In 1.5.4, there is a constant which is defined in "qa-util-string.php" (line 711), which by defaut is defined with the following content.
@define('QA_PREG_INDEX_WORD_SEPARATOR', '[\n\r\t\ \!\"\\\'\(\)\*\+\,\.\/\:\;\<\=\>\?\[\\\\\]\^\`\{\|\}\~]');
These are characters used as word separators as I understand it.
Say you want to be able to use tag 'C++', you then need to remove the character '+' from the word separator list, that is you need to remove it from the entries (along with the antislash escape), i.e you remove '\+'
The constant definition becomes:
@define('QA_PREG_INDEX_WORD_SEPARATOR', '[\n\r\t\ \!\"\\\'\(\)\*\,\.\/\:\;\<\=\>\?\[\\\\\]\^\`\{\|\}\~]');
Same applies to the dot character if you need it for '.htacces'
I would remove characters only when needed and cautiosly though.