Check if lines of the provided files are valid email addresses. Regular expression for checking emails is from the awsome book “Let's Go!” by Alex Edwards — https://lets-go.alexedwards.net/

email_check.go

package main

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

var EmailRX = regexp.MustCompile("^[a-zA-Z0-9\\.!#$%&'*+\\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$")

func main() {
	for _, s := range os.Args[1:] {
		f, err := os.Open(s)
		if err != nil {
			fmt.Fprintln(os.Stderr, err)
			continue
		}
		defer f.Close()

		scanner := bufio.NewScanner(f)
		line := 1
		for scanner.Scan() {
			addr := scanner.Text()
			if !EmailRX.MatchString(addr) {
				fmt.Fprintf(os.Stderr, "error in %s +%d: %q isn't a valid email address\n", f.Name(), line, addr)
			}
			line++
		}
		if err := scanner.Err(); err != nil {
			fmt.Fprintln(os.Stderr, err)
		}
	}
}