[removed] Regular Expression to parse a formula

Question

I have been working on a function to parse a formula for some time, but haven\'t been able to make it work properly. It seems to not always work - it filters some parts of t

Answer 1

You cannot really use a regular expression to match a formula. Formulae are a context-free language and regular expressions are limited to regular languages, the latter being a subset of the former. There are a number of algorithms for recognizing context-free languages such as CYK and LL parsers. I don't recommend studying those if you already haven't since the topic is quite large.

What you can do quickly, efficiently and easy though, is to attempt to calculate the formula using Reverse Polish Notation (RPN) (use the Shunting Yard algorithm to convert your formula to RPN). If the attempt fails (due to parenthesis not maching, invalid functions / constants, w/e), clearly the text is not a formula, otherwise all is good. Shunting yard is not a particularly difficult algorithm and you should have no trouble implementing it. Even if you do, the wikipedia page I linked above has pseudo code and there a good number of questions in SO as well to help you.