Re: Word boundary anchors \< and \> not parsed correctly on the right side of =~

[Paulo Marcel Coelho Aragão]

On Tue, Jul 10, 2018 at 08:27:18AM -0400, Greg Wooledge wrote:
> Bash uses ERE (Extended Regular Expressions) here.  There is no \< or \>
> in an ERE.
> 
> http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04

Thanks for the reference. The document lists only ^ and $ as anchors. However, 
this indicates to me that bash recognizes \< \> as word anchors:

    paulo:~$ re='\<foo\>'
    paulo:~$ [[ 'foo bar' =~ $re ]] && echo yes || echo no
    yes
    paulo:~$ [[ 'foobar' =~ $re ]] && echo yes || echo no
    no

It baffles me that literal \< \> are not evaluated correctly

    paulo:~$ [[ 'foo bar' =~ \<foo\> ]] && echo yes || echo no
    no

This indicates that bash also recognizes \b as word anchor:

    paulo:~$ re='\bfoo\b'
    paulo:~$ [[ 'foo bar' =~ $re ]] && echo yes || echo no
    yes
    paulo:~$ [[ 'foobar' =~ $re ]] && echo yes || echo no
    no

but it also baffles me literal \b is not evaluated correctly:

    paulo:~$ [[ 'foo bar' =~ \bfoo\b ]] && echo yes || echo no
    no

Empirically all this demonstrates:

1. bash recognizes \< \> \b as word anchors
2. bash doesn't evaluate them correctly when used as literals
3. bash evaluates them correctly when used with parameter expansion