Backslash Bug in ARE Class-Shorthand Escape?

Kind people,

I have a little puzzlement. In the first select, I double the
backslash and return true. In the second, I don't and get false.
Have I missed something important in the docs?

Cheers,
D

test=> select version();
version
-------------------------------------------------------------------------------------------------------
PostgreSQL 7.4 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.2
20030222 (Red Hat Linux 3.2.2-5)
(1 row)

test=> select ('123' ~ '\\d');
?column?
----------
t
(1 row)

test=> select ('123' ~ '\d');
?column?
----------
f
(1 row)

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