Regex: Underscore (_) makes replace group not found

[mikhailkolesnik]

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

$ go version
go version go1.11.4 darwin/amd64

Does this issue reproduce with the latest release?

Assuming playground uses it - yes: https://play.golang.org/p/THIwTK5zXpW

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/...Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/.../.../gocode"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.4/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.4/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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/th/6k5d1zsx18z5dnzlrsrskxgc0000gp/T/go-build913576916=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://play.golang.org/p/THIwTK5zXpW

package main

import (
	"fmt"
	"regexp"
)

func main() {
	r := regexp.MustCompile(`(\w+)`)
	fmt.Println(r)

	orig := "somerandomtext"
	fmt.Println(r.MatchString(orig))

	fmt.Println(r.ReplaceAllString(orig, "$1"))
	fmt.Println(r.ReplaceAllString(orig, "$1_"))
	fmt.Println(r.ReplaceAllString(orig, "$1 _"))
}

What did you expect to see?

second output should produce somerandomtext_ but produces nothing

(\w+)
true
somerandomtext
somerandomtext_
somerandomtext _

What did you see instead?

(\w+)
true
somerandomtext

somerandomtext _

RE2 issue: google/re2#209

[ianlancetaylor]

The docs for regexp.ReplaceAllString say that it interprets $ as in Expand. The docs for Expand say "a variable is denoted by a substring of the form $name or ${name}, where name is a non-empty sequence of letters, digits, and underscores." The names are for use with the (?P<name>re) syntax. So when you write $1_, ReplaceAllString is substituting the value of the variable 1_, which is empty. You need to write ${1}_.