re = /\b(?=(?:[0-9]*[a-zA-Z]){2})(?=(?:[a-zA-Z]*[0-9]){2})[a-zA-Z0-9]{8}\b/
str = 'papave23 ciao il mio pin papaver1 è reeredji332ji con vaff8loe 1234567o 123t123t papavero 9o 123t123y'
subst = '********'
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