module ActiveSupport::Inflector

'SSLError'.underscore.camelize # => "SslError"

+camelize+, though there are cases where that does not hold:
As a rule of thumb you can think of +underscore+ as the inverse of

'ActiveModel::Errors'.underscore # => "active_model/errors"
'ActiveModel'.underscore # => "active_model"

Changes '::' to '/' to convert namespaces to paths.

Makes an underscored, lowercase form from the expression in the string.

def underscore(camel_cased_word)
  word = camel_cased_word.to_s.dup
  word.gsub!('::', '/')
  word.gsub!(/(?:([A-Za-z\d])|^)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1}#{$1 && '_'}#{$2.downcase}" }
  word.gsub!(/([A-Z\d]+)([A-Z][a-z])/,'\1_\2')
  word.gsub!(/([a-z\d])([A-Z])/,'\1_\2')
  word.tr!("-", "_")
  word.downcase!
  word
end