Use regular expression to handle nested parenthesis in math equation?

Question

If I have:

statement = "(2*(3+1))*2"

I want to be able to handle multiple parentheses within parentheses for a math reader I'm writing. Perhaps I'm going about this the wrong way, but my goal was to recursively go deeper into the parentheses until there were none, and then I would perform the math operations. Thus, I would first want to focus on

"(2*(3+1))" 

then focus on

"(3+1)"

I hoped to do this by assigning the focus value to the start index of the regex and the end index of the regex. I have yet to figure out how to find the end index, but I'm more interested in first matching the regex

r"\(.+\)" 

failed to match. I wanted it to read as "any one or more characters contained within a set of parentheses". Could someone explain why the above expression will not match to the above statement in python?

Answer 1

I love regular expressions. I use them all the time.

Don't use regular expressions for this.

You want an actual parser that will actually parse your math expressions. You might want to read this:

http://effbot.org/zone/simple-top-down-parsing.htm

Once you have actually parsed the expression, it's trivial to walk the parse tree and compute the result.

EDIT: @Lattyware suggested pyparsing, which should also be a good way to go, and might be easier than the EFFBot solution posted above.

https://github.com/pyparsing/pyparsing

Here's a direct link to the pyparsing sample code for a four-function algebraic expression evaluator:

http://pyparsing.wikispaces.com/file/view/fourFn.py