regex: ReplaceAllString and ReplaceAll broken group replacement

[yunier-rojas]

Go version

go version go1.20.2 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE="on"
GOARCH="arm64"
GOBIN=""
GOCACHE="~/Library/Caches/go-build"
GOENV="~/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="~/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="~/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/yunier/go/go1.20.2"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="~/go/go1.20.2/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.20.2"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="~/sample/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/tr/jxpk2zl51n9d_tkg9yw439nm0000gp/T/go-build3831294269=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

When replacing a string with a match group, if any text is put after the replacement, all text is ignore afterwards. regex replacements work as expected for $1 bar while $1bar does not.

bug example

What did you see happen?

When using regex replacements, if any word (\w+) is used after the replacement, the complete text is ignored

re := regexp.MustCompile(`(foo)`)
s := re.ReplaceAllString("foo", "$1bar.baz")

outputs: .baz

What did you expect to see?

Given that the above example, I would expect it to return foobar.baz

[yunier-rojas]

in python at least this is the expected result:

import re

re.sub('(foo)', '\\1bar', "foo")

# foobar

[cespare]

This is working as documented. Inside the documentation for the function you called, ReplaceAllString, it says:

Inside repl, $ signs are interpreted as in Expand,

and then the Expand documentation says:

In the template, 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. A purely numeric name like $1 refers to the submatch with the corresponding index; other names refer to capturing parentheses named with the (?P...) syntax. A reference to an out of range or unmatched index or a name that is not present in the regular expression is replaced with an empty slice.

In the $name form, name is taken to be as long as possible: $1x is equivalent to ${1x}, not ${1}x, and, $10 is equivalent to ${10}, not ${1}0.

So you want to write ${1}bar.baz: https://go.dev/play/p/a222CVP2XB7

[yunier-rojas]

Thanks @cespare for your response.