Add regexp.Regexp support

It's useful to support validated regular expressions in configuration
files. Previously, we could represent these fields as raw pattern
strings and compile them in application code. By supporting Regexp as a
first-class field type, we get a smaller application code and more
immediate error reporting without complicating the API.

README.md

@@ -20,7 +20,7 @@ fig is a tiny library for loading an application's config file and its environme
 - Define your **configuration**, **validations** and **defaults** in a single location
 - Optionally **load from the environment** as well
 - Only **3** external dependencies
-- Full support for`time.Time` & `time.Duration`
+- Full support for`time.Time`, `time.Duration` & `regexp.Regexp`
 - Tiny API
 - Decoders for `.yaml`, `.json` and `.toml` files
 
@@ -64,8 +64,9 @@ type Config struct {
     Cleanup time.Duration `fig:"cleanup" default:"30m"`
   }
   Logger struct {
-    Level string `fig:"level" default:"info"`
-    Trace bool   `fig:"trace"`
+    Level   string         `fig:"level" default:"info"`
+    Pattern *regexp.Regexp `fig:"pattern" default:".*"`
+    Trace   bool           `fig:"trace"`
   }
 }
 

fig.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -289,6 +290,12 @@ func (f *fig) setValue(fv reflect.Value, val string) error {
 				return err
 			}
 			fv.Set(reflect.ValueOf(t))
+		} else if _, ok := fv.Interface().(regexp.Regexp); ok {
+			re, err := regexp.Compile(val)
+			if err != nil {
+				return err
+			}
+			fv.Set(reflect.ValueOf(*re))
 		} else {
 			return fmt.Errorf("unsupported type %s", fv.Kind())
 		}

fig_test.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"regexp"
 	"strings"
 	"testing"
 	"time"
@@ -221,8 +222,9 @@ func Test_fig_Load_Defaults(t *testing.T) {
 					Host   string `fig:"host" default:"127.0.0.1"`
 					Ports  []int  `fig:"ports" default:"[80,443]"`
 					Logger struct {
-						LogLevel   string `fig:"log_level" default:"info"`
-						Production bool   `fig:"production"`
+						LogLevel   string         `fig:"log_level" default:"info"`
+						Pattern    *regexp.Regexp `fig:"pattern" default:".*"`
+						Production bool           `fig:"production"`
 						Metadata   struct {
 							Keys []string `fig:"keys" default:"[ts]"`
 						}
@@ -236,6 +238,7 @@ func Test_fig_Load_Defaults(t *testing.T) {
 				want.Host = "0.0.0.0"
 				want.Ports = []int{80, 443}
 				want.Logger.LogLevel = "debug"
+				want.Logger.Pattern = regexp.MustCompile(".*")
 				want.Logger.Production = false
 				want.Logger.Metadata.Keys = []string{"ts"}
 				want.Application.BuildDate = time.Date(2020, 1, 1, 12, 0, 0, 0, time.UTC)
@@ -1014,6 +1017,35 @@ func Test_fig_setValue(t *testing.T) {
 		}
 	})
 
+	t.Run("regexp", func(t *testing.T) {
+		var re regexp.Regexp
+		fv := reflect.ValueOf(&re).Elem()
+
+		err := fig.setValue(fv, "[a-z]+")
+		if err != nil {
+			t.Fatalf("unexpected err: %v", err)
+		}
+
+		want, err := regexp.Compile("[a-z]+")
+		if err != nil {
+			t.Fatalf("error parsing time: %v", err)
+		}
+
+		if re.String() != want.String() {
+			t.Fatalf("want %v, got %v", want, re)
+		}
+	})
+
+	t.Run("bad regexp", func(t *testing.T) {
+		var re regexp.Regexp
+		fv := reflect.ValueOf(&re).Elem()
+
+		err := fig.setValue(fv, "[a-")
+		if err == nil {
+			t.Fatalf("expected err")
+		}
+	})
+
 	t.Run("interface returns error", func(t *testing.T) {
 		var i interface{}
 		fv := reflect.ValueOf(i)
@@ -1089,6 +1121,15 @@ func Test_fig_setSlice(t *testing.T) {
 			},
 			Val: "[2019-12-25T10:30:30Z,2020-01-01T00:00:00Z]",
 		},
+		{
+			Name:    "regexps",
+			InSlice: &[]*regexp.Regexp{},
+			WantSlice: &[]*regexp.Regexp{
+				regexp.MustCompile("[a-z]+"),
+				regexp.MustCompile(".*"),
+			},
+			Val: "[[a-z]+,.*]",
+		},
 	} {
 		t.Run(tc.Val, func(t *testing.T) {
 			in := reflect.ValueOf(tc.InSlice).Elem()