Number Identification/Extraction In Text String

In the original field there were more than one occurence of six digits so you could try an if statement to catch the remainder where spaces don't exist:
 
@strip(@if(@regexpr(notes, "\s[0-9][0-9][0-9][0-9][0-9][0-9]\s")="", @regexpr(notes, "[0-9][0-9][0-9][0-9][0-9][0-9]"),@regexpr(notes, "\s[0-9][0-9][0-9][0-9][0-9][0-9]\s")))
 
Or you  could use caret or dollar sign

7. If the caret (^) is at the beginning of the (sub)expression, then the matched string must be at the beginning of the string being searched.
8. If the dollar sign ($) is at the end of the (sub)expression, then the matched string must be at the end of the string being searched.