line.gsub!(/^([A-Z ]+)/, '1'.capitalize)

line.gsub!(/^([A-Z ]+)/, "\1".capitalize)

line.gsub!(/^([A-Z ]+)/) { |w| w.capitalize }

require 'active_support'
'FOO bar'.gsub(/^[A-Z]+/) { |w| w.capitalize }
 => "Foo bar"

'FOO bar'.gsub!(/^([A-Z ]+)/) { |w| w.capitalize }
 => "Foo bar"

'FOO BAR'.gsub!(/^([A-Z ]+ [A-Z]+)/) { |w| w.capitalize }
 => "Foo bar"

'FOO BAR'.gsub!(/^((?<word>[A-Z]+) g<word>)/) { |w| w.capitalize }
 => "Foo bar"

require 'active_support'

line = 'AFOO BFOO CFOO DFOO e f g'
words = line[/^(?:[A-Z]+ )+/].split.map{ |w| w.capitalize } # => ["Afoo", "Bfoo", "Cfoo", "Dfoo"]
[words,line.split[words.size..-1]].join(' ')                # => "Afoo Bfoo Cfoo Dfoo e f g"

line.scan(/w+|W+/).map(&:capitalize).join

>> "FOO BAR foo BAR".gsub(/^([A-Z ]+)/,"#{$1.capitalize}")
=> "Foo bar foo BAR"