Post-merged code review:

You store FormatTags as typedef QPair<QRegExp, QString>

Which is really sensible (well done), especially as compiling a reg-ex can be slow.

but in the actual replacement we turn the regexp into a string, and then back into a (different) regular expression, never using the initial regexp from the FormatTag as a reg-exp, this is just weird and confusing.

Also "addTag" don't convert a QString into a char* and back (it's overly slow, passing a const reference is basically "free") and don't convert a QString into a char! which you also do.