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  1. or download this
    $ perl -le 'print "hello world" =~ /^\w+$/ ? "OK" : "BAD"'
BAD ---> contain spaces
...
OK
$ perl -le 'print "hello_world" =~ /^\w+$/ ? "OK" : "BAD"'
OK ---> underscore is considered alphanumeric character.
  2. or download this
    $ perl -le 'print "hello_world" =~ /^[a-zA-Z0-9]+$/ ? "OK" : "BAD"'
BAD ---> underscore is not in the class
  3. or download this
    $ perl -le 'print "hello world" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
BAD ---> space is not alnum
...
BAD ---> _ is not alnum
$ perl -le 'print "HelloWorld1" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
OK --> nothing there but letters and digits