如何强制正则表达式匹配正则表达式开头和结尾的可选组。例如：
package main
import (
"fmt"
"regexp"
)
func main() {
//here ok as exact match. The matched string starts by one of the keyword and ends by one of them.
re := regexp.MustCompile(`(keyword1)\W*(?:\w+\W+){0,6}(keyword2)\W*(?:\w+\W+){0,6}(keyword3)`)
text := "start text keyword1 optional word here then keyword2 again optional words then keyword3 others words after."
s := re.FindAllString(text, -1)
fmt.Println(s)
//Here not exact match as the result doesn't start by keyword1 and end by keyword3
re = regexp.MustCompile(`(keyword1)*\W*(?:\w+\W+){0,6}(keyword2)\W*(?:\w+\W+){0,6}(keyword3)*`)
text = "start text keyword1 optional word then keyword2 optional words keyword3 others words after."
s = re.FindAllString(text, -1)
fmt.Println(s)
}
这里，keyword1和keywrod3分别位于正则表达式的开头和结尾，是可选的。我按此顺序查找keyword1，keyword2和keyword3。关键字之间最多可以有6个单词，但匹配的字符串应该以一个关键字开头，并以其中一个关键字结束。不需要额外的字符串。有没有办法做到这一点。任何建议都是受欢迎的。以下是代码和指向Go Playground的链接：
{{1}}
由于