map: Fix regex mappings

It didn't really make sense how we were doing them before. See https://caddy.community/t/map-directive-and-regular-expressions/13866/6?u=matt

modules/caddyhttp/map/map.go

@@ -136,7 +136,11 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhtt
 					if output := m.Outputs[destIdx]; output == nil {
 						continue
 					} else {
-						output = m.re.ReplaceAllString(input, m.Outputs[destIdx].(string))
+						var result []byte
+						for _, submatches := range m.re.FindAllStringSubmatchIndex(input, -1) {
+							result = m.re.ExpandString(result, m.Outputs[destIdx].(string), input, submatches)
+						}
+						output = string(result)
 						return output, true
 					}
 				}

modules/caddyhttp/map/map_test.go

@@ -0,0 +1,96 @@
+package maphandler
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"reflect"
+	"testing"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+)
+
+func TestHandler(t *testing.T) {
+	for i, tc := range []struct {
+		handler Handler
+		reqPath string
+		expect  map[string]interface{}
+	}{
+		{
+			reqPath: "/foo",
+			handler: Handler{
+				Source:       "{http.request.uri.path}",
+				Destinations: []string{"{output}"},
+				Mappings: []Mapping{
+					{
+						Input:   "/foo",
+						Outputs: []interface{}{"FOO"},
+					},
+				},
+			},
+			expect: map[string]interface{}{
+				"output": "FOO",
+			},
+		},
+		{
+			reqPath: "/abcdef",
+			handler: Handler{
+				Source:       "{http.request.uri.path}",
+				Destinations: []string{"{output}"},
+				Mappings: []Mapping{
+					{
+						InputRegexp: "(/abc)",
+						Outputs:     []interface{}{"ABC"},
+					},
+				},
+			},
+			expect: map[string]interface{}{
+				"output": "ABC",
+			},
+		},
+		{
+			reqPath: "/ABCxyzDEF",
+			handler: Handler{
+				Source:       "{http.request.uri.path}",
+				Destinations: []string{"{output}"},
+				Mappings: []Mapping{
+					{
+						InputRegexp: "(xyz)",
+						Outputs:     []interface{}{"...${1}..."},
+					},
+				},
+			},
+			expect: map[string]interface{}{
+				"output": "...xyz...",
+			},
+		},
+	} {
+		if err := tc.handler.Provision(caddy.Context{}); err != nil {
+			t.Fatalf("Test %d: Provisioning handler: %v", i, err)
+		}
+
+		req, err := http.NewRequest(http.MethodGet, tc.reqPath, nil)
+		if err != nil {
+			t.Fatalf("Test %d: Creating request: %v", i, err)
+		}
+		repl := caddyhttp.NewTestReplacer(req)
+		ctx := context.WithValue(req.Context(), caddy.ReplacerCtxKey, repl)
+		req = req.WithContext(ctx)
+
+		rr := httptest.NewRecorder()
+		noop := caddyhttp.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) error { return nil })
+
+		if err := tc.handler.ServeHTTP(rr, req, noop); err != nil {
+			t.Errorf("Test %d: Handler returned error: %v", i, err)
+			continue
+		}
+
+		for key, expected := range tc.expect {
+			actual, _ := repl.Get(key)
+			if !reflect.DeepEqual(actual, expected) {
+				t.Errorf("Test %d: Expected %#v but got %#v for {%s}", i, expected, actual, key)
+			}
+		}
+	}
+}