Regex: Possible combine list operator [a-z] with " "{2, } in one expression? Just letters a-z and not more than one whitespace consecutivly

I'm searching for a regular expression that let me replace all chars but letters and digits and one whitespace consecutively.

For example:

string = "a b c         e f g 1 2 3 !"

should be replaced in ruby to "a b c e f g 1 2 3 "

matching letters and digits is not that problem with [a-zA-Z0-9] with the list operator. but how to combine the interval operator for " "{2,} with the list operator, since intervals seem they can't be used in l开发者_开发知识库ist operators? Or is there another approach.

You could simply replace all sequences of non-alphanumeric characters by a single space:

string.gsub(/[^a-zA-Z0-9]+/, " ")

irb(main):001:0> string = "abc         e f g 1 2 3 !"
=> "abc         e f g 1 2 3 !"
irb(main):002:0> string.gsub(/[^[:alnum:]]/,"").gsub(/(.)/,'\1 ')
=> "a b c e f g 1 2 3 "

irb(main):002:0> string = "abc         e f g 1 2 3 !"
=> "abc         e f g 1 2 3 !"
irb(main):003:0> string.gsub(/[^a-zA-Z0-9]+/, " ")
=> "abc e f g 1 2 3 "