Representation of chemical formulas as strings? Is there a standard manner of representing chemical formulas as linear strings?

For example, $H_2O$ can be represented with the string H2O. Isotopes are usually specified with a number before the element. But this must be preceded by a space, or it can be get mixed with the number right before: O4 13CH, which is equivalent to $O_4{^{13}C}H$.

I'm just wondering if there is any set of accepted conventions when representing chemical formulas as linear strings.
If I understand your question correctly, the simplified molecular-input line-entry system (SMILES) and the IUPAC International Chemical Identifier (InChI) are two examples of "human-readable" string notations for chemical formulas your are looking for.