$ perl -le 'print "hello world" =~ /^\w+$/ ? "OK" : "BAD"'
BAD ---> contain spaces
$ perl -le 'print "helloworld" =~ /^\w+$/ ? "OK" : "BAD"'
OK
$ perl -le 'print "hello_world" =~ /^\w+$/ ? "OK" : "BAD"'
OK ---> underscore is considered alphanumeric character.

##</code><code>##

$ perl -le 'print "hello_world" =~ /^[a-zA-Z0-9]+$/ ? "OK" : "BAD"'
BAD ---> underscore is not in the class

##</code><code>##

$ perl -le 'print "hello world" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
BAD ---> space is not alnum
$ perl -le 'print "hello_world" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
BAD ---> _ is not alnum
$ perl -le 'print "HelloWorld1" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
OK --> nothing there but letters and digits