RegEx for parsing chemical formulas

(PO4)2 really sits aside from all.

Let's start from simple, match items without parenthesis:

[A-Z][a-z]?\d*

Using regex above we can successfully parse Ag3PO4, H2O, CH3OOH.

Then we need to somehow add expression for group. Group by itself can be matched using:

\(.*?\)\d+

So we add or condition:

[A-Z][a-z]?\d*|\(.*?\)\d+

Demo

Which works for given cases. But may be you have some more samples.

Note: It will have problems with nested parenthesis. Ex. Co3(Fe(CN)6)2

If you want to handle that case, you can use the following regex:

[A-Z][a-z]?\d*|(?<!\([^)]*)\(.*\)\d+(?![^(]*\))

For Objective-C you can use the expression without lookarounds:

[A-Z][a-z]?\d*|\([^()]*(?:\(.*\))?[^()]*\)\d+

Demo

Or regex with repetitions (I don't know such formulas, but in case if there is anything like A(B(CD)3E(FG)4)5 - multiple parenthesis blocks inside one.

[A-Z][a-z]?\d*|\((?:[^()]*(?:\(.*\))?[^()]*)+\)\d+

Demo

This should just about work:

/(\(?)([A-Z])([a-z]*)([0-9]*)(\))?([0-9]*)/g

Play around with it here: http://refiddle.com/

When you encounter a parenthesis group, you don't want to parse what's inside, right?

If there are no nested parenthesis groups you can simply use

[A-Z][a-z]*\d*|\([^)]+\)\d*

\d is a shorcut for [0-9], [^)] means anything but a parenthesis.

See demo here.