Quering by Regular Expressions stored in table

Hi! I have a table with a character column (called match_regex)
containing a POSIX regular expression, hoping to have Postgres use it
while evaluating the "where" predicate. The following error occurs:

SELECT *
from patterns
where 'blah blah blah' ~ match_regex;

ERROR: invalid regular expression: quantifier operand invalid

We are using Postgres 7.4.8. I just tried the above on Postgres 8.0.4
and it seems to work! Is there a way to do this on my older version? If
not, maybe I can leverage this to get Postgres upgraded ;-)

Is seems that I need a regular expression type on the right of the
operator. Perhaps I can cast this 'character varying(100) column' to a
regular expression type if there is such a thing... though I have not
found one in the docs?
where 'blah blah blah' ~ match_regex::regularexpression;

Thanks,
Allen

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