@@ -125,6 +125,16 @@ func (r *Replacer) replace(input, empty string,
 	// iterate the input to find each placeholder
 	var lastWriteCursor int
 	for i := 0; i < len(input); i++ {
+
+		// check for the escape character
+		if i > 0 && input[i-1] == phEscape {
+			if input[i] == phClose || input[i] == phOpen {
+				sb.WriteString(input[lastWriteCursor : i-1])
+				lastWriteCursor = i
+			}
+			continue
+		}
+
 		if input[i] != phOpen {
 			continue
 		}
// Copyright 2015 Matthew Holt and The Caddy Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package caddy

import (
	"fmt"
	"os"
	"path/filepath"
	"runtime"
	"strings"
	"time"
)

// NewReplacer returns a new Replacer.
func NewReplacer() *Replacer {
	rep := &Replacer{
		static: make(map[string]string),
	}
	rep.providers = []ReplacerFunc{
		globalDefaultReplacements,
		rep.fromStatic,
	}
	return rep
}

// Replacer can replace values in strings.
// A default/empty Replacer is not valid;
// use NewReplacer to make one.
type Replacer struct {
	providers []ReplacerFunc
	static    map[string]string
}

// Map adds mapFunc to the list of value providers.
// mapFunc will be executed only at replace-time.
func (r *Replacer) Map(mapFunc ReplacerFunc) {
	r.providers = append(r.providers, mapFunc)
}

// Set sets a custom variable to a static value.
func (r *Replacer) Set(variable, value string) {
	r.static[variable] = value
}

// Get gets a value from the replacer. It returns
// the value and whether the variable was known.
func (r *Replacer) Get(variable string) (string, bool) {
	for _, mapFunc := range r.providers {
		if val, ok := mapFunc(variable); ok {
			return val, true
		}
	}
	return "", false
}

// Delete removes a variable with a static value
// that was created using Set.
func (r *Replacer) Delete(variable string) {
	delete(r.static, variable)
}

// fromStatic provides values from r.static.
func (r *Replacer) fromStatic(key string) (val string, ok bool) {
	val, ok = r.static[key]
	return
}

// ReplaceOrErr is like ReplaceAll, but any placeholders
// that are empty or not recognized will cause an error to
// be returned.
func (r *Replacer) ReplaceOrErr(input string, errOnEmpty, errOnUnknown bool) (string, error) {
	return r.replace(input, "", false, errOnEmpty, errOnUnknown, nil)
}

// ReplaceKnown is like ReplaceAll but only replaces
// placeholders that are known (recognized). Unrecognized
// placeholders will remain in the output.
func (r *Replacer) ReplaceKnown(input, empty string) string {
	out, _ := r.replace(input, empty, false, false, false, nil)
	return out
}

// ReplaceAll efficiently replaces placeholders in input with
// their values. All placeholders are replaced in the output
// whether they are recognized or not. Values that are empty
// string will be substituted with empty.
func (r *Replacer) ReplaceAll(input, empty string) string {
	out, _ := r.replace(input, empty, true, false, false, nil)
	return out
}

// ReplaceFunc calls ReplaceAll  efficiently replaces placeholders in input with
// their values. All placeholders are replaced in the output
// whether they are recognized or not. Values that are empty
// string will be substituted with empty.
func (r *Replacer) ReplaceFunc(input string, f ReplacementFunc) (string, error) {
	return r.replace(input, "", true, false, false, f)
}

func (r *Replacer) replace(input, empty string,
	treatUnknownAsEmpty, errOnEmpty, errOnUnknown bool,
	f ReplacementFunc) (string, error) {
	if !strings.Contains(input, string(phOpen)) {
		return input, nil
	}

	var sb strings.Builder

	// it is reasonable to assume that the output
	// will be approximately as long as the input
	sb.Grow(len(input))

	// iterate the input to find each placeholder
	var lastWriteCursor int
	for i := 0; i < len(input); i++ {
		if input[i] != phOpen {
			continue
		}

		// find the end of the placeholder
		end := strings.Index(input[i:], string(phClose)) + i
		if end < i {
			continue
		}

		// write the substring from the last cursor to this point
		sb.WriteString(input[lastWriteCursor:i])

		// trim opening bracket
		key := input[i+1 : end]

		// try to get a value for this key, handle empty values accordingly
		val, found := r.Get(key)
		if !found {
			// placeholder is unknown (unrecognized); handle accordingly
			if errOnUnknown {
				return "", fmt.Errorf("unrecognized placeholder %s%s%s",
					string(phOpen), key, string(phClose))
			} else if !treatUnknownAsEmpty {
				// if treatUnknownAsEmpty is true, we'll
				// handle an empty val later; so only
				// continue otherwise
				lastWriteCursor = i
				continue
			}
		}

		// apply any transformations
		if f != nil {
			var err error
			val, err = f(key, val)
			if err != nil {
				return "", err
			}
		}

		// write the value; if it's empty, either return
		// an error or write a default value
		if val == "" {
			if errOnEmpty {
				return "", fmt.Errorf("evaluated placeholder %s%s%s is empty",
					string(phOpen), key, string(phClose))
			} else if empty != "" {
				sb.WriteString(empty)
			}
		} else {
			sb.WriteString(val)
		}

		// advance cursor to end of placeholder
		i = end
		lastWriteCursor = i + 1
	}

	// flush any unwritten remainder
	sb.WriteString(input[lastWriteCursor:])

	return sb.String(), nil
}

// ReplacerFunc is a function that returns a replacement
// for the given key along with true if the function is able
// to service that key (even if the value is blank). If the
// function does not recognize the key, false should be
// returned.
type ReplacerFunc func(key string) (val string, ok bool)

func globalDefaultReplacements(key string) (string, bool) {
	// check environment variable
	const envPrefix = "env."
	if strings.HasPrefix(key, envPrefix) {
		return os.Getenv(key[len(envPrefix):]), true
	}

	switch key {
	case "system.hostname":
		// OK if there is an error; just return empty string
		name, _ := os.Hostname()
		return name, true
	case "system.slash":
		return string(filepath.Separator), true
	case "system.os":
		return runtime.GOOS, true
	case "system.arch":
		return runtime.GOARCH, true
	case "time.now.common_log":
		return nowFunc().Format("02/Jan/2006:15:04:05 -0700"), true
	}

	return "", false
}

// ReplacementFunc is a function that is called when a
// replacement is being performed. It receives the
// variable (i.e. placeholder name) and the value that
// will be the replacement, and returns the value that
// will actually be the replacement, or an error. Note
// that errors are sometimes ignored by replacers.
type ReplacementFunc func(variable, val string) (string, error)

// nowFunc is a variable so tests can change it
// in order to obtain a deterministic time.
var nowFunc = time.Now

// ReplacerCtxKey is the context key for a replacer.
const ReplacerCtxKey CtxKey = "replacer"

const phOpen, phClose = '{', '}'

For some reason, the placement of this block isn't sitting well with me. My bug sense is tingling. Maybe new test cases will help validate or dissolve my concerns. I'm not sure that `continue` is in the right place. Does that skip all backslashes? Doesn't this immediately write all content (minus backslash) since the `lastWriteCursor`, and then skip all the logic below? I feel like we need to maybe put backslash-skipping logic around existing `WriteString` calls. (Does that make sense?) I'm just supposing here -- sorry if it turns out to be wrong!