SMARTS reads a structure as a question rather than a molecule, and several characters change meaning when it does. This table lists every primitive, and says which of the shared ones stop meaning what they mean in a SMILES.
Convert between chemical structures and SMILES in both directions, search a list of structures by substructure or SMARTS, and learn the notation step by step. Everything runs in your browser — no structure you open ever leaves your machine — so the tool needs JavaScript.