What about `'foo' + 'bar'` ?

You need to test for `'` in the middle.

Also, why not just 

    expressionString.substring(1, expressionString.length() - 1));

Finally, what drove you to create this? Evaluating a `LiteralExpression` is clearly faster, but not by all that much (8x in my tests, when you include assigning the result to a local variable).

If you think it's really necessary, maybe it should be folded into spring-expression ?