That behaviour has nothing to do with re.

This line:

    samples = filter(lambda sample: not pttn.match(sample), data)

creates a generator that, when evaluated, will use the value of 'pttn' _at that time_.

However, you then bind 'pttn' to something else.

Here's a simple example:

>>> x = 1
>>> func = lambda: print(x)
>>> func()
1
>>> x = 2
>>> func()
2

A workaround is to capture the current value with a default argument:

>>> x = 1
>>> func = lambda x=x: print(x)
>>> func()
1
>>> x = 2
>>> func()
1