IPv4正则表达式捕获地址的不正确部分

I'm trying to write a program that prints the invalid part or parts of an IPv4 address from terminal input.

Here is my code:

package chapter4

import (
    "bufio"
    "fmt"
    "os"
    "regexp"
    "strings"
    "time"
)

func IPV4() {
    var f *os.File
    f = os.Stdin
    defer f.Close()

    scanner := bufio.NewScanner(f)
    fmt.Println("Exercise 1, Chapter 4 - Detecting incorrect parts of IPv4 Addresses, enter an address!")
    for scanner.Scan() {

        if scanner.Text() == "STOP" {
            fmt.Println("Initializing Level 4...")
            time.Sleep(5 * time.Second)
            break
        }
        expression := "(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])"
        matchMe, err := regexp.Compile(expression)
        if err != nil {
            fmt.Println("Could not compile!", err)
        }

        s := strings.Split(scanner.Text(), ".")

        for _, value := range s {
            fmt.Println(value)
            str := matchMe.FindString(value)
            if len(str) == 0 {
                fmt.Println(value)
            }
        }

    }
}

My thought process is that for every terminal IP address input, I split the string by '.'

Then I iterate over the resulting []string and match each value to the regular expression.

For some reason the only case where the regex expression doesn't match is when there are letter characters in the input. Every number, no matter the size or composition, is a valid match for my expression.

I'm hoping you can help me identify the problem, and if there's a better way to do it, I'm all ears. Thanks!

</div>

Maybe, this expression might be closer to what you might have in mind:

^(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)$

Test

package main

import (
    "regexp"
    "fmt"
)

func main() {
    var re = regexp.MustCompile(`(?m)^(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)$`)
    var str = `127.0.0.1
192.168.1.1
192.168.1.255
255.255.255.255
0.0.0.0
1.1.1.01

30.168.1.255.1
127.1
192.168.1.256
-1.2.3.4
3...3`

    for i, match := range re.FindAllString(str, -1) {
        fmt.Println(match, "found at index", i)
    }
}

The expression is explained on the top right panel of regex101.com, if you wish to explore/simplify/modify it, and in this link, you can watch how it would match against some sample inputs, if you like.

Reference:

Validating IPv4 addresses with regexp

RegEx Circuit

jex.im visualizes regular expressions:

I am pretty sure that your expression needs anchors or the last part of it will match any single digit and succeed. Try using ^ on the front and $ on the back.