[bug-gawk] escaping open curly bracket produces unexpected warning with --posix

[Ed Morton]

I just came across this in gawk 5.0.0 (note "4"):

   1) $ echo 'foo{bar' | awk '/{/'
   foo{bar

   2) $ echo 'foo{bar' | awk --posix '/{/'
   awk: cmd. line:1: error: Invalid preceding regular expression: /{/

   3) $ echo 'foo{bar' | awk '/\{/'
   foo{bar

   4) $ echo 'foo{bar' | awk --posix '/\{/'
   awk: cmd. line:1: warning: regexp escape sequence `\{' is not a
   known regexp operator
   foo{bar

I understand 1, 2, and 3 but I don't understand why I get the warning 
from 4 above. I expected `\{` to simply be treated as a literal `{` with 
no warnings just like other RE metachars are:

   $ echo 'foo[bar' | awk --posix '/\[/'
   foo[bar

   $ echo 'foo+bar' | awk --posix '/\+/'
   foo+bar

Instead I had to put it inside square brackets to get it treated as 
literal with no warnings:

   $ echo 'foo{bar' | awk --posix '/[{]/'
   foo{bar

as no amount of additional preceding backslashes would make it "work" 
without a warning:

   $ echo 'foo{bar' | awk --posix '/\{/'
   awk: cmd. line:1: warning: regexp escape sequence `\{' is not a
   known regexp operator
   foo{bar
   $ echo 'foo{bar' | awk --posix '/\\{/'
   $ echo 'foo{bar' | awk --posix '/\\\{/'
   awk: cmd. line:1: warning: regexp escape sequence `\{' is not a
   known regexp operator
   $ echo 'foo{bar' | awk --posix '/\\\\{/'
   $

Regards,

    Ed.