RegEx for parsing chemical formulas - objective-c

Question

I need a way to separate a chemical formula into its components. The result should look like
this:
Ag3PO4 -> [Ag3, P, O4]
H2O -> [H2, O]
CH3OOH -> [C, H3, O, O, H]
Ca3(PO4)2 -> [Ca3, (PO4)2]
I don't know regex syntax, but I know I need something like this
[An optional parenthesis][A capital letter][0 or more lowercase letters][0 or more numbers][An optional parenthesis][0 or more numbers]
This worked
NSRegularExpression *regex = [NSRegularExpression
regularExpressionWithPattern:#"[A-Z][a-z]*\\d*|\\([^)]+\\)\\d*"
options:0
error:nil];
NSArray *tests = [[NSArray alloc ] initWithObjects:#"Ca3(PO4)2", #"HCl", #"CaCO3", #"ZnCl2", #"C7H6O2", #"BaSO4", nil];
for (NSString *testString in tests)
{
NSLog(#"Testing: %#", testString);
NSArray *myArray = [regex matchesInString:testString options:0 range:NSMakeRange(0, [testString length])] ;
NSMutableArray *matches = [NSMutableArray arrayWithCapacity:[myArray count]];
for (NSTextCheckingResult *match in myArray) {
NSRange matchRange = [match rangeAtIndex:0];
[matches addObject:[testString substringWithRange:matchRange]];
NSLog(#"%#", [matches lastObject]);
}
}

Answer 1

(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

Answer 2

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.

Answer 3

This should just about work:
/(\(?)([A-Z])([a-z]*)([0-9]*)(\))?([0-9]*)/g
Play around with it here: http://refiddle.com/

Answer 4

this pattern should work depending on you RegEx engine
([A-Z][a-z]*\d*)|(\((?:[^()]+|(?R))*\)\d*) with gm option
Demo

Answer 5

Better to limit the set of chars to valid chemical names. In simple form:
^((Ac|Ag|Al|Am|Ar|As|At|Au|B|Ba|Be|Bh|Bi|Bk|Br|C|Ca|Cd|Ce|Cf|Cl|Cm|Co|Cr|Cs|Cu|Ds|Db|Dy|Er|Es|Eu|F|Fe|Fm|Fr|Ga|Gd|Ge|H|He|Hf|Hg|Ho|Hs|I|In|Ir|K|Kr|La|Li|Lr|Lu|Md|Mg|Mn|Mo|Mt|N|Na|Nb|Nd|Ne|Ni|No|Np|O|Os|P|Pa|Pb|Pd|Pm|Po|Pr|Pt|Pu|Ra|Rb|Re|Rf|Rg|Rh|Rn|Ru|S|Sb|Sc|Se|Sg|Si|Sm|Sn|Sr|Ta|Tb|Tc|Te|Th|Ti|Tl|Tm|U|V|W|Xe|Y|Yb|Zn|Zr)\d*)+$
This doesn't deal with the parenthesized groups.
This we worked out during the San Diego Python Users Group meeting.