Bash regex comparison issue

Question

I have the following function

checkFormat()
{
        local funcUserName=$1
        if [[ "$funcUserName" != [a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9] ]];then
                echo "1"
        else
                echo "0"
        fi
}

if [[ $string != [a-zA-Z0-9]* ]]

Only returns true if the first character is not [a-zA-Z0-9] if [[ $string != [a-zA-Z0-9]{5} ]]

Never returns true.

if [[ $string != [a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9] ]]

Returns as I want it to. Why is this?

The code is to check that a username is 5 characters long and alphanumeric i.e. Joe12 or 12345 but not %$134.

bash version 4.2.37

Good Q except you should include sample input (both good and bad). Good luck. — shellter
– shellter, Commented Oct 30, 2016 at 16:45
What do you actually want to check with your reg ex?Give some examples — Sulav Timsina
– Sulav Timsina, Commented Oct 30, 2016 at 16:47
None of this is regex. It's a different kind of pattern called globs. They can look similar, but they're not the same. — that other guy
– that other guy, Commented Oct 30, 2016 at 16:49
I managed to emit my regex example. Oops. [[ $string =~ ^[a-zA-Z0-9]{5}*$ ]] didn't work when I reversed the logic. — joemobaggins
– joemobaggins, Commented Oct 30, 2016 at 17:01

Cyrus · Accepted Answer · 2016-10-30 16:55:04Z

3

I suggest to replace

if [[ $string != [a-zA-Z0-9]{5} ]]

by

if [[ ! $string =~ ^[a-zA-Z0-9]{5}*$ ]]

to match a regex.

edited Oct 30, 2016 at 16:55

answered Oct 30, 2016 at 16:49

Cyrus

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4 Comments

joemobaggins Over a year ago

Hey, thanks allot, that works. When I tried [[ $string =~ ^[a-zA-Z0-9]{5}*$ ]] and reversed the logic, It didn't work. Why so?

Cyrus Over a year ago

This works for me with bash version 4: string="a1B2c"; [[ $string =~ ^[a-zA-Z0-9]{5}*$ ]] && echo match

joemobaggins Over a year ago

could you explain the && syntax to me? I realise it's an AND. But I don't understand it in this context.

Cyrus Over a year ago

Yes, && is a boolean AND. If [[ ... ]] is true execute echo match. Long version with an else branch: string=a1B2c; if [[ $string =~ ^[a-zA-Z0-9]{5}*$ ]]; then echo match; else echo "no match"; fi

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