Determine if a string is a valid float value

Here’s one way:

class String
  def valid_float?
    # The double negation turns this into an actual boolean true - if you're 
    # okay with "truthy" values (like 0.0), you can remove it.
    !!Float(self) rescue false
  end
end

"a".valid_float? #false
"2.4".valid_float? #true

If you want to avoid the monkey-patch of String, you could always make this a class method of some module you control, of course:

module MyUtils
  def self.valid_float?(str)
    !!Float(str) rescue false
  end
end
MyUtils.valid_float?("a") #false