Referencing parts captured by round brackets in a regex in 8.4.13

Hello,

how to get rid of this warning
on a PostgreSQL 8.4.13 prompt?

# select 'axxxxxyz' ~ '(.)\1\1';
WARNING: nonstandard use of escape in a string literal
LINE 1: select 'axxxxxyz' ~ '(.)\1\1';
^
HINT: Use the escape string syntax for escapes, e.g., E'\r\n'.
?column?
----------
f
(1 row)

The table 9-18 at
http://www.postgresql.org/docs/8.4/static/functions-matching.html
suggests that using \1 as above should be ok....

Thank you
Alex