Using Regex to make a better comparison for string replacement.

Get's around "int4" conflicts in Postgres and is more sound / cleaner than the multiple replacements we had before.

We are in the process of moving from FrontBase to Postgres. The older ERXExistsQualifier was failing in Postgres as it tried to replace "int4" seeing it as a "T4" and made "inExists4" which is wrong. This new version works well in both FrontBase and Postgres. 

Signed-off-by: Aaron Rosenzweig <aaron@chatnbike.com>