security - How to detect SQL Injection sitting at a reverse proxy?

I am writing a simple reverse proxy in java. So, I have access to all Http requests and responses exchanged between client and server. Sitting at the proxy I am trying to detect SQL Injection Attack.

I got few links - (like for example)

http://www.symantec.com/connect/articles/detection-sql-injection-and-cross-site-scripting-attacks

where some regex are mentioned, but I suppose it's not that simple.
It is impossible to write regex for all possible/valid SQL statements.

Because so many databases are there in the market and SQL statements must follow some grammer rules.

Let me break down the problem to a simple question -

Given a string, can it be checked that whether it contains a valid SQL statement?

Can anyone tell me the best way to do it? Or, any library which does that for me?