alejolp / simple-expr-eval-java

Self-contained pure-Java simple expressions parser, implemented using a recursive descent hand-crafted parser

Self-contained pure-Java simple expressions parser, implemented using a recursive descent hand-crafted parser in ~150 lines of code.

Given a string 2 + 3 * 4 it returns 14 and (2 + 3) * 4 gives 20. The parser also has the unary minus - which can be tricky to get for things like 10 / -(-(2))). The parser is interesting because it's done as a recursive descent parser code which is much simpler to understand, follow and debug than the stack parsing algorithm (Shunting-yard algorithm). This project also has a working pom.xml for Spring Boot and JUint 5.

The recursive code follows the EBNF grammar. The EBNF grammar is:

Grammar: (the {...} means 0+ repetition)

expr: term | term { + term } | term { - term }

term: factor | factor { * factor } | factor { / factor } 

factor: - factor | ( expr ) | value

This parser assumes that expressions are always valid. Thus, error detection is done with RuntimeException, NullPointerException or Stack Overflows.

See the SimpleExprParserTest class for tests and examples.