simple rewrite

foooos.go

package main

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

func main() {
    re, err := regexp.Compile("fo+")
    if err != nil {
        panic(err)
    }
    if len(os.Args) == 0 {
        panic("no input file")
    }
    f, err := os.Open(os.Args[1])
    if err != nil {
        panic(err)
    }
    defer func() {
        if err := f.Close(); err != nil {
            panic(err)
        }
    }()
    scanner := bufio.NewScanner(f)
    for scanner.Scan() {
        str := scanner.Bytes()
        if re.Match(str) {
            fmt.Print("foooo")
        } else {
            panic("syntax error")
        }
    }
    fmt.Println()
}