Flags

d
Generate indices for substring matches.
g
Global search.
i
Case-insensitive search.
m
Allows ^ and $ to match newline characters.
s
Allows . to match newline characters.
u
"Unicode"; treat a pattern as a sequence of Unicode code points.
y
Perform a "sticky" search that matches starting at the current position in the target string.

Groups and backreferences

(x)
Capturing group
(?<Name>x)
Named capturing group
(?:x)
Non-capturing group
\N
Where "N" is a positive integer. A back reference to the last substring matching the N parenthetical in the regular expression (counting left parentheses).
\k<Name>
A back reference to the last substring matching the named capture group specified by <Name>.

Character classes

[xyz]
[a-c]
A character class.
[^xyz]
[^a-c]
A negated or complemented character class.
.
Matches any single character except line terminators: \n, \r, \u2028 or \u2029.
Inside a character class, the dot loses its special meaning and matches a literal dot.
Note that the m multiline flag doesn't change the dot behavior. So to match a pattern across multiple lines, the character class [^] can be used — it will match any character including newlines.
The s "dotAll" flag allows the dot to also match line terminators.
\d
Matches any digit. Equivalent to [0-9].
\D
Matches any character that is not a digit. Equivalent to [^0-9].
\w
Matches any alphanumeric character from the basic Latin alphabet, including the underscore. Equivalent to [A-Za-z0-9_].
\W
Matches any character that is not a word character from the basic Latin alphabet. Equivalent to [^A-Za-z0-9_].
\s
Matches a single white space character, including space, tab, form feed, line feed, and other Unicode spaces.
\S
Matches a single character other than white space.
\t
Matches a horizontal tab.
\r
Matches a carriage return.
\n
Matches a linefeed.
\v
Matches a vertical tab.
\f
Matches a form-feed.
[\b]
Matches a backspace.
\0
Matches a NUL character. Do not follow this with another digit.
\cX
Matches a control character using caret notation, where "X" is a letter from A–Z (corresponding to code points U+0001–U+001A).
\xhh
Matches the character with the code hh (two hexadecimal digits).
\uhhhh
Matches a UTF-16 code-unit with the value hhhh (four hexadecimal digits).
\u{hhhh} or
\u{hhhhh}
(Only when the u flag is set.) Matches the character with the Unicode value U+hhhh or U+hhhhh (hexadecimal digits).
\p{UnicodeProperty},
\P{UnicodeProperty}
Matches a character based on its Unicode character properties.

Assertions

^
Matches the beginning of input. If the multiline flag is set to true, also matches immediately after a line break character.
$
Matches the end of input. If the multiline flag is set to true, also matches immediately before a line break character.
\b
Matches a word boundary. This is the position where a word character is not followed or preceded by another word-character, such as between a letter and a space. Note that a matched word boundary is not included in the match.
\B
Matches a non-word boundary.
x(?=y)
Lookahead assertion: Matches "x" only if "x" is followed by "y".
x(?!y)
Negative lookahead assertion: Matches "x" only if "x" is not followed by "y".
(?<=y)x
Lookbehind assertion: Matches "x" only if "x" is preceded by "y".
(?<!y)x
Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y".

Quantifiers

x*
Matches the preceding item "x" 0 or more times.
x+
Matches the preceding item "x" 1 or more times.
x?
Matches the preceding item "x" 0 or 1 times.
x{n}
Where "n" is a positive integer, matches exactly "n" occurrences of the preceding item "x".
x{n,}
Where "n" is a positive integer, matches at least "n" occurrences of the preceding item "x".
x{n,m}
Where "n" is 0 or a positive integer, "m" is a positive integer, and m > n, matches at least "n" and at most "m" occurrences of the preceding item "x".
x*?
x+?
x??
x{n}?
x{n,}?
x{n,m}?
The ? character after the quantifier makes the quantifier "non-greedy": meaning that it will stop as soon as it finds a match.

Source: MDN Web Docs