How can I tell, in a trigger, if a value in a string is an integer.

I have a trigger function that, for every Update or Insert of
a table inserts a row in a "audit" table that contains some trace
data, the key of the first table and the changed columns (unchanged
cols are NULL) I have this working with a plpgsql function execpt
for one of the trace fields (called the audit_session) which is an
integer. The value of this should be -1 unless a variable is set
for the session using

 set_config('session.sessionid','23',false);

which can be read with

current_setting('session.sessionid')

Which will allow the web server to link a action to a "session"
(data stored in another table but starts from 1 and goes up as
a integer)

I was not able to make the trigger function work in plperl (where
the forcing of the invalid, unset, or unknown to -1 would be easy).

any ideas on the best way to handle this.

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