Parentheses (...) in a pattern create a capturing group. A group lets you extract one part of the match:

• group() or group(0) — the whole match.
• group(n) — the text captured by group number n. Groups are numbered from 1, left to right.
• groups() — a tuple with every captured group, in order.

re.finditer(pattern, string, flags=0) returns an iterator of match objects, one per occurrence, so you can read each match in a for loop:

• finditer accepts the same pattern, string and flags as search.
• It is lazy: it produces one match at a time, left to right, without overlapping.

The r"..." prefix makes a raw string: backslashes are kept for re instead of being interpreted by Python. Use a raw string whenever the pattern contains a backslash; it is also fine to use one every time.

Coming from JavaScript? match.group(1) is close to match[1] in JS; match.groups() is match.slice(1), but Python returns a tuple instead of an array.