What form of Linux are you on? This works for me (on RHEL 8 with BigFix 10.0.8) , matching on both tabs and spaces… the ‘%09’ in the ‘lines of file’ result below is a tab character.
Q: if exists files "/tmp/Query.txt" whose (exists lines whose (it contains regex("123456[[:space:]]+FE[[:space:]]+FI[[:space:]]+FO[[:space:]]+FUM[[:space:]]+BUM[[:space:]]+0")) of it) then "Yes" else "No"
A: Yes
T: 249
I: string
Q: lines of files "/tmp/Query.txt"
A:
A: 123456%09 FE FI FO FUM BUM 0
Can you try the ‘lines of file’ in the qna utility ( /opt/BESClient/bin/qna ) and paste in the line that you expect should match? I wonder if there are other unprintable characters in the line, that would be represent by a %xx, that may not match against [[:space:]]
edit: also what version of client is running