Exercise 3 solution

    import re

    expr = "a1+b2*(c-3)"
    parts = re.split(r"([+\-*/()])", expr)
    print(parts)
    # ['a1', '+', 'b2', '*', '', '(', 'c', '-', '3', ')', '']
    print([p for p in parts if p])
    # ['a1', '+', 'b2', '*', '(', 'c', '-', '3', ')']

    doc = "Para one\nline two\n\n\nPara two\n\nPara three"
    paras = re.split(r"\n\s*\n", doc)
    print(paras, len(paras))
    # ['Para one\nline two', 'Para two', 'Para three'] 3
    print(re.split(r"\n\s*\n", doc, maxsplit=1))
    # ['Para one\nline two', 'Para two\n\nPara three']

(All lines were run and printed exactly what is shown.)

Explanation: a capturing group in the split pattern makes re.split keep the
separators in the result. Two separators in a row ("*" then "(") leave an empty
string between them, and a separator at the very end leaves one after it, so
the list comprehension filters the empties out. The paragraph pattern \n\s*\n
treats one or more blank lines as a single break. maxsplit=1 stops after the
first break.