Markup:

<p><b>CHAPTER 1</b><p>TEST HELLO</p><p>TEST</p>
<p><b>SUBCHAPTER A</b><p>TEST</p><p>TESTEE</p>


<p><b>CHAPTER 20</b><p>W3SCHOOL</p><p><b>SUBCHAPTER B</b><p>HELLO</p>

Output:

<p CLASS="C1">CHAPTER 1. TEST HELLO TEST</p>
<p CLASS="C1">SUBCHAPTER A. TEST TESTEE</p>

p CLASS="C1">CHAPTER 20. W3SCHOOL</p>
<p CLASS="C1">SUBCHAPTER B. HELLO</p>

Using re.sub module