# BRACKETS:
[...]: Any one character between the brackets.
[^...]: Any one character not between the brackets.
[0-9]: It matches any decimal digit from 0 through 9.
[a-z]: It matches any character from lowercase a through lowercase z.
[A-Z]: It matches any character from uppercase A through uppercase Z.
[a-Z]: It matches any character from lowercase a through uppercase Z.


# QUANTIFIERS:
p+: It matches any string containing at least one p.
p*: It matches any string containing zero or more ps.
p?: It matches any string containing one or more ps.
p{N}: It matches any string containing a sequence of N ps.
p{2,3}: It matches any string containing a sequence of two or three ps.
p{2, }: It matches any string containing a sequence of at least two ps.
p$: It matches any string with p at the end of it.
^p: It matches any string with p at the beginning of it.


# LITERAL CHARACTERS:
Alphanumeric: Itself.
\0: The NUL character (\u0000).
\t: Tab (\u0009).
\n: Newline (\u000A).
\v: Vertical tab (\u000B).
\f: Form feed (\u000C).
\r: Carriage return (\u000D).


# METACHARACTERS:
.: a single character.
\s: a whitespace character (space, tab, newline).
\S: non-whitespace character.
\d: a digit (0-9).
\D: a non-digit.
\w: a word character (a-z, A-Z, 0-9, _).
\W: a non-word character.
[\b]: a literal backspace (special case).
[aeiou]: matches a single character in the given set.
[^aeiou]: matches a single character outside the given set.
(foo|bar|baz): matches any of the alternatives specified.

# Common REGULAR EXPRESSIONS:
Matching a Username: ^[a-z0-9_-]{3,16}$
Matching a Password: ^[a-z0-9_-]{6,18}$
Matching a Hex Value: ^#?([a-f0-9]{6}|[a-f0-9]{3})$
Matching a Slug: ^[a-z0-9-]+$
Matching an Email: ^([a-z0-9_\.-]+)@([\da-z\.-]+)\.([a-z\.]{2,6})$
Matching a URL: ^(https?:\/\/)?([\da-z\.-]+)\.([a-z\.]{2,6})([\/\w \.-]*)*\/?$
Matching an IP Address: ^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$
Matching an HTML Tag: ^<([a-z]+)([^<]+)*(?:>(.*)<\/\1>|\s+\/>)$