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// Copyright 2012 The Gorilla Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package reverse
import (
// Regexp stores a regular expression that can be "reverted" or "built":
// outermost capturing groups become placeholders to be filled by variables.
type Regexp struct {
compiled *regexp.Regexp // compiled regular expression
template string // reverse template
groups []string // order of positional and named capturing groups;
// names for named and empty strings for positional
indices []int // indices of the outermost groups
// CompileRegexp compiles a regular expression pattern and creates a template
// to revert it.
func CompileRegexp(pattern string) (*Regexp, error) {
compiled, err := regexp.Compile(pattern)
if err != nil {
return nil, err
re, err := syntax.Parse(pattern, syntax.Perl)
if err != nil {
return nil, err
tpl := &template{buffer: new(bytes.Buffer)}
return &Regexp{
compiled: compiled,
template: tpl.buffer.String(),
groups: tpl.groups,
indices: tpl.indices,
}, nil
// Compiled returns the compiled regular expression to be used for matching.
func (r *Regexp) Compiled() *regexp.Regexp {
return r.compiled
// Template returns the reverse template for the regexp, in fmt syntax.
func (r *Regexp) Template() string {
return r.template
// Groups returns an ordered list of the outermost capturing groups found in
// the regexp.
// Positional groups are listed as an empty string and named groups use
// the group name.
func (r *Regexp) Groups() []string {
return r.groups
// Indices returns the indices of the outermost capturing groups found in
// the regexp.
// Not all indices may be present because nested capturing groups are ignored.
func (r *Regexp) Indices() []int {
return r.indices
// Match returns whether the regexp matches the given string.
func (r *Regexp) MatchString(s string) bool {
return r.compiled.MatchString(s)
// Values matches the regexp and returns the results for positional and
// named groups. Positional values are stored using an empty string as key.
// If the string doesn't match it returns nil.
func (r *Regexp) Values(s string) url.Values {
match := r.compiled.FindStringSubmatch(s)
if match != nil {
values := url.Values{}
for k, v := range r.groups {
values.Add(v, match[r.indices[k]])
return values
return nil
// Revert builds a string for this regexp using the given values. Positional
// values use an empty string as key.
// The values are modified in place, and only the unused ones are left.
func (r *Regexp) Revert(values url.Values) (string, error) {
vars := make([]interface{}, len(r.groups))
for k, v := range r.groups {
if len(values[v]) == 0 {
return "", fmt.Errorf(
"Missing key %q to revert the regexp "+
"(expected a total of %d variables)", v, len(r.groups))
vars[k] = values[v][0]
values[v] = values[v][1:]
return fmt.Sprintf(r.template, vars...), nil
// RevertValid is the same as Revert but it also validates the resulting
// string matching it against the compiled regexp.
// The values are modified in place, and only the unused ones are left.
func (r *Regexp) RevertValid(values url.Values) (string, error) {
reverse, err := r.Revert(values)
if err != nil {
return "", err
if !r.compiled.MatchString(reverse) {
return "", fmt.Errorf("Resulting string doesn't match the regexp: %q",
return reverse, nil
// template builds a reverse template for a regexp.
type template struct {
buffer *bytes.Buffer
groups []string // outermost capturing groups: empty string for
// positional or name for named groups
indices []int // indices of outermost capturing groups
index int // current group index
level int // current capturing group nesting level
// write writes a reverse template to the buffer.
func (t *template) write(re *syntax.Regexp) {
switch re.Op {
case syntax.OpLiteral:
if t.level == 0 {
for _, r := range re.Rune {
if r == '%' {
case syntax.OpCapture:
if t.level == 1 {
t.groups = append(t.groups, re.Name)
t.indices = append(t.indices, t.index)
for _, sub := range re.Sub {
case syntax.OpConcat:
for _, sub := range re.Sub {