regex.rb

allowed = 'A-Za-z0-9'
q = /^(([#{allowed}]+)([#{allowed}\-]+)([#{allowed}]+)|([#{allowed}]{1,2}))$/
p q =~ 'aa'
p q =~ 'aaa'
p q =~ 'aa-aa'
p q =~ '9aa-aa'
p q =~ '90'

p '---- not from down -----'

p q =~ '-aa'
p q =~ 'aaa-'
p q =~ 'aa_aa'
p q =~ '__'