The problem here is that we're using the
word 'convert' when what Ashvini is really talking about should be describe with the word 'parse'.
Converting a string into a number implies a 1-to-1 type of relationship (most of the time). But parsing, implies 1-many relationships that can vary depending on your parsing engine. For example, what order of operation do you use in your example? If you go left to right + would be evaluated before & but if you use scientific notation, & would be evaluated before +.
What you want to do is either write your own parser (your 'small' then becomes a big problem as writing a numeric parser is non-trivial) or find a numeric equation parser out there that most closely matches your data and use it by converting your data. For example, other parsers might interpret Pi as "pi" or have special symbols like "@pi".