What you are trying to do is involves complex logic that is not going to be possible, even in an advanced text editor like Notepad++. In addition, your problem is not precisely defined (for example, is "12.3" a number or not?). Only you know your exact requirements.
Depending on how technical you are, you could adapt the following C# code to do exactly what you want. You'll need Visual Studio Express or similar. You can of course write this in another language or try and find an alternate solution. Note that the code will delete lines starting with "Volume ", even if the following text is not a number. You'll need to deal with exactly what processing you want. Also, the code deals with files specifically by name. If you want to recurse all files in a folder, say, that's another detail you may need to consider:
// Read the file line by line
string szOriginal = @"C:\Tmp\Test.txt";
string szCopy = @"C:\Tmp\Test.txt.tmp";
StreamReader fText = new StreamReader(szOriginal);
StreamWriter fCopy = new StreamWriter(szCopy);
string szLine = string.Empty;
int nIndex = 0;
while ((szLine = fText.ReadLine()) != null)
// Don't copy lines starting with "Volume " (note space character)
nIndex = szLine.IndexOf("Volume ");
if (nIndex != 0)
// Delete original and rename copy
Create an empty Windows Forms project, add a button, double-click the button (to view the code in the editor), paste in the above code (with any modifications, if required), run the code and click the button. Good luck!