Validate only an email address username with BASH regex-CodePudding

In: [email protected]...

I need to make sure that 'johnson' is valid by server/client standards (probably RFC 5322) for the username part of an email address. That is, Gmail and Thunderbird would accept them.

This question addresses full email addresses, which I don't need: How can I validate an email address using a regular expression?

This unpopular question is about JavaScript and doesn't have answers: Validating username part of email address

This answer to the afirst question above offers a semi-acceptable regex for a full email address, which I don't need, though it seems there might be room for improvement, but I might not need improvement: https://stackoverflow.com/a/201378/10343144

My current best solution would be to take:

emailusername="$1"
testemail="${emailusername}@nonsense.com"
regex='some existing full-email regex'
if [[ "${testemail}" =~ ${regex} ]]; then
  echo "it works"
fi

But, that doesn't address the email username part specifically, and it's more expensive than validating only the username part.

Is adding a nonsense domain to the username for a regex check the best way?

Or is there a regex that can handle only only the username part of an email address?

CodePudding user response：

Gmail is more restrictive than the RFC in respect of the accepted usernames (see Create a username):

“Abuse” and “Postmaster” are reserved

6–30 characters long

can contain letters (a-z), numbers (0-9), and periods (.)

cannot contain [...] more than one period (.) in a row

can begin or end with [...] except periods (.)

periods (dots) don’t matter in Gmail addresses

remark: the length of a username doesn't take the dots into account.

Then, for validating a Google username with bash you could do:

#!/bin/bash

username="$1"
username_nodots="${username//./}" 

if ! {
    (( ${#username_nodots} >=  6 )) && # this rule also excludes 'Abuse' 
    (( ${#username_nodots} <= 30 )) &&
    [[ $username =~ ^[[:alnum:]] (\.[[:alnum:]] )*$ ]] &&
    [[ $username != 'Postmaster' ]]
}
then
    echo "error: illegal google username: $username" >&2
    exit 1
fi

Edit: following @tripleee advice, i.e. using standard shell constructs:

username="$1"
length=$(printf %s "$username" | tr -d '.' | wc -c)

[ "$length" -ge 6 ] || {
    printf '%s\n' 'too short' >&2
    exit 1
}
[ "$length" -le 30 ] || {
    printf '%s\n' 'too long' >&2
    exit 1
}
case $username in
    *[^[:alnum:].]*)
        printf '%s\n' 'illegal character' >&2
        exit 1
    ;;
    .*)
        printf '%s\n' 'starts with dot' >&2
        exit 1
    ;;
    *.)
        printf '%s\n' 'ends with dot' >&2
        exit 1
    ;;
    *..*)
        printf '%s\n' 'multiple dots in a row' >&2
        exit 1
    ;;
    Abuse|Postmaster)
        printf '%s\n' 'reserved username' >&2
        exit 1
    ;;
esac

CodePudding user response：

If your locale is C the following may work for you. It is inspired by the last regex you mention (which has not been checked against the RFC), and was not extensively tested:

atext="A-Za-z0-9!#\$%&'* /=?^_\`{|}~-"
qs1=$'\x09\x0a\x0d\x20\x22\x5c'
qs2=$'\x0a\x0d'
[[ "$localpart" =~ ^([$atext] (\.[$atext] )*|\"([^$qs1]|\\[^$qs2])*\")$ ]] && echo "yes"