ScrolledText (over Tkinter.Text) validation

[Jeff Epler]

By returning the string "break" from a binding, you can stop further
processing of events, as described in the Tk "bind" manpage.

You would start with something like 
    def myfunc(evt):
	k = evt.char
	if k in "PERU": return
	return "break"

    aText = Text()
    aText.bind("<KeyPress>", myfunc)
However, 'myfunc' will be executed even when the key is one such as
<Tab>, <Home>, or another key to be treated specially.  For each such
key, you need a binding of the form
    aText.bind("<Tab>", "# nothing")
as well as bindings to ignore modified keys
    aText.bind("<Shift-Key>", "# nothing")

If you use an Entry instead of a Text, you can use textvariable= to link
the value in the entry to some Python instance.  I'm not familiar with
doing this in Python, but the code might look something like this:

    import Tkinter, re

    class MyVariable(Tkinter.StringVar):
	def __init__(self, master=None):
	    Tkinter.StringVar.__init__(self, master)
	    self.value = ""
	    self.trace = self.trace_variable("w", self._validate)

	def __del__(self):
	    self.trace_vdelete("w", self.trace)

	def _validate(self, name1, name2, op):
	    self.validate(self.get(), self.value)
	    self.value = self.get()

	def validate(self, new_value, old_value):
	    self.set(new_value)

    class PERUVariable(MyVariable):
	def validate(self, new_value, old_value):
	    if re.match("^[PERU]*$", new_value):
		self.set(new_value)
	    else:
		self.set(old_value)

    app = Tkinter.Tk()
    aEntry = Tkinter.Entry(textvariable=PERUVariable(app))
    aEntry.pack()
    app.mainloop()

Jeff