I have a search that returns a field called "Administrators"
for every entry that has a \ at the beginning I would like to put the word "Domain"
This should work for you:
*|eval administrators=if(match(administrators, "^\\\.+"), "Domain".administrators, administrators)|table administrators
Probably want to put a carat ^ at the start of that, so it only matches at the beginning of the string.
"Match" returns true if the REGEX can find a match against any substring of SUBJECT.
In @nickhillscpl's answer, replace first
* with whatever search you've right now. Just add that
| eval administrator... to end of your search.