WARNING: nonstandard use of escape in a string literal

I just ran something like:
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UPDATE my_table SET colname = NULL WHERE colname ~ '^\s*$'
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I got the following warnings/hints as a result:
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WARNING: nonstandard use of escape in a string literal
HINT: USE the escape string syntax for escapes, e.g., E'\r\n'.
UPDATE 500
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Oddly it actually updated, and did just exactly what I wanted! :-)

So what am I to make of those weird hints and warning?