import re
 
_char = re.compile(r'\w')
 
''.join([chr(i) for i in xrange(65,122) if _char.match(chr(i))])
 
# or just:
import string
 
string.ascii_letters
string.ascii_lowercase
string.ascii_uppercase

(65..122).to_a.map{ |e| e.chr =~ /\w/ && e.chr }.compact.to_s

$chars .= chr() =~ /\w/ ? chr() : q{} for 65..122;