How to Exclude a Specific String When Matching in Go Regular Expressions?

Matching Everything Except a Constant String in Go Regular Expressions

In Go, utilizing the regular expression engine RE2, there is no direct way to employ lookahead assertions to match all characters except a specific string. However, there are alternative approaches to achieve this functionality:

Web Service to Generate Negated Patterns

1. Use the online service [Non-match REGEX](http://www.formauri.es/personal/pgimeno/misc/non-match-regex) to generate a negated pattern for the string you wish to exclude ("somestring" in this case).
2. In your regular expression, replace the last (.*) with the generated negated pattern (ensure there are no backslashes in the copied negated pattern).

For example, if the generated negated pattern is:

[^([^s]|s(s|o(s|m(s|es(omes)*(s|t(s|r(s|i(s|ns)))|o(s|ms)))))*([^os]|o([^ms]|m([^es]|e([^s]|s(omes)*([^ost]|t([^rs]|r([^is]|i([^ns]|n[^gs])))|o([^ms]|m([^es]|e[^s]))))))))*(s(s|o(s|m(s|es(omes)*(s|t(s|r(s|i(s|ns)))|o(s|ms)))))*(o((me?)?|mes(omes)*(t(r?|rin?)|o(me?)?)?))?)?$

Your final regex will look like:

/[^/]*/[^/]*/(([^s]|s(s|o(s|m(s|es(omes)*(s|t(s|r(s|i(s|ns)))|o(s|ms)))))*([^os]|o([^ms]|m([^es]|e([^s]|s(omes)*([^ost]|t([^rs]|r([^is]|i([^ns]|n[^gs])))|o([^ms]|m([^es]|e[^s]))))))))*(s(s|o(s|m(s|es(omes)*(s|t(s|r(s|i(s|ns)))|o(s|ms)))))*(o((me?)?|mes(omes)*(t(r?|rin?)|o(me?)?)?))?)?)$

Capturing All Parts

Alternatively, you can capture all three parts of the input string (separated by forward slashes) into a slice. Then, check if the third part (val[1]) equals the string you want to exclude ("somestring" in this case). If it does not match, use val[1] as the expected result:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    s := "anything/anything/somestring"
    r := regexp.MustCompile(`^[^/]+/[^/]+/(.*)`)
    val := r.FindStringSubmatch(s)
    // fmt.Println(val[1]) // -> somestring
    if len(val) > 1 && val[1] != "somestring" { // val has more than 1 element and is not equal to somestring?
        fmt.Println(val[1])      // Use val[1]
    } else {
        fmt.Println("No match")  // Else, report no match
    }
}

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