This sure looks like a bug to me but I am not sure whose responsibility so am dual posting to clips and scripts.
^!Find "^.*[\H].*$" RH (or equivalent) ^!Find "^.*[^\h].*$" RH
should throw an error condition for any line consisting solely of horizontal white space (HT, NBSP, SP), such as spaces, but it does not.
Trying to isolate the "found" non-white space character via
^!Find "^.*\K[\H](?=.*)$" RH (or equivalent) ^!Find "^.*\K[^\h](?=.*)$" RH
results in no matches but no error condition either.
Substituting an "S" or "s" for "H" or "h" respectively yields the correct results.
However, the "S" meta character includes vertical white space characters (LF, VT, CR, NL) which is not always wanted.
So is this a bug in RegEx or a bug in Notetab? (tested NTB 5.8/fv & 6.2/fv).