Just The Extension

Capture the file extension of a filename: report.q3.pdf gives pdf, not q3.pdf.

Your first capture group. Round brackets mark the part of the match you want handed back — capture mode compares group 1, group 2 and so on against the expected list, and the rest of the match is just there to find the right spot. The trap is the dot: a filename can have several, and you want the LAST one. Anchoring the end with $ is what forces the engine past the earlier dots. A name with nothing after the final dot has no extension at all, so it should not match.