Bug in std::regex? - c++

Question

Here is code :
#include <string>
#include <regex>
#include <iostream>
int main()
{
std::string pattern("[^c]ei");
pattern = "[[:alpha:]]*" + pattern + "[[:alpha:]]*";
std::regex r(pattern);
std::smatch results;
std::string test_str = "cei";
if (std::regex_search(test_str, results, r))
std::cout << results.str() << std::endl;
return 0;
}
Output :
cei
The compiler used is gcc 4.9.1.
I'm a newbie learning regular expression.I expected nothing should be output,since "cei" doesn't match the pattern here. Am I doing it right? What's the problem?
Update:
This one has been reported and confirmed as a bug, for detail please visit here :
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63497

Answer 1

It's a bug in the implementation. Not only do a couple other tools I tried agree that your pattern does not match your input, but I tried this:
#include <string>
#include <regex>
#include <iostream>
int main()
{
std::string pattern("([a-z]*)([a-z])(e)(i)([a-z]*)");
std::regex r(pattern);
std::smatch results;
std::string test_str = "cei";
if (std::regex_search(test_str, results, r))
{
std::cout << results.str() << std::endl;
for (size_t i = 0; i < results.size(); ++i) {
std::ssub_match sub_match = results[i];
std::string sub_match_str = sub_match.str();
std::cout << i << ": " << sub_match_str << '\n';
}
}
}
This is basically similar to what you had, but I replaced [:alpha:] with [a-z] for simplicity, and I also temporarily replaced [^c] with [a-z] because that seems to make it work correctly. Here's what it prints (GCC 4.9.0 on Linux x86-64):
cei
0: cei
1:
2: c
3: e
4: i
5:
If I replace [a-z] where you had [^c] and just put f there instead, it correctly says the pattern doesn't match. But if I use [^c] like you did:
std::string pattern("([a-z]*)([^c])(e)(i)([a-z]*)");
Then I get this output:
cei
0: cei
1: cei
terminate called after throwing an instance of 'std::length_error'
what(): basic_string::_S_create
Aborted (core dumped)
So it claims to match successfully, and results[0] is "cei" which is expected. Then, results[1] is "cei" also, which I guess might be OK. But then results[2] crashes, because it tries to construct a std::string of length 18446744073709551614 with begin=nullptr. And that giant number is exactly 2^64 - 2, aka std::string::npos - 1 (on my system).
So I think there is an off-by-one error somewhere, and the impact can be much more than just a spurious regex match--it can crash at runtime.

Answer 2

The regex is correct and should not match the string "cei".
The regex can be tested and explained best in Perl:
my $regex = qr{ # start regular expression
[[:alpha:]]* # 0 or any number of alpha chars
[^c] # followed by NOT-c character
ei # followed by e and i characters
[[:alpha:]]* # followed by 0 or any number of alpha chars
}x; # end + declare 'x' mode (ignore whitespace)
print "xei" =~ /$regex/ ? "match\n" : "no match\n";
print "cei" =~ /$regex/ ? "match\n" : "no match\n";
The regex will first consume all chars to the end of the string ([[:alpha:]]*), then backtrack to find the NON-c char [^c] and proceed with the e and i matches (by backtracking another time).
Result:
"xei" --> match
"cei" --> no match
for obvious reasons. Any discrepancies to this in various C++ libraries and testing tools are the problem of the implementation there, imho.