# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/::\K\s+|\s*\n(?=[^:]*(\n|$))\s*//g;t' <<< "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"
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 SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html