Repetitions:

x*             zero or more x, prefer more
x+             one or more x, prefer more
x?             zero or one x, prefer one
x{n,m}         n or n+1 or ... or m x, prefer more
x{n,}          n or more x, prefer more
x{n}           exactly n x
x*?            zero or more x, prefer fewer
x+?            one or more x, prefer fewer
x??            zero or one x, prefer zero
x{n,m}?        n or n+1 or ... or m x, prefer fewer
x{n,}?         n or more x, prefer fewer
x{n}?          exactly n x

Grouping:

(re)           numbered capturing group (submatch)
(?P<name>re)   named & numbered capturing group (submatch)
(?:re)         non-capturing group
(?flags)       set flags within current group; non-capturing
(?flags:re)    set flags during re; non-capturing

Flag syntax is xyz (set) or -xyz (clear) or xy-z (set xy, clear z). The flags are:

i              case-insensitive (default false)
m              multi-line mode: ^ and $ match begin/end line in addition to begin/end text (default false)
s              let . match \n (default false)
U              ungreedy: swap meaning of x* and x*?, x+ and x+?, etc (default false)

Perl character classes (all ASCII-only):

\d             digits (== [0-9])
\D             not digits (== [^0-9])
\s             whitespace (== [\t\n\f\r ])
\S             not whitespace (== [^\t\n\f\r ])
\w             word characters (== [0-9A-Za-z_])
\W             not word characters (== [^0-9A-Za-z_])

Empty strings:

^              at beginning of text or line (flag m=true)
$              at end of text (like \z not Perl's \Z) or line (flag m=true)
\A             at beginning of text
\b             at ASCII word boundary (\w on one side and \W, \A, or \z on the other)
\B             not at ASCII word boundary
\z             at end of text

Escape sequences:

\a             bell (== \007)
\f             form feed (== \014)
\t             horizontal tab (== \011)
\n             newline (== \012)
\r             carriage return (== \015)
\v             vertical tab character (== \013)
\*             literal *, for any punctuation character *
\123           octal character code (up to three digits)
\x7F           hex character code (exactly two digits)
\x{10FFFF}     hex character code
\Q...\E        literal text ... even if ... has punctuation

Character class elements:

x              single character
A-Z            character range (inclusive)
\d             Perl character class
[:foo:]        ASCII character class foo
\p{Foo}        Unicode character class Foo
\pF            Unicode character class F (one-letter name)