ToExpression and strings like "5--1", "5 is not a variable with a value"?

Questions
1

I’ve got some code that does some string manipulation to assemble small equations, e.g. “(5+3)*8”, and from these assembled strings I’d like to convert them to expressions for evaluation. This works in most circumstances, except when the string assembly produces something like “5–1”, which should be read as “Five minus negative one”. Instead, Mathematica’s ToExpression function evaluates “–” as the Decrement function, which of course won’t work since 5 is an integer, not a variable pointing to an integer that could be decremented.

Any thoughts on ways to change this behavior, apart from adding a space, e.g. “5- -1”?

2

You can get the effect you want by using StringReplace to insert a space between the two minus signs as,

s = "5--1";
ToExpression[StringReplace[s, "--" -> "- -"]]
3

The main reason why it does what it does is because of the precedence of the operators:

Precedence[Decrement]
Precedence[Minus]
Precedence[Subtract]

I’m not sure if you can alter these because it will have huge effect on all the internal code as well…