There is a problem with the documentation vs. the implementation of Topics in Mailman 2.1. This is corrected in Mailman 2.1.19.
The problem is the documentation says that you can put keywords in the regexp box for a topic with one word per line, but the contents of the regexp box is actually compiled as a single regexp in VERBOSE mode. This means that all the lines are just concatenated and all unescaped whitespace is removed. Thus, if one puts
word1 word2 a phrase
the resultant regexp is
which is unlikely to match anything. To achieve the intent of this example, you need something like
word1| word2| a\ phrase
In other words you need to escape with a '\' any significant whitespace and put the 'or' operator ('|') at the end of all but the last line.
Note that the match is always case insensitive.
For Mailman 2.1.19, the lines are joined by Mailman with the '|' operator into a single line which is compiled in non-VERBOSE mode so whitespace will also be preserved. Existing pre-2.1.19 regexps will be converted to the equivalent 2.1.19 regexp by Mailman's upgrade process on initial upgrade to 2.1.19.