How Can I Simulate Negative Lookbehind in Go Regular Expressions?

Negative Lookbehind Simulation in Go

In Go, negative lookbehind assertions are not supported for performance reasons. This can present challenges when attempting to match patterns using lookbehind operators.

For example, a negative lookbehind might be used to extract a command from a string, excluding any leading characters from the set [@#/]. Using a negative lookbehind assertion:

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

However, this regex will not compile in Go due to the lack of support for negative lookbehind.

Alternative Approach

Instead, we can replace the negative lookbehind with a negated character set, which matches any character not in the set.

Updated Regex:

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

If leading characters from the set [@#/] are permitted at the start of the string, we can use the ^ anchor:

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

Filter Function

Alternatively, we can use a filter function in Go to filter out strings that begin with characters from the set [@#/].

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
}

Process Function

We can also create a process function that makes use of the filter function:

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, " ")
}

This process function can be used to transform input strings, removing any words that begin with characters from the set [@#/].

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