re = /(?<!\w\.\w.)(?<![A-Z][a-z]\.)(?<=\.|\?)\s/m
str = 'Mr. Smith bought cheapsite.com for 1.5 million dollars, i.e. he paid a lot for it. Did he mind? Adam Jones Jr. thinks he didn\'t. In any case, this isn\'t true... Well, with a probability of .9 it isn\'t.'
subst = '\\n'
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html