如何在正则表达式中精确匹配部分字符串并忽略模式前后的任何字符？

I am using golang regexp and I want to extract a pattern in a string.

For example, I can have the following name value pairs in the string:

"name1=val1;name2=val2;needle=haystack;name3=val3"

I am looking for exactly the string "needle=haystack" and discard anything else.

It would be even better if I could just get the result to be exactly haystack.

How do I do this with regexp in golang?

I'm not entirely clear on what the goal is. If you're always looking for needle=haystack then you can use strings.Contains(str, "needle=haystack").

If you really want to do it with regexes then it would be something like the code below.

package main

import (
    "fmt"
    "regexp"
)

func main() {
    str := "name1=val1;name2=val2;needle=haystack;name3=val3"

    r := regexp.MustCompile("needle=([a-z]+);")
    found := r.FindString(str)
    if found == "" {
        fmt.Println("No match found")
        return
    }
    fmt.Println(found) // needle=haystack;

    submatches := r.FindStringSubmatch(str)
    fmt.Println(submatches) // [needle=haystack; haystack]
    if len(submatches) < 2 {
        fmt.Println("No submatch found")
        return
    }
    fmt.Println(submatches[1]) // haystack
}