RegEx

Introduction

This is a quick cheat sheet to getting started with regular expressions.

• Regex in Python (cheatsheets.zip)

• Regex in JavaScript (cheatsheets.zip)

• Regex in PHP (cheatsheets.zip)

• Regex in Java (cheatsheets.zip)

• Regex in MySQL (cheatsheets.zip)

• Online regex tester (regex101.com)

Character Classes

Pattern	Description
[abc]	A single character of: a, b or c
[^abc]	A character except: a, b or c
[a-z]	A character in the range: a-z
[^a-z]	A character not in the range: a-z
[0-9]	A digit in the range: 0-9
[a-zA-Z]	A character in the range: a-z or A-Z
[a-zA-Z0-9]	A character in the range: a-z, A-Z or 0-9

Quantifiers

Pattern	Description
a?	Zero or one of a
a*	Zero or more of a
a+	One or more of a
[0-9]+	One or more of 0-9
a{3}	Exactly 3 of a
a{3,}	3 or more of a
a{3,6}	Between 3 and 6 of a
a*	Greedy quantifier
a*?	Lazy quantifier
a*+	Possessive quantifier

Common Metacharacters

| Pattern | Description | | ——- | :———————————————————– | ——————————————– | | ^ | Matches the start of a string. | | { | Starts a quantifier for the number of occurrences. | | + | Matches one or more of the preceding element. | | < | Not a standard regex meta character (commonly used in HTML). | | [ | Starts a character class. | | * | Matches zero or more of the preceding element. | | ) | Ends a capturing group. | | > | Not a standard regex meta character (commonly used in HTML). | | . | Matches any character except a newline. | | ( | Starts a capturing group. | |       | | Acts as a logical OR within a regex pattern. | | $ | Matches the end of a string. | | \ | Escapes a meta character, giving it literal meaning. | | ? | Matches zero or one of the preceding element. |

Escape these special characters with \

Meta Sequences

Pattern	Description
.	Any single character
\s	Any whitespace character
\S	Any non-whitespace character
\d	Any digit, Same as [0-9]
\D	Any non-digit, Same as [^0-9]
\w	Any word character
\W	Any non-word character
\X	Any Unicode sequences, linebreaks included
\C	Match one data unit
\R	Unicode newlines
\v	Vertical whitespace character
\V	Negation of \v - anything except newlines and vertical tabs
\h	Horizontal whitespace character
\H	Negation of \h
\K	Reset match
\n	Match nth subpattern
\pX	Unicode property X
\p{...}	Unicode property or script category
\PX	Negation of \pX
\P{...}	Negation of \p
\Q...\E	Quote; treat as literals
\k<name>	Match subpattern name
\k'name'	Match subpattern name
\k{name}	Match subpattern name
\gn	Match nth subpattern
\g{n}	Match nth subpattern
\g<n>	Recurse nth capture group
\g'n'	Recurses nth capture group.
\g{-n}	Match nth relative previous subpattern
\g<+n>	Recurse nth relative upcoming subpattern
\g'+n'	Match nth relative upcoming subpattern
\g'letter'	Recurse named capture group letter
\g{letter}	Match previously-named capture group letter
\g<letter>	Recurses named capture group letter
\xYY	Hex character YY
\x{YYYY}	Hex character YYYY
\ddd	Octal character ddd
\cY	Control character Y
[\b]	Backspace character
\	Makes any character literal

Anchors

Pattern	Description
\G	Start of match
^	Start of string
$	End of string
\A	Start of string
\Z	End of string
\z	Absolute end of string
\b	A word boundary
\B	Non-word boundary

Substitution

Pattern	Description
\0	Complete match contents
\1	Contents in capture group 1
$1	Contents in capture group 1
${foo}	Contents in capture group foo
\x20	Hexadecimal replacement values
\x{06fa}	Hexadecimal replacement values
\t	Tab
\r	Carriage return
\n	Newline
\f	Form-feed
\U	Uppercase Transformation
\L	Lowercase Transformation
\E	Terminate any Transformation

Group Constructs

Pattern	Description
(...)	Capture everything enclosed
(a|b)	Match either a or b
(?:...)	Match everything enclosed
(?>...)	Atomic group (non-capturing)
(?|…)	Duplicate subpattern group number
(?#...)	Comment
(?'name'...)	Named Capturing Group
(?<name>...)	Named Capturing Group
(?P<name>...)	Named Capturing Group
(?imsxXU)	Inline modifiers
(?(DEFINE)...)	Pre-define patterns before using them

Assertions

-	-
(?(1)yes|no)	Conditional statement
(?(R)yes|no)	Conditional statement
(?(R#)yes|no)	Recursive Conditional statement
(?(R&name\yes|no)	Conditional statement
(?(?=…)yes|no)	Lookahead conditional
(?(?<=…)yes|no)	Lookbehind conditional

Lookarounds

-	-
(?=...)	Positive Lookahead
(?!...)	Negative Lookahead
(?<=...)	Positive Lookbehind
(?<!...)	Negative Lookbehind

Lookaround lets you match a group before (lookbehind) or after (lookahead) your main pattern without including it in the result.

Flags/Modifiers

Pattern	Description
g	Global
m	Multiline
i	Case insensitive
x	Ignore whitespace
s	Single line
u	Unicode
X	eXtended
U	Ungreedy
A	Anchor
J	Duplicate group names

Recurse

-	-
(?R)	Recurse entire pattern
(?1)	Recurse first subpattern
(?+1)	Recurse first relative subpattern
(?&name)	Recurse subpattern name
(?P=name)	Match subpattern name
(?P>name)	Recurse subpattern name

POSIX Character Classes

Character Class	Same as	Meaning
[[:alnum:]]	[0-9A-Za-z]	Letters and digits
[[:alpha:]]	[A-Za-z]	Letters
[[:ascii:]]	[\x00-\x7F]	ASCII codes 0-127
[[:blank:]]	[\t ]	Space or tab only
[[:cntrl:]]	[\x00-\x1F\x7F]	Control characters
[[:digit:]]	[0-9]	Decimal digits
[[:graph:]]	[[:alnum:][:punct:]]	Visible characters (not space)
[[:lower:]]	[a-z]	Lowercase letters
[[:print:]]	[ -~] == [ [:graph:]]	Visible characters
[[:punct:]]	[!”#$%&’()*+,-./:;<=>?@[]^_`{|}~]	Visible punctuation characters
[[:space:]]	[\t\n\v\f\r ]	Whitespace
[[:upper:]]	[A-Z]	Uppercase letters
[[:word:]]	[0-9A-Za-z_]	Word characters
[[:xdigit:]]	[0-9A-Fa-f]	Hexadecimal digits
[[:<:]]	[\b(?=\w)]	Start of word
[[:>:]]	[\b(?<=\w)]	End of word

Control verb

-	-
(*ACCEPT)	Control verb
(*FAIL)	Control verb
(*MARK:NAME)	Control verb
(*COMMIT)	Control verb
(*PRUNE)	Control verb
(*SKIP)	Control verb
(*THEN)	Control verb
(*UTF)	Pattern modifier
(*UTF8)	Pattern modifier
(*UTF16)	Pattern modifier
(*UTF32)	Pattern modifier
(*UCP)	Pattern modifier
(*CR)	Line break modifier
(*LF)	Line break modifier
(*CRLF)	Line break modifier
(*ANYCRLF)	Line break modifier
(*ANY)	Line break modifier
\R	Line break modifier
(*BSR_ANYCRLF)	Line break modifier
(*BSR_UNICODE)	Line break modifier
(*LIMIT_MATCH=x)	Regex engine modifier
(*LIMIT_RECURSION=d)	Regex engine modifier
(*NO_AUTO_POSSESS)	Regex engine modifier
(*NO_START_OPT)	Regex engine modifier

Regex examples

Characters

Pattern	Matches
ring	Match ring springboard etc.
.	Match a, 9, + etc.
h.o	Match hoo, h2o, h/o etc.
ring\?	Match ring?
\(quiet\)	Match (quiet)
c:\\windows	Match c:\windows

Use \ to search for these special characters:
[ \ ^ $ . | ? * + ( ) { }

Alternatives

Pattern	Matches
cat|dog	Match cat or dog
id|identity	Match id or identity
identity|id	Match id or identity

Order longer to shorter when alternatives overlap

Character classes

Pattern	Matches
[aeiou]	Match any vowel
[^aeiou]	Match a NON vowel
r[iau]ng	Match ring, wrangle, sprung, etc.
gr[ae]y	Match gray or grey
[a-zA-Z0-9]	Match any letter or digit
[\u3a00-\ufa99]	Match any Unicode Hàn (中文)

In [ ] always escape . \ ] and sometimes ^ - .

Shorthand classes

Pattern	Meaning
\w	“Word” character (letter, digit, or underscore)
\d	Digit
\s	Whitespace (space, tab, vtab, newline)
\W, \D, or \S	Not word, digit, or whitespace
[\D\S]	Means not digit or whitespace, both match
[^\d\s]	Disallow digit and whitespace

Occurrences

Pattern	Matches
colou?r	Match color or colour
[BW]ill[ieamy's]*	Match Bill, Willy, William’s etc.
[a-zA-Z]+	Match 1 or more letters
\d{3}-\d{2}-\d{4}	Match a SSN
[a-z]\w{1,7}	Match a UW NetID

Greedy versus lazy

Pattern	Meaning
*  + {n,} greedy	Match as much as possible
<.+>	Finds 1 big match in <b>bold</b>
*?  +? {n,}? lazy	Match as little as possible
<.+?>	Finds 2 matches in <b>bold</b>

Scope

Pattern	Meaning
\b	“Word” edge (next to non “word” character)
\bring	Word starts with “ring”, ex ringtone
ring\b	Word ends with “ring”, ex spring
\b9\b	Match single digit 9, not 19, 91, 99, etc..
\b[a-zA-Z]{6}\b	Match 6-letter words
\B	Not word edge
\Bring\B	Match springs and wringer
^\d*$	Entire string must be digits
^[a-zA-Z]{4,20}$	String must have 4-20 letters
^[A-Z]	String must begin with capital letter
[\.!?"')]$	String must end with terminal puncutation

Modifiers

Pattern	Meaning
(?i)[a-z]*(?-i)	Ignore case ON / OFF
(?s).*(?-s)	Match multiple lines (causes . to match newline)
(?m)^.*;$(?-m)	^ & $ match lines not whole string
(?x)	#free-spacing mode, this EOL comment ignored
(?-x)	free-spacing mode OFF
/regex/ismx	Modify mode for entire string

Groups

Pattern	Meaning
(in|out)put	Match input or output
\d{5}(-\d{4})?	US zip code (”+ 4” optional)

Parser tries EACH alternative if match fails after group.
Can lead to catastrophic backtracking.

Back references

Pattern	Matches
(to) (be) or not \1 \2	Match to be or not to be
([^\s])\1{2}	Match non-space, then same twice more   aaa, …
\b(\w+)\s+\1\b	Match doubled words

Non-capturing group

Pattern	Meaning
on(?:click|load)	Faster than: on(click|load)

Use non-capturing or atomic groups when possible

Atomic groups

Pattern	Meaning
(?>red|green|blue)	Faster than non-capturing
(?>id|identity)\b	Match id, but not identity

“id” matches, but \b fails after atomic group, parser doesn’t backtrack into group to retry ‘identity’

If alternatives overlap, order longer to shorter.

Lookaround

Pattern	Meaning
(?= )	Lookahead, if you can find ahead
(?! )	Lookahead,if you can not find ahead
(?<= )	Lookbehind, if you can find behind
(?<! )	Lookbehind, if you can NOT find behind
\b\w+?(?=ing\b)	Match warbling, string, fishing, …
\b(?!\w+ing\b)\w+\b	Words NOT ending in ing
(?<=\bpre).*?\b	Match pretend, present, prefix, …
\b\w{3}(?<!pre)\w*?\b	Words NOT starting with pre
\b\w+(?<!ing)\b	Match words NOT ending in ing

If-then-else

Match “Mr.” or “Ms.” if word “her” is later in string

M(?(?=.*?\bher\b)s|r)\.

requires lookaround for IF condition

RegEx in Python

Getting started

Import the regular expressions module

import re

Examples

re.search()

>>> sentence = 'This is a sample string'
>>> bool(re.search(r'this', sentence, flags=re.I))
True
>>> bool(re.search(r'xyz', sentence))
False

re.findall()

>>> re.findall(r'\bs?pare?\b', 'par spar apparent spare part pare')
['par', 'spar', 'spare', 'pare']
>>> re.findall(r'\b0*[1-9]\d{2,}\b', '0501 035 154 12 26 98234')
['0501', '154', '98234']

re.finditer()

>>> m_iter = re.finditer(r'[0-9]+', '45 349 651 593 4 204')
>>> [m[0] for m in m_iter if int(m[0]) < 350]
['45', '349', '4', '204']

re.split()

>>> re.split(r'\d+', 'Sample123string42with777numbers')
['Sample', 'string', 'with', 'numbers']

re.sub()

>>> ip_lines = "catapults\nconcatenate\ncat"
>>> print(re.sub(r'^', r'* ', ip_lines, flags=re.M))
* catapults
* concatenate
* cat

re.compile()

>>> pet = re.compile(r'dog')
>>> type(pet)
<class '_sre.SRE_Pattern'>
>>> bool(pet.search('They bought a dog'))
True
>>> bool(pet.search('A cat crossed their path'))
False

Functions

Function	Description
re.findall	Returns a list containing all matches
re.finditer	Return an iterable of match objects (one for each match)
re.search	Returns a Match object if there is a match anywhere in the string
re.split	Returns a list where the string has been split at each match
re.sub	Replaces one or many matches with a string
re.compile	Compile a regular expression pattern for later use
re.escape	Return string with all non-alphanumerics backslashed

Flags

-	-	-
re.I	re.IGNORECASE	Ignore case
re.M	re.MULTILINE	Multiline
re.L	re.LOCALE	Make \w,\b,\s locale dependent
re.S	re.DOTALL	Dot matches all (including newline)
re.U	re.UNICODE	Make \w,\b,\d,\s unicode dependent
re.X	re.VERBOSE	Readable style

Regex in JavaScript

test()

let textA = 'I like APPles very much';
let textB = 'I like APPles';
let regex = /apples$/i;

// Output: false
console.log(regex.test(textA));

// Output: true
console.log(regex.test(textB));

search()

let text = 'I like APPles very much';
let regexA = /apples/;
let regexB = /apples/i;

// Output: -1
console.log(text.search(regexA));

// Output: 7
console.log(text.search(regexB));

exec()

let text = 'Do you like apples?';
let regex = /apples/;

// Output: apples
console.log(regex.exec(text)[0]);

// Output: Do you like apples?
console.log(regex.exec(text).input);

match()

let text = 'Here are apples and apPleS';
let regex = /apples/gi;

// Output: [ "apples", "apPleS" ]
console.log(text.match(regex));

split()

let text = 'This 593 string will be brok294en at places where d1gits are.';
let regex = /\d+/g;

// Output: [ "This ", " string will be brok", "en at places where d", "gits are." ]
console.log(text.split(regex));

matchAll()

let regex = /t(e)(st(\d?))/g;
let text = 'test1test2';
let array = [...text.matchAll(regex)];

// Output: ["test1", "e", "st1", "1"]
console.log(array[0]);

// Output: ["test2", "e", "st2", "2"]
console.log(array[1]);

replace()

let text = 'Do you like aPPles?';
let regex = /apples/i;

// Output: Do you like mangoes?
let result = text.replace(regex, 'mangoes');
console.log(result);

replaceAll()

let regex = /apples/gi;
let text = 'Here are apples and apPleS';

// Output: Here are mangoes and mangoes
let result = text.replaceAll(regex, 'mangoes');
console.log(result);

Regex in PHP

Functions

-	-
preg_match()	Performs a regex match
preg_match_all()	Perform a global regular expression match
preg_replace_callback()	Perform a regular expression search and replace using a callback
preg_replace()	Perform a regular expression search and replace
preg_split()	Splits a string by regex pattern
preg_grep()	Returns array entries that match a pattern

preg_replace

$str = "Visit Microsoft!";
$regex = "/microsoft/i";

// Output: Visit CheatSheets!
echo preg_replace($regex, "CheatSheets", $str);

preg_match

$str = "Visit CheatSheets";
$regex = "#cheatsheets#i";

// Output: 1
echo preg_match($regex, $str);

preg_matchall

$regex = "/[a-zA-Z]+ (\d+)/";
$input_str = "June 24, August 13, and December 30";
if (preg_match_all($regex, $input_str, $matches_out)) {

    // Output: 2
    echo count($matches_out);

    // Output: 3
    echo count($matches_out[0]);

    // Output: Array("June 24", "August 13", "December 30")
    print_r($matches_out[0]);

    // Output: Array("24", "13", "30")
    print_r($matches_out[1]);
}

preg_grep

$arr = ["Jane", "jane", "Joan", "JANE"];
$regex = "/Jane/";

// Output: Jane
echo preg_grep($regex, $arr);

preg_split

$str = "Jane\tKate\nLucy Marion";
$regex = "@\s@";

// Output: Array("Jane", "Kate", "Lucy", "Marion")
print_r(preg_split($regex, $str));

Regex in Java

Styles

First way

Pattern p = Pattern.compile(".s", Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher("aS");
boolean s1 = m.matches();
System.out.println(s1);   // Outputs: true

Second way

boolean s2 = Pattern.compile("[0-9]+").matcher("123").matches();
System.out.println(s2);   // Outputs: true

Third way

boolean s3 = Pattern.matches(".s", "XXXX");
System.out.println(s3);   // Outputs: false

Pattern Fields

-	-
CANON_EQ	Canonical equivalence
CASE_INSENSITIVE	Case-insensitive matching
COMMENTS	Permits whitespace and comments
DOTALL	Dotall mode
MULTILINE	Multiline mode
UNICODE_CASE	Unicode-aware case folding
UNIX_LINES	Unix lines mode

Methods

Pattern

• Pattern compile(String regex [, int flags])

• boolean matches([String regex, ] CharSequence input)

• String[] split(String regex [, int limit])

• String quote(String s)

Matcher

• int start([int group | String name])

• int end([int group | String name])

• boolean find([int start])

• String group([int group | String name])

• Matcher reset()

String

• boolean matches(String regex)

• String replaceAll(String regex, String replacement)

• String[] split(String regex[, int limit])

There are more methods …

Examples

Replace sentence:

String regex = "[A-Z\n]{5}$";
String str = "I like APP\nLE";

Pattern p = Pattern.compile(regex, Pattern.MULTILINE);
Matcher m = p.matcher(str);

// Outputs: I like Apple!
System.out.println(m.replaceAll("pple!"));

Array of all matches:

String str = "She sells seashells by the Seashore";
String regex = "\\w*se\\w*";

Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher(str);

List<String> matches = new ArrayList<>();
while (m.find()) {
    matches.add(m.group());
}

// Outputs: [sells, seashells, Seashore]
System.out.println(matches);

Regex in MySQL

Functions

Name	Description
REGEXP	Whether string matches regex
REGEXP_INSTR()	Starting index of substring matching regex (NOTE: Only MySQL 8.0+)
REGEXP_LIKE()	Whether string matches regex (NOTE: Only MySQL 8.0+)
REGEXP_REPLACE()	Replace substrings matching regex (NOTE: Only MySQL 8.0+)
REGEXP_SUBSTR()	Return substring matching regex (NOTE: Only MySQL 8.0+)

REGEXP

expr REGEXP pat

Examples

mysql> SELECT 'abc' REGEXP '^[a-d]';
1
mysql> SELECT name FROM cities WHERE name REGEXP '^A';
mysql> SELECT name FROM cities WHERE name NOT REGEXP '^A';
mysql> SELECT name FROM cities WHERE name REGEXP 'A|B|R';
mysql> SELECT 'a' REGEXP 'A', 'a' REGEXP BINARY 'A';
1   0

REGEXP_REPLACE

REGEXP_REPLACE(expr, pat, repl[, pos[, occurrence[, match_type]]])

Examples

mysql> SELECT REGEXP_REPLACE('a b c', 'b', 'X');
a X c
mysql> SELECT REGEXP_REPLACE('abc ghi', '[a-z]+', 'X', 1, 2);
abc X

REGEXP_SUBSTR

REGEXP_SUBSTR(expr, pat[, pos[, occurrence[, match_type]]])

Examples

mysql> SELECT REGEXP_SUBSTR('abc def ghi', '[a-z]+');
abc
mysql> SELECT REGEXP_SUBSTR('abc def ghi', '[a-z]+', 1, 3);
ghi

REGEXP_LIKE

REGEXP_LIKE(expr, pat[, match_type])

Examples

mysql> SELECT regexp_like('aba', 'b+')
1
mysql> SELECT regexp_like('aba', 'b{2}')
0
mysql> # i: case-insensitive
mysql> SELECT regexp_like('Abba', 'ABBA', 'i');
1
mysql> # m: multi-line
mysql> SELECT regexp_like('a\nb\nc', '^b$', 'm');
1

REGEXP_INSTR

REGEXP_INSTR(expr, pat[, pos[, occurrence[, return_option[, match_type]]]])

Examples

mysql> SELECT regexp_instr('aa aaa aaaa', 'a{3}');
2
mysql> SELECT regexp_instr('abba', 'b{2}', 2);
2
mysql> SELECT regexp_instr('abbabba', 'b{2}', 1, 2);
5
mysql> SELECT regexp_instr('abbabba', 'b{2}', 1, 3, 1);
7