How to convert a regex pattern that uses Lookahead? - regex

Question

I'm having trouble converting this regex to something Golang supports. Could I get some help? It's originally from this SO question.
^(?=.{1,24}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?<![_.])$
Here's the criteria:
Only contains alphanumeric characters,
underscore and dot.
Underscore and dot can't be at the end or
start of a username (e.g _username / username_ / .username / username.).
Underscore and dot can't be next to each other (e.g user_.name).
Underscore or dot can't be used multiple times in a row (e.g user__name / user..name).

Answer 1

I don't have any experience with Go, so perhaps someone could come up with a better solution.
Here are the two options I found:
1. Write a regex that covers everything except the length restriction
You can use something like this:
^(?:[a-zA-Z0-9]+[._]?[a-zA-Z0-9]+)+$
Regex101 demo.
And you can use len to check for the string length. Here's a full example:
func main() {
var re = regexp.MustCompile(`(?m)^(?:[a-zA-Z0-9]+[._]?[a-zA-Z0-9]+)+$`)
var str = `username
user_name
user.name
user.name_123
username$$$
_username
username_
user_.name
user._name
user__name
user..name
VeryLongUserNameThatExceedsTheLimit
`
for i, match := range re.FindAllString(str, -1) {
if len(match) <= 24 {fmt.Println(match, "found at index", i)}
}
}
Output:
username found at index 0
user_name found at index 1
user.name found at index 2
user.name_123 found at index 3
Test it online.
2. Use a third-party engine
I found this .NET-based engine which should support Lookarounds. If the previous solution doesn't work for you, you may look into this. Note that the author of that engine recommends using the built-in engine whenever possible:
You'll likely be better off with the RE2 engine from the regexp
package and should only use this if you need to write very complex
patterns or require compatibility with .NET.