Regex based unmarshaler

unmarshal.go

package main

import (
	"encoding"
	"fmt"
	"reflect"
	"regexp"
	"strconv"
)

var tagIndex = map[reflect.Type]map[string]int{}

func unmarshal[T any](r *regexp.Regexp, text string) (T, error) {
	t := reflect.TypeOf(*new(T))
	v := reflect.New(t)

	if _, ok := tagIndex[t]; !ok {
		tagIndex[t] = map[string]int{}
	}

	matches := r.FindStringSubmatch(text)
	for i, name := range r.SubexpNames() {
		index, ok := tagIndex[t][name]
		if !ok {
			index = -1
			for i := 0; i < t.NumField(); i++ {
				if t.Field(i).Tag.Get("match") == name {
					tagIndex[t][name] = i
					index = i
					break
				}
			}
			if index == -1 {
				continue
			}
		}
		f := v.Elem().Field(tagIndex[t][name])
		v, err := caster(f.Type(), matches[i])
		if err != nil {
			return *new(T), err
		}
		f.Set(v)
	}

	return v.Elem().Interface().(T), nil
}

func caster(t reflect.Type, s string) (reflect.Value, error) {
	switch t.Kind() {
	case reflect.String:
		return reflect.ValueOf(s), nil
	case reflect.Int:
		i, err := strconv.Atoi(s)
		if err != nil {
			return reflect.Value{}, err
		}
		return reflect.ValueOf(i), nil
	case reflect.Struct:
		v := reflect.New(t)
		decoder, ok := v.Interface().(encoding.TextUnmarshaler)
		if !ok {
			return reflect.Value{}, fmt.Errorf("unable to decode %v", t)
		}
		if err := decoder.UnmarshalText([]byte(s)); err != nil {
			return reflect.Value{}, err
		}
		return v.Elem(), nil
	default:
		return reflect.Value{}, fmt.Errorf("unsupported type: %v", t.Kind())
	}
}

unmarshal_test.go

package main

import (
	"regexp"
	"testing"
	"time"

	"github.com/stretchr/testify/assert"
)

func TestUnmarshal(t *testing.T) {
	var (
		userExample = regexp.MustCompile(`username=(?P<username>.*)\sage=(?P<age>\d*)\semail=(?P<email>.*)`)
	)

	type example struct {
		Username string `match:"username"`
		Age      int    `match:"age"`
		Email    string `match:"email"`
	}

	for name, test := range map[string]struct {
		Input  string
		Regex  *regexp.Regexp
		Expect example
	}{
		"basic 1": {
			Input: `username=bob age=21 email=bob@bob.com`,
			Regex: userExample,
			Expect: example{
				Username: "bob",
				Age:      21,
				Email:    "bob@bob.com",
			},
		},
		"basic 2": {
			Input: `username=sally age=22 email=sally@example.com`,
			Regex: userExample,
			Expect: example{
				Username: "sally",
				Age:      22,
				Email:    "sally@example.com",
			},
		},
	} {
		t.Run(name, func(t *testing.T) {
			e, err := unmarshal[example](userExample, test.Input)
			if err != nil {
				t.Fatal(err)
			}
			assert.Equal(t, test.Expect.Username, e.Username)
			assert.Equal(t, test.Expect.Age, e.Age)
			assert.Equal(t, test.Expect.Email, e.Email)
		})
	}
}

func TestComplexUnmarshal(t *testing.T) {
	var (
		timeExample = regexp.MustCompile(`time=(?P<time>.*)`)
	)

	type example struct {
		Time time.Time `match:"time"`
	}

	for name, test := range map[string]struct {
		Input  string
		Regex  *regexp.Regexp
		Expect example
	}{
		"basic 1": {
			Input: `time=2024-04-01T00:00:00Z`,
			Regex: timeExample,
			Expect: example{
				Time: time.Date(2024, time.April, 1, 0, 0, 0, 0, time.UTC),
			},
		},
	} {
		t.Run(name, func(t *testing.T) {
			e, err := unmarshal[example](timeExample, test.Input)
			if err != nil {
				t.Fatal(err)
			}
			assert.Equal(t, test.Expect.Time, e.Time)
		})
	}
}

Some improvements that can be made:

• Support more types
  • Support UnmarshalText interface
• Default to field name matching (similiar to how json works)
• Tag Index might be unsafe. Since we're only adding to it, chances are it's fine but still, should add some safety