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Golang doesn't support positive lookbehind assertion? #11

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Golang doesn't support positive lookbehind assertion?#11
@thinkhy

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@thinkhy
thinkhy
opened on Apr 2, 2016

Seems Golang doesn't support positive lookbehind assertion, below code output "No matched!":

   package main
    import (
        "fmt"
        "regexp"
    )
    func main() {
        s := "abcdef"

        if matched, str := regexp.MatchString(`(?<=abc)def`, s); matched {
            fmt.Println("It's matched ", str)
        } else {
            fmt.Println("Nothing matched!")
        }
    }

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StefanSchroeder

StefanSchroeder commented on Apr 8, 2016

@StefanSchroeder
StefanSchroeder
on Apr 8, 2016
Owner

That is correct. This is not a bug. Refer to
https://github.com/google/re2/wiki/Syntax
to check the supported syntax.

thinkhy

thinkhy commented on Apr 9, 2016

@thinkhy
thinkhy
on Apr 9, 2016
Author

Thanks for your answer. I also notice Russ Cox's statement in Golang-nuts

The lack of generalized assertions, like the lack of backreferences, 
is not a statement on our part about regular expression style.  It is 
a consequence of not knowing how to implement them efficiently.  If 
you can implement them while preserving the guarantees made by the 
current package regexp, namely that it makes a single scan over the 
input and runs in O(n) time, then I would be happy to review and 
approve that CL.  However, I have pondered how to do this for five 
years, off and on, and gotten nowhere.
thinkhy
closed this as completedon Apr 9, 2016
sergeevabc

sergeevabc commented on Dec 25, 2016

@sergeevabc
sergeevabc
on Dec 25, 2016 · edited by sergeevabc

Oh, fellas, it makes life so harder.
For example, (\w+)(\.\w+)+(?!.*(\w+)(\.\w+)+) to get a filename from URL does not work.
More or less workaround is [^\/?#]*\.[^\/?#]*(\?.*)?(\#.*)?$.

rasom
mentioned this on Dec 30, 2016
StefanSchroeder

StefanSchroeder commented on Jan 4, 2017

@StefanSchroeder
StefanSchroeder
on Jan 4, 2017
Owner

When retrieving a filename from a URL I would use 'split' at '/' and then take the last item. No need for regex.

kirsle
mentioned this on Aug 25, 2017
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          Golang doesn't support positive lookbehind assertion? · Issue #11 · StefanSchroeder/Golang-Regex-Tutorial