What regular expression can be used to select letters that stand alone, and not in a word?

Question

Regular Expressions

There are several substances in one line
MnCl2, NaCl, Cl2, Cl2O
Which regular expression can only select Cl2 without affecting MnCl2 or Cl2O ?

Answer 1

In sense, "letters separately"?
Perhaps you meant the designation of molecules consisting of atoms of only one substance.
\b[A-Z][a-z]?\d?\b