. — any char (dotAll=s affects newline)
^ $ — start / end (m = multiline)
\d \D \w \W \s \S — digit, word, space classes
[abc] [^abc] — class / negated class
a|b — alternation
() — capturing group
(?: … ) — non-capturing
(?<n> … ) — named capture
(?= … ) (?! … ) — lookahead
(?<= … ) (?<! … ) — lookbehind
* + ? — 0+, 1+, 0/1 (greedy; add ? for lazy)
{n} {n,} {n,m} — bounded counts
\b \B — word boundary / not
\1 \k<n> — numbered / named backref
$1 $$ $& — replacement inserts ($& whole match)