How to simulate negative lookbehind in Go

Question

I'm trying to write a regex that can extract a command, here's what I've got so far using a negative lookbehind assertion:

\b(?<![@#\/])\w.*

So with the input:

/msg @nickname #channel foo bar baz
/foo #channel @nickname foo bar baz 
foo bar baz

foo bar baz is extracted every time. See working example https://regex101.com/r/lF9aG7/3

In Go however this doesn't compile http://play.golang.org/p/gkkVZgScS_

It throws:

panic: regexp: Compile(`\b(?<![@#\/])\w.*`): error parsing regexp: invalid or unsupported Perl syntax: `(?<`

I did a bit of research and realized negative lookbehinds are not supported in the language to guarantee O(n) time.

How can I rewrite this regex so that it does the same without negative lookbehind?

Answer 1

Since in your negated lookbehind, you are only using a simple character set; you can replace it with a negated character-set:

\b[^@#/]\w.*

If the are allowed at the start of the string, then use the ^ anchor:

(?:^|[^@#\/])\b\w.*

---

Based on the samples in Go playground link in your question, I think you're looking to filter out all words beginning with a character from [#@/]. You can use a filter function:

func Filter(vs []string, f func(string) bool) []string {
    vsf := make([]string, 0)
    for _, v := range vs {
        if f(v) {
            vsf = append(vsf, v)
        }
    }
    return vsf
}

and a Process function, which makes use of the filter above:

func Process(inp string) string {
    t := strings.Split(inp, " ")
    t = Filter(t, func(x string) bool {
        return strings.Index(x, "#") != 0 &&
            strings.Index(x, "@") != 0 &&
            strings.Index(x, "/") != 0
    })
    return strings.Join(t, " ")
}

It can be seen in action on playground at http://play.golang.org/p/ntJRNxJTxo