re = /^(?:\+?7|8)?(?:[\s\-(_]+)?(\d{3})(?:[\s\-_)]+)?(\d{3})(?:[\s\-_]+)?(\d{2})(?:[\s\-_]+)?(\d{2})$/m
str = '+78005553535
88005553535
8005553535
78005553535
+7(800)555-35-35
+7(800)5553535
+7(800)555 35 35
+7 (800) 555 35 35
+7 (800) 555-35-35
+7-(800)-555-35-35
8 (800) 555 35 35
800 555 35 35
800-555-35-35
8(800)5553535
8(800)555 35 35
8(800) 555-35-35
8-800-555-35-35'
subst = '$1$2$3$4'
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