re = /::\K\s+|\s*\n(?=[^:]*(\n|$))\s*/
str = 'dn: CN=Gast,CN=Users,DC=question,DC=local
changetype: add
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user
cn: Gast
description::
Vm9yZGVmaW5pZXJ0ZXMgS29udG8gZsO8ciBHYXN0enVncmlmZiBhdWYgZGVuIENvbXB1dGVyIGJ6dy
4gZGllIERvbcOkbmU=
name: Gast
sAMAccountName: Gast
unicodePwd::IgA2AEcATQBNAHQANwBoADcAIgA=
userAccountControl:512'
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