Using Regex in PostgreSQL

I used regex to check if a string has any letters before I convert it to a number.

CASE WHEN SUBSTRING(tr.transsn,3,5) !~ '^(\+|-)?[[:digit:]]+$'
 THEN NULL::DATE
 ELSE TO_DATE(CONCAT('20'
   ,SUBSTRING(tr.transsn,3,2)::TEXT
   ,'0101')
   ,'YYYYMMDD')::DATE + SUBSTRING(tr.transsn,5,3)::INT - 1
 END AS "BuildDate"

This query will take the Julian date from the Transmission serial number (ex. 22115) and convert to a date but first it will check that the Julian date extracted from the serial number is actually a number. I need to do more research so that I can incorporate this into more of my queries.

See https://stackoverflow.com/questions/61134182/postgres-cast-column-to-type-or-null-if-unable-to-cast is where I got the solution. See https://stackoverflow.com/questions/1684291/sql-like-condition-to-check-for-integer and https://www.postgresql.org/docs/current/functions-matching.html#FUNCTIONS-POSIX-TABLE for more info.