pattern: unmatched opening bracket should be treated as a literal #1372

Version: mvdan.cc/sh/v3 v3.13.1

The public pattern.Regexp API rejects an unmatched opening bracket, while Bash treats it as a literal character in shell pattern positions.

To reproduce, create an empty directory and save this as main.go:

package main

import (
	"fmt"

	"mvdan.cc/sh/v3/pattern"
)

func main() {
	regexp, err := pattern.Regexp("[", pattern.EntireString)
	fmt.Printf("regexp=%q error=%T %v\n", regexp, err, err)
}

Run it with:

go mod init example.com/mvdan-repro
go get mvdan.cc/sh/v3@v3.13.1
go run .

Actual output:

regexp="" error=*pattern.SyntaxError [ was not matched with a closing ]

The corresponding Bash pattern matches the literal opening bracket:

case "[" in
	[) echo match ;;
	*) echo miss ;;
esac

Bash output:

match

Would it be appropriate for shell-pattern compilation to treat an unmatched [ as a literal, while retaining errors for malformed constructs that Bash also rejects?