golang regex replace problem

[kzhui125]

https://play.golang.org/p/G7VF0QgUxGi

package main

import (
    "regexp"
    "fmt"
)

func main() {
    // var re = regexp.MustCompile(`(=\s*)D\s+('.*?')`)
    var re = regexp.MustCompile(`(?m)(=\s*)D\s+('.*?')`)
    var str = `a = D '123' b`
    var substitution = "$1xx$2"
    
    fmt.Println(re.ReplaceAllString(str, substitution))
}

expected: a = xx'123' b
actual: a '123' b

[ianlancetaylor]

ReplaceAllString treats the substitution string as though passed to Expand, and in Expand a $ may be followed by a name, and a name is a non-empty sequence of letters, digits, and underscores. So in your substitution string you are substituting $1xx (which is not defined) and then $2. Write instead ${1}xx${2}.

[kzhui125]

@iamoryanmoshe Thanks very much.

I saw the comment "so for instance $1", so I used $1.

And in my experience $1 is more general than ${1}
(php support ${1}, not support $1).

c#

string s1= Regex.Replace("123456", @"(12)34(56)", @"$1xxx$2"); // 12xxx56
string s2 = Regex.Replace("123456", @"(12)34(56)", @"${1}xxx${2}"); // 12xxx56

js:

'123456'.replace(/(12)34(56)/g,'$1xxx$2') // "12xxx56"
'123456'.replace(/(12)34(56)/g,'${1}xxx${2}') // "${1}xxx${2}"

python:

import re 

s = '123456'
re_test = re.compile(r'(12)34(56)')
print(re_test.sub(r'\1xxx\2',s)) # 12xxx56
print(re_test.sub(r'\{1}xxx\{2}',s)) # \{1}xxx\{2}

java:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class RegexMatches {

    public static void main(String[] args) {
    
        Pattern p = Pattern.compile("(12)34(56)");
        // get a matcher object
        Matcher m = p.matcher("123456"); 
        System.out.println(m.replaceAll("$1xxx$2")); // 12xxx56
        // System.out.println(m.replaceAll("${1}xxx${2}")); // invalid
   }
}