[libc++] Treat trailing backslashes in a regex pattern as invalid.

[jpetrie]

Summary

std::regex should throw std::regex_error if constructed with a pattern ending in a trailing backslash, since a trailing backlash is not a valid escape sequence.

This addresses bug #26175.

[mclow.lists]

This looks good to me. A quick search for \\ in regex didn't find any other obvious instances of this anti-pattern. For the record, the bug is https://llvm.org/bugs/show_bug.cgi?id=26175

Can you commit this yourself, or would you rather I did it?

[jpetrie]

In D16262#329328, @mclow.lists wrote:

This looks good to me. A quick search for \\ in regex didn't find any other obvious instances of this anti-pattern. For the record, the bug is https://llvm.org/bugs/show_bug.cgi?id=26175

Can you commit this yourself, or would you rather I did it?

I don't have commit access; if you could commit it, that would be great.

[Eugene.Zelenko]

Committed in r258107.

include/regex

 template <class _ForwardIterator>
 _ForwardIterator
 basic_regex<_CharT, _Traits>::__parse_atom_escape(_ForwardIterator __first,
                                                   _ForwardIterator __last)
 {
     if (__first != __last && *__first == '\\')
     {
         _ForwardIterator __t1 = _VSTD::next(__first);
+        if (__t1 == __last)
+            __throw_regex_error<regex_constants::error_escape>();
+
         _ForwardIterator __t2 = __parse_decimal_escape(__t1, __last);
         if (__t2 != __t1)
             __first = __t2;
         else
         {
             __t2 = __parse_character_class_escape(__t1, __last);
             if (__t2 != __t1)
                 __first = __t2;

test/std/re/re.regex/re.regex.construct/bad_escape.pass.cpp

static bool error_escape_thrown(const char *pat)
     }
     return result;
 }
 
 int main() 
 {
     assert(error_escape_thrown("[\\a]"));
     assert(error_escape_thrown("\\a"));
+    assert(error_escape_thrown("\\"));
 
     assert(error_escape_thrown("[\\e]"));
     assert(error_escape_thrown("\\e"));
 
     assert(error_escape_thrown("[\\c:]"));
     assert(error_escape_thrown("\\c:"));
     assert(error_escape_thrown("\\c"));
     assert(!error_escape_thrown("[\\cA]"));
     assert(!error_escape_thrown("\\cA"));
 
 }