regexp.rb

s_full = "South"
tests = [
  ["Some text with S", "S", "South", "Some text with South"],
  ["Some text with S/wrong", "S/wrong", "Something wrong", "Some text with Something wrong"],
  ["Some text with S/wrong", "S", "South", "Some text with S/wrong"]
]
def match(where, what, replace)
  where.gsub(/\b#{Regexp.quote(what)}(\s|\z|$)/, replace + '\1')
end

tests.each do |where, what, replace, expected|
  replaced = match(where, what, replace)
  if replaced == expected
    puts "matched!"
  else
    puts "not matching: - #{where} - #{what} - #{replace}"
    puts "#{replaced} - #{expected}"
  end
end