Dim q = From word In thetext.Split(" ")
Where keywords.Any(Function(s) word.StartsWith(s))
Select word
RichTextBox1.Text = String.Join(Environment.NewLine, q.ToArray())

Answer 2

If don't know the keywords in advance but know in which context they occur, you can find them with a Regex expression. Two very handy Regex expressions allow you to find occurences succeeding or preceeding another:
(?<=prefix)find finds a pattern that follows another.
find(?=suffix) finds a pattern that comes before another.
If your number keyword (774) always preceeds " SIZE" you can find it like this: \w+(?=\sSIZE).
If the other keywords are always between "EX " and " DETAILS" you can find them like this: (?<=EX\s)(\w+\s)+(?=DETAILS).
You can put the whole thing together like this: \w+(?=\sSIZE)|(?<=EX\s)(\w+\s)+(?=DETAILS).
The disadvantage is that the keywords between "EX " and "DETAILS" will be returned as one match. But you can split the matches afterwards as in:
Const input As String = "2 3 3 4 4 A A B B SHEET 1 OF 1 774 SIZE SCALE 24.000-47.999 12.000-23.999 CON BAG WIRE 90in. EX Bos00232940 Bos00320491 Das1234 Das3216 DETAILS 1 2 RAGE"
Dim matches = Regex.Matches(input, "\w+(?=\sSIZE)|(?<=EX\s)(\w+\s)+(?=DETAILS)")
For Each m As Match In matches
Dim words = m.Value.Split(" "c)
For Each word As String In words
If word.Length > 0 Then ' Suppress the last empty word.
Console.WriteLine(word)
End If
Next
Next
Output:
774
Bos00232940
Bos00320491
Das1234
Das3216

Answer 3

How to do it with regular expression...
Dim re As New Regex("[\t ](?<w>((774)|(Bos)|(Das))[a-z0-9]*)[\t ]", RegexOptions.ExplicitCapture Or RegexOptions.IgnoreCase Or RegexOptions.Compiled)
Private Sub test()
Dim Lines() As String = {"2 3 3 4 4 A A B B SHEET 1 OF 1 774 SIZE SCALE 24.000-47.999 12.000-23.999 CON BAG WIRE 90in. EX Bos00232940 Bos00320491 Das1234 Das3216 DETAILS 1 2 RAGE"}
Dim words As New List(Of String)
For Each s As String In Lines
Dim mc As MatchCollection = re.Matches(s)
For Each m As Match In mc
words.Add(m.Groups("w").Value)
Next
Next
End Sub
Regex break down...
[\t ] Single tab or space (there is an alternative for whitespace too)
(?<w> Start of capture group called "w" This the the text returned later in the "m.Groups"
((774)|(Bos)|(Das)) one of the 3 blobs of text
[a-z0-9]* any a-z or 0-9 character, * = any number of them
) End of Capture group "w" from above.