Grab a REGEDIT cheat sheet (and a CSS one while you are at it
)
I like cheatography ones.
a period is a REGEDIT special character You saw
.+? all 3 have special meaning
Calibre and Sigil use PCRE flavor, so not all on the sheet apply
special characters need to be 'escaped', thus
\.
\s is any kind of space
\d any digit