can anyone tell me how Perl treats this pattern? r'((((((((((a))))))))))\41' in SRE, this is currently a couple of nested groups, surrounding a single literal, followed by a back reference to the fourth group, followed by a literal "1" (since there are less than 41 groups) in PRE, it turns out that this is a syntax error; there's no group 41. however, this test appears in the test suite under the section "all test from perl", but they're commented out: # Python does not have the same rules for \\41 so this is a syntax error # ('((((((((((a))))))))))\\41', 'aa', FAIL), # ('((((((((((a))))))))))\\41', 'a!', SUCCEED, 'found', 'a!'), if I understand this correctly, Perl treats as an *octal* escape (chr(041) == "!"). now, should I emulate PRE, Perl, or leave it as it is... </F> PS. in case anyone wondered why I haven't seen this before, it's because I just discovered that the test suite masks syntax errors under some circumstances...