import re

html_doc = """
<td class="center" id="a_b_ahmet">x</td>
<td class="center" id="a_b_mehmet">y</td>
<td class="center" id="a_b_ali">z</td> 
"""

soup = BeautifulSoup(html_doc, 'html.parser')

td_elements = soup.find_all('td')
for td in td_elements:
    if re.match(r'a_b_.*', td['id']):
        print(td)