Date pattern regex doesn't match the records in PostgreSQL query?
PostgreSQL does not implement Perl regular expression, it implements POSIX regular expressions.  One difference among many is that \b represents a backspace character, not a word boundary. You can use \y to represent a word boundary, although I don't know if that has the exact same meaning as \b has in Perl.

If you want to use Perl regular expressions exactly, you could use the PL/Perl language