[Patch] Let ordinary escaping in POSIX regex be valid

[Paolo Carlini]

On 9/27/13 4:34 AM, Jonathan Wakely wrote:
> On 27 September 2013 03:15, Tim Shen wrote:
>> POSIX ERE says that escaping an ordinary char, say R"\n" is not
>> permitted, because 'n' is not a special char. However, they also say
>> that : "Implementations are permitted to extend the language to allow
>> these. Conforming applications cannot use such constructs."
>>
>> So let's support it not to make users surprised.
>>
>> Booted and tested under -m32 and -m64
> I'm wondering whether we want to have a stricter mode that doesn't
> allow them, to help users avoid creating non-portable programs.  We
> could check the value of the preprocessor macro __STRICT_ANSI__, which
> is set by -std=c++11 but not by -std=gnu++11, although that's not
> really the right flag. We want something more like the GNU shell
> utils' POSIXLY_CORRECT.
Indeed. I think that for now __STRICT_ANSI__ can do, it's important to 
manage to accept those otherwise, as we discovered yesterday, we easily 
reject quite a few rather sensible regex users can write or find in 
examples: this started when Tim, upon my suggestion, tried the examples 
in the new edition of Nicolai Josuttis book and found in one those an 
escaped closed curly bracket (note, closed, open are definitely fine), 
which apparently most of the other implementations do not reject.

Paolo.