One bracket, five opinions

bash's rule for a [ that never finds its ] is one sentence: it is an ordinary character. huck matched patterns in five places — [[ == ]], case, the ${…#…} family, completion's -X filter and filename globbing — and only globbing had that sentence. The others had each grown their own way of coping with the pattern library refusing the [, so a case arm or a ${v/…/…} that should have matched simply did nothing:

# before
$ huck -c 'case "a[b" in a[b) echo matched;; *) echo no;; esac'
no
$ huck -c 'v="a[b"; echo "${v/[/X}"'
a[b

# after
$ huck -c 'case "a[b" in a[b) echo matched;; *) echo no;; esac'
matched
$ huck -c 'v="a[b"; echo "${v/[/X}"'
aXb

[[ ]] was the loud one, turning the library's complaint into an error and an exit status:

# before
$ huck -c '[[ "[x" == [x ]] && echo yes || echo no'
huck: line 1: [[: bad pattern: Pattern syntax error near position 0: invalid range pattern
no

# after
$ huck -c '[[ "[x" == [x ]] && echo yes || echo no'
yes

The fix is one place all five now call, which decides which engine matches the pattern and applies bash's bracket rules first. Reading those rules in bash's source turned up more than the one sentence. A range with no end, [a- at the end of a pattern, matches nothing — not even itself. Inside an extended glob like @(…), the search for the closing ) runs the bracket scan first, so @([x) is five literal characters rather than a group. And ${v/pat/rep} does not look for the longest match when the pattern has no * outside a bracket: it tries substrings of exactly one length, counted from the pattern, and a lone [ counts every character it swallows — which is why ${v/[*/X} replaces exactly two characters in bash and now in huck.

The same pass fixed what set -x shows for a quoted pattern. huck's internal spelling of "this * is literal" had been leaking into the trace:

# before
$ huck -c 'set -x; [[ abc == "a*" ]]'
+ [[ abc == a[*] ]]

# after
$ huck -c 'set -x; [[ abc == "a*" ]]'
+ [[ abc == \a\* ]]

The pattern text huck carries around is now written the way bash writes it — a backslash before each quoted character — and the one matching chokepoint translates it for the library. That also made completion's -X filter honour [[:alpha:]] and friends, which it had been passing straight to a library that does not know them.

Still open: x[ at the very end of a word is read by the lexer as the start of a subscript (#75), so it never reaches the matcher; and [[ =~ ]] regexes are a separate engine with their own open issue (#51).