Bash subdomain regex validation never matches

Question

I'm trying to verify if a subdomain entered by a user is valid, but whatever I pass in, it's never valid. I know the regex is ok, so the problem is my "if" logic, however I'm new to shell/bash

#!/bin/bash
#
echo Enter the subdomain\'s name to configure.
read SUBDOMAIN
if [[ ! $SUBDOMAIN =~ [A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])? ]]; then
 echo "$SUBDOMAIN is not a valid domain"
fi

Examples:
Would be accepted (regular subdomain names): test
Would not be accepted (invalid subdomain name): -
Would not be accepted (invalid subdomain name): (Empty)
Would not be accepted (invalid subdomain name): #$??&@#&?$##$

I would prefer using shell, but the parentheses in the regex make the script throw an error.

I'm not sure if it can be done with grep, but I never understood how to use grep and it always confused me.

• steeldriver: Likely related: Bash =~ regex and https://regex101.com/
• NaturalBornCamper: @roaima DONE! :)
• NaturalBornCamper: @steeldriver I checked it out but "set -o rematchpcre" doesn't work
• NaturalBornCamper: @roaima Because subdomains can contain dashes for example, but cannot start with a dash

Answer 1

If you're trying to match "alphanumeric" followed by "alphanumeric or dash", ensuring there's not a dash at the end, such that there is a total of 1..62 characters, this RE will work for you

^[[:alnum:]](([[:alnum:]]|-){0,61}[[:alnum:]])?$

This binds to the beginning and end of the string, so the RE must match the string in its entirety.

• Start of line ^
• A single alphanumeric, any case [[:alnum:]]
• An optional block (bracketed (...) and terminated with ?)
  • [[:alnum:]] or a dash -, repeated 0..60 times
  • [[:alnum:]]
• End of line $

As has been recommended in the comments under this answer, I should point out that the [[:alnum:]] range is affected by the current locale. If you want to ensure that it matches only "ASCII" A-Z, a-z and 0-9 you need to ensure it's running with LANG=C. Otherwise you may find that additional characters are accepted, such as á é ø ß and others.

• NaturalBornCamper: Thanks friend! Your regex looks much better! I just have to change the regex a bit so subdomains can't end with a dash as well and It's all good :)
• Chris Davies: @NaturalBornCamper that's actually a little more complicated than it sounds
• NaturalBornCamper: Nope, what you gave me got me started, I just changed your answer a bit and it's working: if [[ ! $SUBDOMAIN =~ ^[[:alnum:]]([[:alnum:]]|-){0,61}[[:alnum:]]$ ]]
• Chris Davies: @NaturalBornCamper that will fail with a single character entry. It will also accept a 63 character string. Please see the amended answer for my suggestion.
• user232326: @roaima Since you are writing an answer about what you do know it follows that it is reasonable that you should make a note about the a-z ranges matching many UNICODE characters and not leave that hidden.