Interpretation of escapes in expansions in pattern matching contexts

[Dan Douglas]

I couldn't find anything obvious in POSIX that implies which interpretation is
correct. Assuming it's unspecified.

Bash (4.2.45) uniquely does interpret such escapes for [[, which makes me 
think this test should say "no":

    x=\\x; if [[ x == $x ]]; then echo yes; else echo no; fi

bash: yes
ksh:  no
mksh: no
zsh:  no

However, ksh93 (AJM 93v- 2013-03-17) is unique in that it flips the result
depending on "[[ ]]" or "case..esac" (bug?), but otherwise it looks like a
fairly random spread:

    x=\\x; case x in $x) echo yes;; *) echo no; esac

bash: yes
ksh:  yes
mksh: no
posh: no
zsh:  no
dash: yes
bb:   no
jsh:  no

18:42:44 jilles: ormaaj, I'm not sure if that's actually a bug
18:43:15 ormaaj: dunno. Bash seems unique in that respect
18:43:23 jilles: you're asking the shell to check if the string x matches the 
pattern stored in the variable x
19:32:51 jilles: freebsd sh and kmk_ash say no, dash says yes
19:33:40 jilles: Bourne shell says no

-- 
Dan Douglas