Golang: Sum, sort, and print values by User from data given in a file

main.go

package main

import (
	"os"
	"fmt"
	"io/ioutil"
	"strings"
	"regexp"
	"strconv"
	"sort"
)

func main() {
	file, err := os.Open("users.txt")
	if err != nil {
		fmt.Errorf("Could not open input file")
	}
	data, err := ioutil.ReadAll(file)
	if err != nil {
		fmt.Errorf("Unable to read file")
	}
	user_times := map[string]int{}
	reg := regexp.MustCompile(`^\*?\s*(\w+) (\d+)`)
	keys := []string{}

	for _, line := range strings.Split(string(data), "\n") {
		matches := reg.FindStringSubmatch(line)
		if len(matches) >= 3 {  // whole string + groups
			if _, ok := user_times[matches[1]]; !ok {
				user_times[matches[1]] = 0
				keys = append(keys, matches[1])
			}
			user_time, err := strconv.Atoi(matches[2])
			if err != nil {
				fmt.Errorf("Error converting '%s'", matches[2])
			}
			user_times[matches[1]] += user_time
		}
	}
	sort.Strings(keys)
	for _, key := range keys {
		fmt.Println(key,":", user_times[key])
	}
}

// Users.txt will contain this data listed here for convenience
var file_data string = `
# Format:  <user> <time>
# Asterisk: secure connection used.
Alice 13
John 440
Bob 2
Carl 49
* Peter 90
Alice 219
Bob 8
Alice 72
* Jim 1
Peter 97
Jim 199
`