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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