My personal work on my lectures at ETH Zurich.
Located in eprog/ExpressionParser
An expression parser and evaluator written in Java. Uses an unambiguous grammar with some limitations (see below) to parse expressions by creating an expression tree which can be inspected at runtime.
Supports arbitrary variable and function definitions.
Built-in functions: sin, cos, log, abs, +, -, *, /, ^
Grammar used to parse stuff:
expr -> term [op term]
term -> `(` expr `)` | fun `(` expr `)` | atom
atom -> num | var
op -> `+` | `-` | `*` | `/` | `^`
num -> int `.` int
int -> [0-9]+
var -> [A-Za-z][A-Za-z0-9]*
Negative numbers are entered as -x = (0 - x).
Builds on double arithmetic, so may suffer from double precision errors.
The example evaluator reads an expression and resolves all undefined subexpressions by querying the user. May be expanded to REPL later.