Characters

CharacterLegendExampleSample Match
\dMost engines: one digit
from 0 to 9		file_\d\dfile_25
\d.NET, Python 3: one Unicode digit in any scriptfile_\d\dfile_9੩
\wMost engines: “word character”: ASCII letter, digit or underscore\w-\w\w\wA-b_1
\w.Python 3: “word character”: Unicode letter, ideogram, digit, or underscore\w-\w\w\w字-ま_۳
\w.NET: “word character”: Unicode letter, ideogram, digit, or connector\w-\w\w\w字-ま‿۳
\sMost engines: “whitespace character”: space, tab, newline, carriage return, vertical taba\sb\sca b
c
\s.NET, Python 3, JavaScript: “whitespace character”: any Unicode separatora\sb\sca b
c
\DOne character that is not a digit as defined by your engine’s \d\D\D\DABC
\WOne character that is not a word character as defined by your engine’s \w\W\W\W\W\W*-+=)
\SOne character that is not a whitespace character as defined by your engine’s \s\S\S\S\SYoyo

Quantifiers

QuantifierLegendExampleSample Match
+One or moreVersion \w-\w+Version A-b1_1
{3}Exactly three times\D{3}ABC
{2,4}Two to four times\d{2,4}156
{3,}Three or more times\w{3,}regex_tutorial
*Zero or more timesA*B*C*AAACC
?Once or noneplurals?plural

More Characters

CharacterLegendExampleSample Match
.Any character except line breaka.cabc
.Any character except line break.*whatever, man.
\.A period (special character: needs to be escaped by a \)a\.ca.c
\Escapes a special character\.\*\+\?    \$\^\/\\.*+?    $^/\
\Escapes a special character\[\{\(\)\}\][{()}]

Logic

LogicLegendExampleSample Match
|Alternation / OR operand22|3333
( … )Capturing groupA(nt|pple)Apple (captures “pple”)
\1Contents of Group 1r(\w)g\1xregex
\2Contents of Group 2(\d\d)\+(\d\d)=\2\+\112+65=65+12
(?: … )Non-capturing groupA(?:nt|pple)Apple

More White-Space

CharacterLegendExampleSample Match
\tTabT\t\w{2}T     ab
\rCarriage return charactersee below
\nLine feed charactersee below
\r\nLine separator on WindowsAB\r\nCDAB
CD
\NPerl, PCRE (C, PHP, R…): one character that is not a line break\N+ABC
\hPerl, PCRE (C, PHP, R…), Java: one horizontal whitespace character: tab or Unicode space separator
\HOne character that is not a horizontal whitespace
\v.NET, JavaScript, Python, Ruby: vertical tab
\vPerl, PCRE (C, PHP, R…), Java: one vertical whitespace character: line feed, carriage return, vertical tab, form feed, paragraph or line separator
\VPerl, PCRE (C, PHP, R…), Java: any character that is not a vertical whitespace
\RPerl, PCRE (C, PHP, R…), Java: one line break (carriage return + line feed pair, and all the characters matched by \v)

More Quantifiers

QuantifierLegendExampleSample Match
+The + (one or more) is “greedy”\d+12345
?Makes quantifiers “lazy”\d+?1 in 12345
*The * (zero or more) is “greedy”A*AAA
?Makes quantifiers “lazy”A*?empty in AAA
{2,4}Two to four times, “greedy”\w{2,4}abcd
?Makes quantifiers “lazy”\w{2,4}?ab in abcd

Character Classes

CharacterLegendExampleSample Match
[ … ]One of the characters in the brackets[AEIOU]One uppercase vowel
[ … ]One of the characters in the bracketsT[ao]pTap or Top
–Range indicator[a-z]One lowercase letter
[x-y]One of the characters in the range from x to y[A-Z]+GREAT
[ … ]One of the characters in the brackets[AB1-5w-z]One of either: A,B,1,2,3,4,5,w,x,y,z
[x-y]One of the characters in the range from x to y[ -~]+Characters in the printable section of the ASCII table.
[^x]One character that is not x[^a-z]{3}A1!
[^x-y]One of the characters not in the range from x to y[^ -~]+Characters that are not in the printable section of the ASCII table.
[\d\D]One character that is a digit or a non-digit[\d\D]+Any characters, inc-
luding new lines, which the regular dot doesn’t match
[\x41]Matches the character at hexadecimal position 41 in the ASCII table, i.e. A[\x41-\x45]{3}ABE

Anchors and Boundaries

AnchorLegendExampleSample Match
^Start of string or start of line depending on multiline mode. (But when [^inside brackets], it means “not”)^abc .*abc (line start)
$End of string or end of line depending on multiline mode. Many engine-dependent subtleties..*? the end$this is the end
\ABeginning of string
(all major engines except JS)		\Aabc[\d\D]*abc (string…
…start)
\zVery end of the string
Not available in Python and JS		the end\zthis is…\n…the end
\ZEnd of string or (except Python) before final line break
Not available in JS		the end\Zthis is…\n…the end\n
\GBeginning of String or End of Previous Match
.NET, Java, PCRE (C, PHP, R…), Perl, Ruby
\bWord boundary
Most engines: position where one side only is an ASCII letter, digit or underscore		Bob.*\bcat\bBob ate the cat
\bWord boundary
.NET, Java, Python 3, Ruby: position where one side only is a Unicode letter, digit or underscore		Bob.*\b\кошка\bBob ate the кошка
\BNot a word boundaryc.*\Bcat\B.*copycats

POSIX Classes

CharacterLegendExampleSample Match
[:alpha:]PCRE (C, PHP, R…): ASCII letters A-Z and a-z[8[:alpha:]]+WellDone88
[:alpha:]Ruby 2: Unicode letter or ideogram[[:alpha:]\d]+кошка99
[:alnum:]PCRE (C, PHP, R…): ASCII digits and letters A-Z and a-z[[:alnum:]]{10}ABCDE12345
[:alnum:]Ruby 2: Unicode digit, letter or ideogram[[:alnum:]]{10}кошка90210
[:punct:]PCRE (C, PHP, R…): ASCII punctuation mark[[:punct:]]+?!.,:;
[:punct:]Ruby: Unicode punctuation mark[[:punct:]]+‽,:〽⁆

Inline Modifiers

None of these are supported in JavaScript. In Ruby, beware of (?s) and (?m).

ModifierLegendExampleSample Match
(?i)Case-insensitive mode
(except JavaScript)		(?i)MondaymonDAY
(?s)DOTALL mode (except JS and Ruby). The dot (.) matches new line characters (\r\n). Also known as “single-line mode” because the dot treats the entire input as a single line(?s)From A.*to ZFrom A
to Z
(?m)Multiline mode
(except Ruby and JS) ^ and $ match at the beginning and end of every line		(?m)1\r\n^2$\r\n^3$1
2
3
(?m)In Ruby: the same as (?s) in other engines, i.e. DOTALL mode, i.e. dot matches line breaks(?m)From A.*to ZFrom A
to Z
(?x)Free-Spacing Mode mode
(except JavaScript). Also known as comment mode or whitespace mode		(?x) # this is a
# comment
abc # write on multiple
# lines
[ ]d # spaces must be
# in brackets		abc d
(?n).NET, PCRE 10.30+: named capture onlyTurns all (parentheses) into non-capture groups. To capture, use named groups.
(?d)Java: Unix linebreaks onlyThe dot and the ^ and $ anchors are only affected by \n
(?^)PCRE 10.32+: unset modifiersUnsets ismnx modifiers

Lookarounds

LookaroundLegendExampleSample Match
(?=…)Positive lookahead(?=\d{10})\d{5}01234 in 0123456789
(?<=…)Positive lookbehind(?<=\d)catcat in 1cat
(?!…)Negative lookahead(?!theatre)the\w+theme
(?<!…)Negative lookbehind\w{3}(?<!mon)sterMunster

Character Class Operations

Class OperationLegendExampleSample Match
[…-[…]].NET: character class subtraction. One character that is in those on the left, but not in the subtracted class.[a-z-[aeiou]]Any lowercase consonant
[…-[…]].NET: character class subtraction.[\p{IsArabic}-[\D]]An Arabic character that is not a non-digit, i.e., an Arabic digit
[…&&[…]]Java, Ruby 2+: character class intersection. One character that is both in those on the left and in the && class.[\S&&[\D]]An non-whitespace character that is a non-digit.
[…&&[…]]Java, Ruby 2+: character class intersection.[\S&&[\D]&&[^a-zA-Z]]An non-whitespace character that a non-digit and not a letter.
[…&&[^…]]Java, Ruby 2+: character class subtraction is obtained by intersecting a class with a negated class[a-z&&[^aeiou]]An English lowercase letter that is not a vowel.
[…&&[^…]]Java, Ruby 2+: character class subtraction[\p{InArabic}&&[^\p{L}\p{N}]]An Arabic character that is not a letter or a number

Other Syntax

SyntaxLegendExampleSample Match
\KKeep Out
Perl, PCRE (C, PHP, R…), Python’s alternate regex engine, Ruby 2+: drop everything that was matched so far from the overall match to be returned		prefix\K\d+12
\Q…\EPerl, PCRE (C, PHP, R…), Java: treat anything between the delimiters as a literal string. Useful to escape metacharacters.\Q(C++ ?)\E(C++ ?)