<td xyz="123"><a href="blah.html">This is a line</a></td>

<tr><td>New line</td></tr>
<tr><td xyz="123"><a href="blah.html">CaptureThis</a></td></tr>

<tr><td x?y?z?=?"?(ddd)?"?>?<?a?.*?>?(.*?)?<?/?a?>?</td></tr>

from xml.etree import ElementTree

tree = ElementTree.parse('filename.html')
for elem in tree.findall('tr'):
    print ElementTree.tostring(elem)

>>> line1
'<tr><td>New line</td></tr>'
>>> line2
'<tr><td xyz="123"><a href="blah.html">CaptureThis</a></td></tr>'
>>> pattern2 = re.compile(r'>([ws]+)<')
>>> pattern2.search(line1).group(1)
'New line'
>>> pattern2.search(line2).group(1)
'CaptureThis'

>>> pattern = re.compile(r'<tds+w+="([^"]*)">')
>>> pattern.search(line2).group(1)
'123'

>>> text = '''<tr><td>New line</td></tr>
<tr><td xyz="123"><a href="blah.html">CaptureThis</a></td></tr>
<tr><td xyz="456">CaptureThisAlso</td></tr>
'''

>>> re.findall(r'<tr><td(?: xyz="(d+)")?>(?:<a href=".*?">)?(.*?)(?:</a>)?</td></tr>', text)
[('', 'New line'), ('123', 'CaptureThis'), ('456', 'CaptureThisAlso')]