[LLVMbugs] [Bug 21751] New: regex iterator endless loop

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21751

            Bug ID: 21751
           Summary: regex iterator endless loop
           Product: libc++
           Version: unspecified
          Hardware: Macintosh
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mclow.lists at gmail.com
                CC: llvmbugs at cs.uiuc.edu, mclow.lists at gmail.com
    Classification: Unclassified

Given the following code:

----
#include <iostream>
#include <regex>
#include <string>

void print(const char* t, const std::string& s, const std::ssub_match& sub)
{
    std::cout << "  " << t << ": " << (sub.matched ? "matched  " : "unmatched")
<< ", "
        "length() = " << sub.length() << ", str() = '" << sub.str() << "\', "
        "pair = (" << sub.first - s.begin() << ", " << sub.second - s.begin()
<< "), "
        "'" << std::string(sub.first, sub.second) << '\'' << std::endl;
}

int main()
{
    const std::regex e("z*");
    const std::string s("ab");

    int i = 0;
    for (auto&& it = std::sregex_iterator(s.begin(), s.end(), e), end =
std::sregex_iterator();
         it != end; ++it) {
        std::cout << i++ << ':' << std::endl;
        print("prefix", s, it->prefix());
        print("match ", s, (*it)[0]);
        std::cout << std::endl;
    }
}
----

The loop runs forever.

Suggested output from <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64140>:

----
0:
  prefix: unmatched, length() = 0, str() = '', pair = (0, 0), ''
  match : matched  , length() = 0, str() = '', pair = (0, 0), ''

1:
  prefix: matched  , length() = 1, str() = 'a', pair = (0, 1), 'a'
  match : matched  , length() = 0, str() = '', pair = (1, 1), ''

2:
  prefix: matched  , length() = 1, str() = 'b', pair = (1, 2), 'b'
  match : matched  , length() = 0, str() = '', pair = (2, 2), ''
----

Thanks to Jonathan Wakely for the report.

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