lookbehind will back up 3 characters and check if the contained pattern matches.
The contained pattern must only match strings of some fixed length, meaning that
``abc`` or ``a|b`` are allowed, but ``a*`` and ``a{3,4}`` are not. Note that
- patterns which start with positive lookbehind assertions will never match at the
+ patterns which start with positive lookbehind assertions will not match at the
beginning of the string being searched; you will most likely want to use the
:func:`search` function rather than the :func:`match` function: