Github username convention using regex - regex

Question

I've been trying to convert the Github username convention using regex in Go for a while now and I couldn't do it. Also the username length shouldn't exceed more than 39 characters.
Below is the username convention from Github
Username may only contain alphanumeric characters or single hyphens, and cannot begin or end with a hyphen.
and for the length
Username is too long (maximum is 39 characters).
Here is the code I've written. You could check here in Go playground
package main
import (
"fmt"
"regexp"
)
func main() {
usernameConvention := "^[a-zA-Z0-9]*[-]?[a-zA-Z0-9]*$"
if re, _ := regexp.Compile(usernameConvention); !re.MatchString("abc-abc") {
fmt.Println("false")
} else {
fmt.Println("true")
}
}
Currently, I could achieve these:
a-b // true - Working!
-ab // false - Working!
ab- // false - Working!
0-0 // true - Working!
But the problem I'm facing is that I couldn't find the regex pattern which should work for the below scenario:
a-b-c // false - Should be true
Also it has to be within 39 characters which I've found that we could use {1,38}, but I don't know where exactly should I add that in the regex pattern.

Answer 1

In Go RE2-based regex, you can't use lookarounds, so checking length limit can only be done either with another regex, or with regular string length checking.
A fully non-regex approach (demo):
package main
import (
"fmt"
"strings"
)
func IsAlnumOrHyphen(s string) bool {
for _, r := range s {
if (r < 'a' || r > 'z') && (r < 'A' || r > 'Z') && (r < '0' || r > '9') && r != '-' {
return false
}
}
return true
}
func main() {
s := "abc-abc-abc"
if len(s) < 40 && len(s) > 0 && !strings.HasPrefix(s, "-") && !strings.Contains(s, "--") && !strings.HasSuffix(s, "-") && IsAlnumOrHyphen(s) {
fmt.Println("true")
} else {
fmt.Println("false")
}
}
Details
len(s) < 40 && len(s) > 0 - Length restriction, from 1 to 39 chars are allowed
!strings.HasPrefix(s, "-") - should not start with -
!strings.Contains(s, "--") - should not contain --
!strings.HasSuffix(s, "-") - should not end with -
IsAlnumOrHyphen(s) - can only contain ASCII alphanumeric and hyphens.
For a partially regex approach, see this Go demo:
package main
import (
"fmt"
"regexp"
)
func main() {
usernameConvention := "^[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*$"
re,_ := regexp.Compile(usernameConvention)
s := "abc-abc-abc"
if len(s) < 40 && len(s) > 0 && re.MatchString(s) {
fmt.Println("true")
} else {
fmt.Println("false")
}
}
Here, the ^[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*$ regex matches
^ - start of string
[a-zA-Z0-9]+ - 1 or more ASCII alphanumeric chars
(?:-[a-zA-Z0-9]+)* - 0 or more repetitions of - and then 1 or more ASCII alphanumeric chars
$ - end of string.