FWIW, the "normalized", though equal form

package main

import (
    "fmt"
    "regexp"
)

func main() {
    pat := regexp.MustCompilePOSIX(`^([^:=]*)(:?=)(.*)$`)
    m := pat.FindStringSubmatch("x:=y")
    fmt.Printf("%q\n", m)
}

prints

["x:=y" "x" ":=" "y"]

(http://play.golang.org/p/2ogIBqdnpd)

AFAIK, match-group should work with specified order. not longest. At least, perl seems to work specified order. ^([^:=]*)(:|:=)(.*)$

My understanding of POSIX leftmost-longest is that it should pick the longest possible alternative. Switching the alternatives in the example from this issue does make it pick a different alternative, which doesn't seem right.

regexp engine of vim works as ordered.

Yes, the leftmost-longest only applies to the overall match. Within the match, it is the "first match" semantics of Perl. When POSIX mandated the submatch leftmost-longest rule they had no idea how to implement it without exponential time. It's not trivial and not worth it.

(It's possible but no one does except maybe Haskell.)