Replace non-word-characters with underscore or dash; without preceding or trailing underscores or dashes.

format_join.rb

def format_join(string, joiner=' ')
  string.split(/\W/).reject(&:empty?).join(joiner.to_s)
end

# $ ruby -v
# ruby 2.1.9p490 (2016-03-30 revision 54437) [x86_64-darwin18.0]
#
# $ irb
# irb(main):001:0> def format_join(string, joiner=' ')
# irb(main):002:1>   string.split(/\W/).reject(&:empty?).join(joiner.to_s)
# irb(main):003:1> end
# => :format_join
# …
# irb(main):005:0> str = 'request.headers["Accept-Language"]'
# => "request.headers[\"Accept-Language\"]"
#
# irb(main):006:0> format_join str
# => "request headers Accept Language"
#
# irb(main):007:0> str = '     req u     est.hea   ders["Accept-Lang   uage"]       '
# => "     req u     est.hea   ders[\"Accept-Lang   uage\"]       "
#
# irb(main):008:0> format_join str
# => "req u est hea ders Accept Lang uage"
#
# irb(main):009:0> format_join str, '_'
# => "req_u_est_hea_ders_Accept_Lang_uage"
#
# irb(main):010:0> format_join str, '-'
# => "req-u-est-hea-ders-Accept-Lang-uage"
#
# irb(main):011:0> format_join str, '_-_-_'
# => "req_-_-_u_-_-_est_-_-_hea_-_-_ders_-_-_Accept_-_-_Lang_-_-_uage"