MustCompile中的Golang正则表达式参考(查找重复字符)

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I am having a hard time with Go's regex. It seems it's different than other language, can someone help me on this.

Obj. I want MustCompile to find all repeated characters in the string. APPLE (where P's repeating)

re := regexp.MustCompile("(\\w)\\${1}\\+")

Above is what I have tried but didn't work at all. Basically I wanted to do:

([A-Za-z])\1+

Can someone tell me what I am doing wrong?

Example below:

https://play.golang.org/p/DeuaIva968

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Apparently Golang doesn't supposed back referencing due to efficiency. :( Thank you everyone for your help.