On 5/10/17 1:48 AM, Sundeep Agarwal wrote:

> my question is whether these regular expression examples come under
> 'obscure regular expressions' mentioned in the man page.

No, they're bugs in grep's regular expression implementation, which is taken
from glibc. I filed a bug report against glibc here:

https://sourceware.org/bugzilla/show_bug.cgi?id=25322

and installed the attached patches to GNU grep to try to document this mess
better. The first patch is the important one; the other two merely standardize
spelling and fix a typo in the first patch. Thanks for reporting the problem.