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The problem is in Lib/idlelib/ReplaceDialog.py:141:
m = prog.match(chars, col)
if not prog:
 return False
new = m.expand(self.replvar.get())
where
prog = re.compile('foo') # i.e. text in the find box
chars = '>>> "foo"\n' # i.e. the text in the IDLE window
col = 5
self.replvar.get() = 'bar\' # i.e. the var with the trailing \
m.expand() searches for groups to expand, like 1円 and 2円 to refer to the first and second matching group, and fails with:
Traceback (most recent call last):
 File "/home/wolf/dev/py/py3k/Lib/sre_parse.py", line 194, in __next
 c = self.string[self.index + 1]
IndexError: string index out of range
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/wolf/dev/py/py3k/Lib/re.py", line 278, in _expand
 template = sre_parse.parse_template(template, pattern)
 File "/home/wolf/dev/py/py3k/Lib/sre_parse.py", line 729, in parse_template
 this = sget()
 File "/home/wolf/dev/py/py3k/Lib/sre_parse.py", line 210, in get
 self.__next()
 File "/home/wolf/dev/py/py3k/Lib/sre_parse.py", line 196, in __next
 raise error("bogus escape (end of line)")
sre_constants.error: bogus escape (end of line)
Using things like foo5円bar also results in a crash because the group 5円 is not found.
I'm not sure what the expected behavior should be. If numeric/named backreferences are not supposed to be supported, I guess a re.escape() might solve the problem. If they are supported the last line should be wrapped in a try/except that looks for sre_constants.error errors and possibly IndexErrors too (this might actually be fixed in sre_parse.py).
BTW, I think that "if not prog" in the snippet I pasted should be "if not m". Pattern objects are always True (I assume prog is always a pattern object).
Do you want to work on a patch?