My first question is "Why are you doing this?"

My first suggestion is to read The Perl Regular Expression Tutorial. Obviously, you would do this with a regexp, possibly something like^*:

@cmd = ($statement =~ m/\b(SELECT|UPDATE|DELETE)\b/);
[download]

Then, @cmd would have to be checked by your rules defining "nested."

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^* "Something like this" means that this is 1) not tested and 2) not guaranteed to even vaguely resemble what you want. I've been reading too many software licenses...