Checking regex pattern validity

Is there a function like IS_VALID_REGEX() to check whether a pattern is
valid (i.e. it compiles)? I'm storing a list of regex patterns in a
table. It would be nice to be able to add a CHECK constraint to ensure
that all the regexes are valid.

If there isn't any, can I suggest Postgres add one? Although I know this
can probably be done in plpgsql using exception handling, or done in
plperl or plruby.

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dave