regex problem ..

[Analog Kid]

Hi All:
I am new to regular expressions in general, and not just re in python. So,
apologies if you find my question stupid :) I need some help with forming a
regex. Here is my scenario ...
I have strings coming in from a list, each of which I want to check against
a regular expression and see whether or not it "qualifies". By that I mean I
have a certain set of characters that are permissible and if the string has
characters which are not permissible, I need to flag that string ... here is
a snip ...

flagged = list()
strs = ['HELLO', 'Hi%20There', '123123@#@']
p =  re.compile(r"""[^a-zA-Z0-9]""", re.UNICODE)
for s in strs:
    if len(p.findall(s)) > 0:
        flagged.append(s)

print flagged

my question is ... if I wanted to allow '%20' but not '%', how would my
current regex (r"""[^a-zA-Z0-9]""") be modified?

TIA,
AK
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