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

Answer 1

I suggest to replace

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

by

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

to match a regex.