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A pattern like r"b{1,3}\Z" matches "b", "bb", and "bbb", as expected. There is no documentation of the behaviour of r"b{1, 3}\Z" -- it matches the LITERAL TEXT "b{1, 3}" in normal mode and "b{1,3}" in verbose mode.
# paste the following at the interactive prompt:
pat = r"b{1, 3}\Z"
bool(re.match(pat, "bb")) # False
bool(re.match(pat, "b{1, 3}")) # True
bool(re.match(pat, "bb", re.VERBOSE)) # False
bool(re.match(pat, "b{1, 3}", re.VERBOSE)) # False
bool(re.match(pat, "b{1,3}", re.VERBOSE)) # True
Suggested change, in decreasing order of preference:
(1) Ignore leading/trailing spaces when parsing the m and n components of {m,n}
(2) Raise an exception if the exact syntax is not followed
(3) Document the existing behaviour
Note: deliberately matching the literal text would be expected to be done by escaping the left brace:
pat2 = r"b\{1, 3}\Z"
bool(re.match(pat2, "b{1, 3}")) # True
and this is not prevented by the suggested changes.

Interesting.
In my regex module (http://pypi.python.org/pypi/regex) I have:
bool(regex.match(pat, "bb", regex.VERBOSE)) # True
bool(regex.match(pat, "b{1,3}", regex.VERBOSE)) # False
because I thought that when the VERBOSE flag is turned on it should ignore whitespace except when it's inside a character class, so "b{1, 3}" would be treated as "b{1,3}".
Apparently re has another exception.

$ echo 'bbbbbaaa' | grep -o 'b\{1,3\}a'
bbba
$ echo 'bbbbbaaa' | grep -o 'b\{1, 3\}a'
grep: Invalid content of \{\}
$ echo 'bbbbbaaa' | egrep -o 'b{1,3}a'
bbba
$ echo 'bbbbbaaa' | egrep -o 'b{1, 3}a'
$ echo 'bbb{1, 3}aa' | LC_ALL=C egrep -o 'b{1, 3}a'
b{1, 3}a
I.e. grep raises error and egrep chooses silent verbatim meaning. I don't know what any standards say about this.

The question is whether re should always treat 'b{1, 3}a' as a literal, even with the VERBOSE flag.
I've checked with Perl 5.14.2, and it agrees with re: adding a space _always_ makes it a literal, even with the 'x' flag (/b{1, 3}a/x is treated as /b\{1,3}a/).

Then let's leave all as is.