MapServer runtime execution: Cannot evaluate expression, no item index defined error

Question

I'm trying MapServer's run-time substitution to dynamically set my Mapfile's CLASS EXPRESSION values.

The expressions take the form of:

CLASS
    EXPRESSION (([my_attr_col] >= num_1) && ([my_attr_col] < num_2))
    ...
END

When manually setting the expression values (not using run-time substitution) it works. (And only seems to do so when I use no quotes at all.)

When trying to pass the expression in dynamically with run-time substitution, then I add a VALIDATION block:

VALIDATION
    'my_attr'    '^[\s\S]*$'
END

and I modify the CLASS EXPRESSIONs along these lines:

CLASS
    EXPRESSION '%my_attr%'
    ...
END

and I then append and send the information via the query URL.

However, I get this error:

msEvalExpression(): General error message. Cannot evaluate expression, no item index defined.

I've tried many permutations of quotes and regex filtering but I can't seem to figure out what would be accepted by MapServer.