Negative look-ahead in Go regular expressions

Negative lookahead isn’t supported for technical reasons, specifically because it conflicts with the O(n)-time guarantees of the library. See the golang-nuts group discussion about this, as well as the Caveats section in Regular Expression Matching in the Wild.

You can express the regular expression you’ve described without negative lookahead:

BBB([^B]|B[^B]|BB[^B])*EEE

Here’s an example to demonstrate:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    re := regexp.MustCompile(`BBB([^B]|B[^B]|BB[^B])*EEE`)
    fmt.Printf("%#v\n", re.FindAllString("BBB EEE BBB..BBB...EEE", -1))
}