How to use regex on a field's value in a search?

index=system* sourcetype=inventory order=829 I am trying to extract the 3 digit field number in this search with rex to search all entries with only the three digit code. I tried: index=system* sourcetype=inventory (rex field=order "\d+") index=system* sourcetype=inventory (rex field=order "(\d+)") index=system* sourcetype=inventory (rex field=order "[0-9]{3}") What is the correct way to do this? Thanks!