Ruby golf; generating random strings

Update: See comments for all the latest/best solutions.

This post got me into some Ruby golfing a few days ago. The challenge is to see how few characters of Ruby code you need to generate a random string of length 4 or more consisting of only the following set of characters a-z, A-Z, 0-9.

The shortest solution posted on the blog entry was 59 chars. I came up with 50, can anyone beat this?

(0..9).map{0until(c=rand(?z).chr)=~/(?!_)\w/;c}*''

I’ll show an expanded version to help explain how it works

char_array = (0..9).map do
  until ( c = rand(?z).chr ) !~ /(?!_)\w/
    0
  end
  c
end
rand_string = char_array*''

(0..9).map just runs the block 10 times and stores the values in an array. *'' is a synonym for .join('') which turns the array into a string. The loop just keeps picking random characters with ascii values between 0 and ?z (122) until it matches one which is in the range we want. The actual body of the loop does nothing, but in the condition the variable c is set to the chosen random character. I used the while/until syntax form which allows you to put the body before the while condition. Using a number (0) as the body allowed me to not even have to use a space.

I described the range of characters with the regular expression /(?!_)\w/ which is equivalent to, but 1 character shorter than /[a-z\d]/i. /\w/ is the same as /[a-z\d_]/i so the problem is it will also match an underscore. I got around this with the negative lookahead (?!_) which means don’t match an underscore.

I think that’s about it. Can anyone beat 50?

Crap I just almost bested myself. This one returns a string ALMOST 100% of the time, but on some rare occasions it will return nil:

(0..9999).map{rand(?z).chr}.join[/[a-z\d]{9}/i]

47 chars, and if you change 9999 to 999 and {9} to {4}, you can get a 4 character string almost 100% of the time with only 46 chars.

Update: Wow Magnus comes in at 42… crazy:

(0..9).map{rand(?z).chr[/[^_\W]/]||redo}*''

props to nonsequitur for the regex /[^_\W]/
mme