\d match a digit
So \d\d will match a 2 digit sequence.
Or more general, \d+ will match any sequence of digits 1 or more long, just as .+ will match any sequence of characters, 1 long or longer.
Start of line can be matched using ^
End of line can be matched using $
You can match titles starting with digits like this:
^\d+.+$
And titles with digits in the middle like this:
^.+\d+.+$
By grouping the matches using parentheses you can do search and replace using the separate parts. Usually it is not necessary to use ^ and $, but sometimes it is. You have to test and see. And make sure you have a backup of your calibre library, and know how to restore it. There is NO UNDO if you make errors. But on the other hand there is a helpful test feature in calibre so you can test the regexp search and replace before you actually do it.
For instance:
Series 32 Title
(.+)(\d+)(.+)
Here group \1 will be "Series ", group \2 will be "32" and group \3 will be " Title".
Or
123 Title
(\d+)(.+)
\1="123", \2=" Title".
Title 456
(.+)(\d+)
\1="Title ", \2="456"
Most likely you would also like to match spaces, \s, outside the groups, like this:
Series 89 Title
(.+)\s(\d+)\s(.+)
\1="Series", \2="89", \3="Title" (Note that the spaces before and after the digits no longer are included in \1 and \3.)
To remove the digits from the beginning of a title yo can do this:
12 Title
Search title for ^(\d+)\s(.+) and replace title with \2.
Read more here:
http://manual.calibre-ebook.com/regexp.html