Remove the b/basic regex flag

[hadrielk]

As part of #1861, this issue is to track removing the support for the "basic" regex flag that sets the pattern to BRE syntax.

The questions are:

1. Should the "b"/"basic" flag be removed altogether, and we pretend it doesn't exist? Or should it still exist in the list of flags, be annotated "deprecated" in the print output, and cause an explicit fatal error if used?

2. Should the "e"/"extend" flag for ERE syntax also be removed, since there is no other syntax option anyway and it's pointless?

[masatake]

Let's remove b for updating the regex engine.

1. ...be annotated "deprecated" in the print output, and cause an explicit fatal error if used?
   Yes.

Should the "e"/"extend" flag for ERE syntax also be removed, since there is no other syntax option anyway and it's pointless?

Please, keep e flag. I assume e is still used in some old .ctags files.
Removing b has very strong reason. However, e has not.
Even if we remove e, we cannot reuse e for another purpose.

Both e and b are kinds of retired numbers.

BTW, though I like using long names for flags, we can still introduce a letter flag(short flag) if it helps writing .ctags. I have not enough experience to decide whether a flag should be short. So I always choose long names.

[masatake]

I found there is a case using b: whiteinge/dotfiles@b18ff64 .

[hadrielk]

Interesting. Well we can either ask that person to change it, or we can implement a convertor for basic->extended syntax.