How To Search for the Last Occurrence of a Substring (168836)

MORE INFORMATION

There are two main methods of searching for the last occurrence of a substring:

1. Searching from the right-hand end of the string and stopping when the first match is found.
2. Searching from the left-hand end of the string until no more matches are found, and remembering the location of the last match.

While the first method would seem the logical approach (and would be in a language such a C that allowed direct pointer manipulation), the second method is nearly always faster in Visual Basic using the InStr() function to search forward through the string.

This is because InStr() is highly optimized and many orders of magnitude faster than emulating the functionality via looping through the string using the Mid$() function to extract each character.

The code example below demonstrates the second method.

WARNING: Microsoft provides code/macro examples for illustration only, without warranty either expressed or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose. This code is provided 'as is' and Microsoft does not guarantee that the following code can be used in all situations. Microsoft does not support modifications of the code to suit customer requirements for a particular purpose.

NOTE: In versions of Basic that don't support the "_" line continuation character, the split lines must be entered on a single line when typing the code into the module.

Step-by-Step Example

1. Create a new project and add a module:

         Function InStrR(ByVal sTarget As String, _
                         ByVal sFind As String, _
                         ByVal iCompare As Long) As Long
         Dim P As Long, LastP As Long, Start As Long
           P = InStr(1, sTarget, sFind, iCompare)
           Do While P
             LastP = P
             P = InStr(LastP + 1, sTarget, sFind, iCompare)
           Loop
           InStrR = LastP
         End Function
   						

2. In Visual Basic only, run the project and when the default Form1 is displayed, pause it.
3. To test this function, type the following line in the Debug/Immediate window, and then press the ENTER key:

   ?InStrR("The quick brown fox jumped over the lazy dog", "the", 0)

   You should see 33 as the result.
   
   Values for iCompare can be:

         0   Binary comparison
         1   Text comparison
         2   Database Comparison (Microsoft Access)