@theducks - you are merciless at times with your acronyms
. Explaining what REGEX is by substituting it with "pcre"... lol
The Modify ePub plugin has other uses for helping automate cleaning things up, inserting/replacing covers etc. But if your sources are not ePub to begin with you will be doing a calibre conversion as your first step, which negates the purpose of probably 75% of this plugin.
You might find it handy to install the Open With plugin to most quickly launch Sigil with the tap of a key from calibre for an ePub you want to edit.
And as mentioned please do use the latest Sigil 0.5.9 beta version rather than the 0.5.3 version, the beta is infinitely more powerful and light years more stable. We intend to have the next Sigil beta iteration out extremely soon (as in a few days) which should speed some important things up massively and polish some areas to really show off the massive effort gone into the 0.6 release...