If you want to match the character string representation of the number ten (and I'm not being pedantic here - regex's only match characters and have no concept of numbers or anything else) the the pattern:
10
will find the two consecutive characters '1' and '0' anywhere within the text. If you don't want it to find the '10' within "341065" then something like:
\b10\b
will work in many cases (but it will still find the '10' in "abc-10-def"). This all goes to show that you need to fully understnad and explain what you aer after.
As for finding each of the sub-strings between the underscores, I would suggest that you use the regex split function using the underscore as the pattern. Note that this will produce 'rdf4er2.pdf' as the last component and not 'rdf4er2' as in your example.
Susan