<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def extract_smiles(s):
 """Return a list of SMILES identifiers extracted from the string."""

    # TODO: This still gets a lot of false positives.
    smiles = []
    for t in s.split():
        if len(t) > 2 and SMILES_RE.match(t) and not t.endswith('.') and bracket_level(t) == 0:
            smiles.append(t)
    return smiles