RegEx for parsing chemical formulas

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(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+

Regular expression visualization

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+(?![^(]*\))

Regular expression visualization

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

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

Regular expression visualization

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+

Regular expression visualization

Demo

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