Regular expressions

Regular expression, often shortened to RegEx or RegExp, is a text-based format used to specify match patterns.

The alternating yellow and orange highlights show results for the following regexp pattern: /[a-z](?<![aeiou])[aeiou]/gi (any consonant-vowel pair)

Syntax

x, y, and z when used under symbols are placeholders for text. Capital Xs, Ys, and Zs are used for number placeholders.

Syntax Reference

Symbol(s)	Name	Description	Example
Groups and backreferences
(x)	Capture group	Separates the content in the output.	"Foo Bar" /(Foo)|(Bar)/g -> [ "Foo", "Bar" ]
(?:x)	Non-capture group	Acts as if the parentheses were not there	"Foo Bar" /(?:Foo)|(?:Bar)/g -> [ "Foo Bar" ]
(?<y>x)	Named capture group	Equivalent to (x), except it remembers the content used.	"Foo Bar" /(?<F>Foo)|(?<B>Bar)/g -> [ "Foo", "Bar" ]
\k<y>	Named backreference	References a previous named capture group, note that \k is literal	"Foo Foo" /(?<Foo>Foo)\s\k<Foo>/g -> [ "Foo Foo" ]
Character classes
[x-z]	Character class	Matches every letter or number from x to z.	"Foo Bar" /[a-f]/gi -> [ "F", "B", "a" ]
[xyz]		References either x, y, or z	"Foo Bar" /[FB]/g -> [ "F", "B" ]
[^x-z]	Negated character class	Matches every letter or number not from x to z.	"Foo Bar" /[^a-f]/gi -> [ "o", "o", " ", "r" ]
[^xyz]		References characters that aren't x, y, or z	"Foo Bar" /[^FB]/g -> [ "o", "o", " ", "a", "r" ]
.	Wildcard	Matches every character besides line terminators. Line terminators include \n, \r, \u2028, and \u2029	"Foo Bar" /./g -> [ "F", "o", "o", " ", "B", "a", "r" ]
x|y	Disjunction	Match something or something else.	"Foo Bar" /Foo|Bar/g -> [ "Foo", "Bar" ]
\	Escape character	If a character is reserved for regex, such as *, |, or .. Note that this is itself a reserve character, so to match for it, you need to use \\.	"Foo.bar apple 78.9 banana" /[A-Za-z0-9]*\.[A-Za-z0-9]*/g -> [ "Foo.bar", "78.9" ]
\d	Digit character class	Equivalent to [0-9]	"78 Foo Bars" /\d/g -> [ "7", "8" ]
\D	Non-digit character class	Equivalent to [^0-9]	"78 Foo Bars" /\d/g -> [ "F", "o"," "o", " ", "B", "a", "r", "s" ]
\w	Word character class	Equivalent to [A-Za-z0-9_]	"_Foo- Bars+" /\d/g -> [ "_", "F"," "o", "o", "B", "a", "r", "s" ]
\W	Non-word character class	Equivalent to [^A-Za-z0-9_]	"_Foo- Bars+" /\d/g -> [ "-", " ", "+" ]
\s	White space character class	Matches all whitespace characters. Equivalent to [\f\n\r\t\v\u0020\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff]	"_Foo- Bars+" /\d/g -> [ "-", " ", "+" ]
\S	Non-white space character class	Matches everything but whitespace characters. Equivalent to [^\f\n\r\t\v\u0020\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff]	"_Foo- Bars+" /\d/g -> [ "-", " ", "+" ]
\t	Horizontal tab	Matches horizontal tab characters.	"a    b" /\t/g -> [ "    " ]
\n	New line	Matches linefeed/new line characters	"a b" /(?:\r?\n)|(?:\v)|(?:\f)/g -> [ "" ]
\r	Carriage return	Matches carriage return characters
\v	Vertical tab	Matches vertical tab characters
\f	Form feed	Matches form feed characters
[\b]	Backspace	Matches backspace	No example can be provided
\0	NUL	Matches the NUL character
\u{YYYY} or \u{YYYY}	Unicode value escape	When the u flag is applied. Here Y represents a hexadecimal number.
\uYYYY		Matches provided UTF-16 hexadecimal value. Represented with Ys here.
\p{x} or \P{x}	Unicode character class	Matches a character based on the Unicode property (x).
\cx	Caret notation escape	Matches the sequence following \c with caret notation. Note that x represents a sequence of characters here, rather than a single one.	"a b" /\cM\cJ//g -> [ "" ]
Assertions
^	Input boundary beginning	Matches the beginning of the input. If the m flag is on, it matches the start of each line.	"Foo Bar" /(^Foo)|(Bar$)/g -> [ "Foo", "Bar" ]
$	Input boundary end	Matches the end of the input. If the m flag is on, it matches the end of each line.
\b	Word boundary	Matches either end of a word.	"Foo Bar" /(\bFoo\b)/ -> [ "Foo" ]
\B	Non-word boundary	Matches the middle of a word.	"Foo Bar" /(B\Bar)/ -> [ "Bar" ]
x(?=y)	Positive lookahead	Matches if y is after x, but doesn't include y in the output.	"Foo Bar" /Foo(?= Bar)/ -> [ "Foo" ]
x(?!y)	Negative lookahead	Matches if y is not after x, but doesn't include y in the output.	"Foo Bar" /Foo(?! Car)/ -> [ "Foo" ]
(?<=x)y	Positive lookbehind	Matches if y is before x, but doesn't include y in the output.	"Foo Bar" /(?<=Foo )Bar/ -> [ "Bar" ]
(?<!x)y	Negative lookbehind	Matches if y is before x, but doesn't include y in the output.	"Foo Bar" /(?<!Moo )Bar/ -> [ "Bar" ]
Quantifiers
x*	Wild-amount	Matches x any number of times, including 0.	"Foo Foo Foo Bar" /(?:Foo )*Bar/g -> [ "Foo Foo Foo Bar" ]
x+	Wild-1-or-more	Matches x if it occurs 1 or more times.	"Foo Bar Bar" /(Foo)+ (Bar)+/ -> [ "Foo Bar Bar" ]
x?	Can occur	Matches x if it occurs, otherwise, ignore it.	"Foo " /Foo (Bar)?/ -> [ "Foo " ]
x{Y}	Occurs set times	Matches if x occurs Y times.	"Foo Bar Bar" /Fo{2} (?:Bar\s?){2}/ [ "Foo Bar Bar" ]
x{Y,Z}	Occurs between set times	Matches if x occurs Y and Z times.	"Foooo Bar Bar Bar Bar Bar" /Fo{2,5} (?:Bar\s?){1,10}/ [ "Foooo Bar Bar Bar Bar Bar" ]
x*?, x+?, x??, x{Y}?, or x{Y,Z}?	Lazy match	Matches x the least number of times possible, in accordance to the base rule.	"Foooo Bar Bar Bar Bar Bar" /Fo{2,5} (?:Bar\s??){1,10}?/ [ "Foooo Bar" ]

Flags

Whilst there are flags other than the following, they are either non-standard, or do not have a baring on PenguinMod.

Flags

Flag	Name	Description
g	global	Search all of a string, rather than stopping once you find an occurrence.
i	Case insensitive	The search will ignore the case of characters, making /[A-Za-z]g and /[a-z]/gi equivalent.
m	multiline	Makes ^ and $ match the start and end of lines rather than the start and end of strings.
s	single line/dot all	Makes . able to match all line terminators: \n, \r, \u2028, and \u2029.
u	unicode	Makes the pattern treated as a sequence of unicode codepoints.
v	unicode upgrade	Similar to u, but updated with more features.

Examples

The following examples will have a RegExp string, and an example string. The places where the example string matches the RegExp will be highlighted with alternating yellow and orange.

Fox and dog

The RegExp (?<=[^a-z])[^aeiou][aeiou][^aeiou](?=[^a-z]|$) will match any 3-letter-word that starts and ends with a consonant, with the middle letter being a vowel.

The quick brown fox jumps over the lazy dog

In this case, the matches are fox and dog.

Word after "l"

The RegExp (?<=(?:^| )[lL])[A-Za-z]* matches every character in a word that starts with the letter L, other than the L at the start.

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

In this case, the matches are "orem," "abore," "aboris," and "aborum."

Non-specific dialects

The RegExp gr[ea]y matches the British-English "grey" and the American-English "gray." this allows for

The colour grey.

The color gray.

Here, both grey and gray are detected.

See also

• (match [foo bar] with regex [foo] [g]:: operators)
• <test regex [foo bar] [g] with text [foo]:: sensing>
• RegExp (extension)

Further reading

• Regular expressions on Wikipedia
• Regular-Expressions.info, a website with many guides and examples for RegExp.
• RegEgg, a tutorial for RegEx.

External links

• regex101, fairly useful little app with some fun challenges to test your knowledge of regex.