Search (batch update) users who have not filled out a specific (text) user field

[Mr Lucky]

I want to show a notice too all users who have not entered something in a custom user field. I don't think a wild card serach is possible (is it?) So far I can only think of a rather cumbersome workaround which is to batch update all users with "a" in the user field to a usergroup, then repeat the process same group with e, i, o , u - I'm assuming there must be a vowel.

Then show notice to those not in that group. Is there another way without messing with database?
Search (batch update) users who have not filled out a specific (text) user field

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