Java Regex - Trying to use regex to get an array of "function" arguments

Question

I have a bunch of strings representing mathematical functions (which could be nested and have any number of arguments), and I want to be able to use regex to return an array of strings, each string being an argument of the outer-most function. Here's an example:

"f1(f2(x),f3(f4(f5(x,y,z))),f(f(1)))"

I would want a regex pattern that I could use to somehow get an array of all the arguments of f1, which in this case are the strings "f2(x)", "f3(f4(f5(x,y,z)))", and "f(f(1))". There will be no spaces in the input string.

Thank you very much to anyone who can help.

Louis Wasserman · Accepted Answer · 2012-02-27 03:23:26Z

4

I don't think this can be done with regexes alone.

This would probably require being able to identify balanced parentheses -- for example, once we've parsed f1(f2(x), the next character could either be a ) or a , -- and that's a canonical example of something that can't be done with regexes, but requires a more sophisticated parser.

answered Feb 27, 2012 at 3:23

Louis Wasserman

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10 Comments

Tim Over a year ago

Dang, really? This is gonna be rough then. Know of any Java utilities that already do anything close to this?

Tony Ennis Over a year ago

The reason this is generally impossible with regexp is that regexp have no memory. There is no general way to let it match nested items, and that's the case with the f3(...) construct above.

Louis Wasserman Over a year ago

The most general way to do this sort of thing is with a tool like ANTLR. Alternatively, roll your own stack-based solution, or a recursive descent parser...but it's not doable with regexes.

Tony Ennis Over a year ago

The bright side is that a recursive descent parser will do this easily and are easy to write.

Louis Wasserman Over a year ago

You could, for example, use a regex to match on just the first and last parentheses in a string, and then recurse on the things between the parentheses.

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