How To Find Multiple Occurrences of a String in a RichTextBox (176643)

SUMMARY

Finding a single occurrence of a text string in a RichTextBox is fairly straightforward. The Instr() function was designed for this. In order to find multiple occurrences of a block of text, however, you need to write a routine that will call the Instr() function as many times as necessary to search the entire block of text for the desired string, while progressively moving the starting point of the search forward to avoid finding the same occurrence multiple times.

You can accomplish this by using a recursive routine, which calls itself from within its own code. If you are not careful, however, recursion can cause problems, such as infinite loops and stack faults. But recursion does allow for elegant solutions to otherwise difficult situations.

This article illustrates recursion by example. The FindIt() routine calls itself recursively as long as the return value of the Instr() function does not equal zero. If this value is non-zero, it means Instr() found another occurrence of the string it was looking for. If the value is zero, either the string is not contained in the block of text being searched or all occurrences of the string have already been found. Checking this value each time the routine runs prevents an endless loop.

MORE INFORMATION

The following steps create a working example of finding multiple occurrences of the same string within a larger block of text.

Step-by-Step Example

Start a new Visual Basic Standard EXE project. Form1 is created by default.
Resize the form to be as large as possible, and add a RichTextBox and a CommandButton to the form.
Make the RichTextBox as large as possible and set its Scrollbars property to 3-rtfBoth.

Add the following code to the form:

     Private Sub Form_Load()

         'Edit the following line to point to your license.txt or another
         'text file

         RichTextBox1.LoadFile "license.txt"

      End Sub

      Private Sub Command1_Click()

      Dim strval As String    'Inputbox returns a string
      Dim nStrings As Long

         'Reload the file to undo the previous selections
         'Edit the following line to point to your license.txt or another
         'text file

         RichTextBox1.LoadFile "license.txt"

         'Ask the user what string to find. Defaults to finding all
         'occurrences of the word "the." In this example, you are
         'concatenating spaces onto each end of the search string to
         'prevent the FindIt routine from finding occurrences of the
         'search string within other words, ie; finding "the" in "there"
         'or "these." If you want to find the string within other strings,
         'simply eliminate the concatenation.

         strval = " " & InputBox("Enter the string to find.", "Findit", _
            "the") & " "

         If strval <> "" Then

         'the user didn't click cancel call the FindIt routine
         'and pass it the

            nStrings = FindIt(RichTextBox1, strval)
            MsgBox (Str$(nStrings) & " instances found.")
         End If

      End Sub

      '********************************************************************
      'Findit takes three arguments; two required, and one optional. The
      'required arguments, Box and Srch, are a RichTextBox object and a
      'string to search for. The optional argument, Start, is a Long
      'integer that is used in the recursive calls.
      '********************************************************************

      Private Function FindIt(Box As RichTextBox, Srch As String, _
         Optional Start As Long)

      Dim retval As Long      'Instr returns a long
      Dim Source As String    'variable used in Instr

         Source = Box.Text   'put the text to search into the variable

         If Start = 0 Then Start = 1 'the initial call doesn't pass a value
                                     'for Start, so it will equal 0

            retval = InStr(Start, Source, Srch) 'do the first search,
                                                'starting at the beginning
                                                'of the text

            If retval <> 0 Then  'there is at least one more occurrence of
                                 'the string

            'the RichTextBox doesn't support multiple active selections, so
            'this section marks the occurrences of the search string by
            'making them Bold and Red

               With Box
                  .SelStart = retval - 1
                  .SelLength = Len(Srch)
                  .SelColor = vbRed
                  .SelBold = True
                  .SelLength = 0          'this line removes the selection
                                          'highlight
               End With

               Start = retval + Len(Srch) 'move the starting point past the
                                          'first occurrence

               'FindIt calls itself with new arguments
               'this is what makes it Recursive
               FindIt = 1 + FindIt(Box, Srch, Start)
            End If
      End Function

Run the project and click the CommandButton. You can accept the default of searching for occurrences of the word "the" or enter your own string to search for. Remember to include a space before and after short words to prevent FindIt() from finding your search string in the middle of a longer word.