ISO8601 duration format parser

iso8601-duration-parser.go

package main

import (
	"fmt"
	"regexp"
	"strconv"
	"time"
)

func main() {
	durationString := "P2YT1H8.001S"
	fmt.Println("duration: ", durationString)

	re := regexp.MustCompile(`^(-?)P(?:T*|\d)(?:(-?\d+)Y)?(?:(-?\d+)M)?(?:(-?\d+)([DW]))?(?:T(?:(-?\d+)H)?(?:(-?\d+)M)?(?:(-?\d+(?:\.\d+)?)S)?)?$`)
	matches := re.FindStringSubmatch(durationString)
	if len(matches) == 0 {
		panic(fmt.Errorf("your regular expression does not match the subject string"))
	}

	var (
		numOffset int
		years     int
		months    int
		days      int
		hours     int
		minutes   int
		seconds   float64
	)

	if s := matches[1]; s != "" {
		numOffset = -1
	} else {
		numOffset = 1
	}

	if i32, err := strconv.Atoi(matches[2]); err == nil {
		years = numOffset * i32
	}
	if i32, err := strconv.Atoi(matches[3]); err == nil {
		months = numOffset * i32
	}
	if i32, err := strconv.Atoi(matches[4]); err == nil {
		switch matches[5] {
		case "W":
			days = numOffset * i32 * 7
		case "D":
			days = numOffset * i32
		}
	}
	if i32, err := strconv.Atoi(matches[6]); err == nil {
		hours = numOffset * i32
	}
	if i32, err := strconv.Atoi(matches[7]); err == nil {
		minutes = numOffset * i32
	}
	if f64, err := strconv.ParseFloat(matches[8], 64); err == nil {
		seconds = float64(numOffset) * f64
	}

	fmt.Println("Group 1 (offset)", numOffset)
	fmt.Println("Group 2 (year)", years)
	fmt.Println("Group 3 (month)", months)
	fmt.Println("Group 4 (day)", days)
	fmt.Println("Group 6 (hour)", hours)
	fmt.Println("Group 7 (minute)", minutes)
	fmt.Println("Group 8 (second)", seconds)

	now := time.Now()
	fmt.Println("current time: ", now.Format(time.RFC3339))

	result := now.AddDate(years, months, days)
	result = result.Add(time.Hour * time.Duration(hours))
	result = result.Add(time.Minute * time.Duration(minutes))
	result = result.Add(time.Second * time.Duration(seconds))

	fmt.Println("expiration time: ", result.Format(time.RFC3339))
}