What version of Go are you using (go version)?

go version devel +7e054553ad Tue Jan 16 15:11:05 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

Use some shell patterns with path.Match.

https://play.golang.org/p/aTUkgwmS-Z6

What did you expect to see?

true <nil>
true <nil>
true <nil>
true <nil>

What did you see instead?

true <nil>
false syntax error in pattern
false syntax error in pattern
false <nil>

For reference, this is a program one can use with Bash to reproduce the expected output:

$ cat f3.sh
[[ '[' == [[ab] ]]; echo $?
[[ ']' == []ab] ]]; echo $?
[[ '-' == [-ab] ]]; echo $?
[[ '\' == [\\ab] ]]; echo $?
$ bash f3.sh
0
0
0
0

There are three small issues here. I'll be quoting the POSIX spec that covers bracket expressions, as seen in 9.3.5: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05

In order:

• ] should not have special meaning if it's the first in the list. This is not done properly, as can be seen by the []ab] test case. Quoting:

The ( ']' ) shall lose its special meaning and represent itself in a bracket expression if it occurs first in the list (after an initial ( '^' ), if any). Otherwise, it shall terminate the bracket expression

• - should not have special meaning if it's the first or last in the list. The relevant test case is [-ab]. Quoting:

The character shall be treated as itself if it occurs first (after an initial '^', if any) or last in the list, or as an ending range point in a range expression. As examples, the expressions "[-ac]" and "[ac-]" are equivalent

• Escaping should not happen within a bracket expressions. The relevant test case is [\ab] (Bash source needs double quoting to get the correct pattern). Quoting:

The special characters '.', '*', '[', and '\' ( , , , and , respectively) shall lose their special meaning within a bracket expression.

I know that not all POSIX Shell pattern matching rules are implemented, like collating symbols and locale. However, these seem like fairly basic stuff that would be easy to fix. I am more than happy to submit patches to fix these in Go 1.11.

The first two changes would simply fix some patterns that used to error. All programs working today would continue to work.

The third change is a bit trickier. If someone is using [\ab] as a pattern nowadays, it currently means "a, or b". But with the change, it would then mean "\, or a, or b" (as in a shell pattern match). This could potentially break some programs.

I would argue that we should apply the third change anyway. The change can be carefully noted in the release notes, and the previous behavior is likely confusing users that expect pattern matching to work like in the shell. I realise that the syntax is specified in the godoc, but still.