Regex Memo

Normal characters

Expression	Description
. or [^

] | any character excluding a newline or carriage return [A-Za-z] | alphabet [a-z] | lowercase alphabet [A-Z] | uppercase alphabet d or [0-9] | digit D or [^0-9] | non-digit _ | underscore w or [A-Za-z0-9_] | alphabet, digit or underscore W or [^A-Za-z0-9_] | inverse of w S | inverse of s

Whitespace characters

Expression	Description
space

 | tab

| newline

| carriage return s | space, tab, newline or carriage return

Character set

Expression	Description
[xyz]	either x, y or z
[^xyz]	neither x, y nor z
[1-3]	either 1, 2 or 3
[^1-3]	neither 1, 2 nor 3

• Think of a character set as an OR operation on the single characters that are enclosed between the square brackets.
• Use ^ after the opening [ to "negate" the character set.
• Within a character set, . means a literal period.

Characters that require escaping

Outside a character set

Expression	Description
.	period
^	caret
$	dollar sign
|	pipe
\	back slash
/	forward slash
(	opening bracket
)	closing bracket
[	opening square bracket
]	closing square bracket
{	opening curly bracket
}	closing curly bracket

Inside a character set

Expression	Description
\	back slash
]	closing square bracket

• A ^ must be escaped only if it occurs immediately after the opening [ of the character set.
• A - must be escaped only if it occurs between two alphabets or two digits.

Quantifiers

Expression	Description
{2}	exactly 2
{2,}	at least 2
{2,7}	at least 2 but no more than 7

• | 0 or more

• | 1 or more ? | exactly 0 or 1

• The quantifier goes after the expression to be quantified.

Boundaries

Expression	Description
^	start of string
$	end of string
	word boundary

• How word boundary matching works:
  • At the beginning of the string if the first character is w.
  • Between two adjacent characters within the string, if the first character is w and the second character is W.
  • At the end of the string if the last character is w.

Matching

Expression	Description
foo	bar
foo(?=bar)	match foo if it's before bar
foo(?!bar)	match foo if it's not before bar
(?<=bar)foo	match foo if it's after bar
(?<!bar)foo	match foo if it's not after bar

Grouping and capturing

Expression	Description
(foo)	capturing group; match and capture foo
(?:foo)	non-capturing group; match foo but without capturing foo
(foo)bar\1	\1 is a backreference to the 1st capturing group; match foobarfoo

• Capturing groups are only relevant in the following methods:
  • string.match(regexp)
  • string.matchAll(regexp)
  • string.replace(regexp, callback)
• \N is a backreference to the Nth capturing group. Capturing groups are numbered starting from 1.

References and tools

• MDN
• RegExplained