regexp: Find makes it easy to unintentionally overwrite slices

[campoy]

What version of Go are you using (go version)?

$ go version
go version devel +e1b49ad608 Sun Feb 10 18:40:06 2019 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/francesc/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/francesc"
GOPROXY=""
GORACE=""
GOROOT="/Users/francesc/go"
GOTMPDIR=""
GOTOOLDIR="/Users/francesc/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/francesc/go/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/tm/816df9c9399_1fnz90k6gfq80000gn/T/go-build072052763=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I created a little program using the slices returned by FindAll by appending some extra characters before printing them.

package main

import (
	"fmt"
	"regexp"
)

func main() {
	input := []byte(`one two three four five`)

	r := regexp.MustCompile("[a-zA-Z]+")

	ss := r.FindAll(input, -1)
	for _, s := range ss {
		fmt.Printf("%s\n", append(s, '.', '.', '.'))
	}
}

What did you expect to see?

I expected the output to be:

one...
two...
three...
four...
five...

What did you see instead?

The output was instead:

one...
..o...
..ree...
..ur...
..ve...

This is similar to the fixes already done in bytes.Fields.

[gopherbot]

Change https://golang.org/cl/161877 mentions this issue: regexp: limit the capacity of slices of bytes returned by FindX