python regex: variable length of positive lookbehind assertion

[Yubin Ruan]

Hi everyone, 
I am struggling writing a right regex that match what I want:

Problem Description:

Given a string like this:

    >>>string = "false_head <a>aaa</a> <a>bbb</a> false_tail \
             true_head some_text_here <a>ccc</a> <a>ddd</a> <a>eee</a> true_tail"

I want to match the all the text surrounded by those "<a> </a>",
but only if those "<a> </a>" locate **in some distance** behind "true_head". That is, I expect to result to be like this:

    >>>import re
    >>>result = re.findall("the_regex",string)
    >>>print result
    ["ccc","ddd","eee"]

How can I write a regex to match that?
I have try to use the **positive lookbehind assertion** in python regex,
but it does not allowed variable length of lookbehind.

Thanks in advance,
Ruan