From 54b7ccd514f6a689347c8d1f876bec90613f28f8 Mon Sep 17 00:00:00 2001
From: "Erik St. Martin" <alakriti@gmail.com>
Date: Sat, 22 Dec 2012 11:14:56 -0500
Subject: [PATCH]       regexp: fix index panic in Replace

When using subexpressions ($1) as replacements, when they either don't exist or values weren't found causes a panic.
This patch ensures that the match location isn't -1, to prevent out of bounds errors.
Fixes #3816.

R=franciscossouza, rsc
CC=golang-dev
https://golang.org/cl/6931049
---
 src/pkg/regexp/all_test.go | 4 ++++
 src/pkg/regexp/regexp.go   | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/pkg/regexp/all_test.go b/src/pkg/regexp/all_test.go
index 3596573b4f..9c4d64f582 100644
--- a/src/pkg/regexp/all_test.go
+++ b/src/pkg/regexp/all_test.go
@@ -196,6 +196,10 @@ var replaceTests = []ReplaceTest{
 	{"a+", "${oops", "aaa", "${oops"},
 	{"a+", "$$", "aaa", "$"},
 	{"a+", "$", "aaa", "$"},
+
+	// Substitution when subexpression isn't found
+	{"(x)?", "$1", "123", "123"},
+	{"abc", "$1", "123", "123"},
 }
 
 var replaceLiteralTests = []ReplaceTest{
diff --git a/src/pkg/regexp/regexp.go b/src/pkg/regexp/regexp.go
index bcf354b44d..c516a1566f 100644
--- a/src/pkg/regexp/regexp.go
+++ b/src/pkg/regexp/regexp.go
@@ -767,7 +767,7 @@ func (re *Regexp) expand(dst []byte, template string, bsrc []byte, src string, m
 		}
 		template = rest
 		if num >= 0 {
-			if 2*num+1 < len(match) {
+			if 2*num+1 < len(match) && match[2*num] >= 0 {
 				if bsrc != nil {
 					dst = append(dst, bsrc[match[2*num]:match[2*num+1]]...)
 				} else {
-- 
2.48.1