UltimateRegexResource

📝 A compilation of Regex syntax and resources for the Google DSC Regex Event

Watch a recording of the Regex presentation here!

Regex, or regular expressions, are patterns used to match strings. Regex is commonly used for searching/filtering strings for information, input validation, and web scraping. "Real-world" examples include everything from validating email addresses to formatting class names in a grades app.

Regex is incredibly powerful, but due to its seemingly unintelligible nature, it's also often intimidating to learn and difficult to remember.

For that reason, I've compiled a selection of the most helpful and commonly used regex syntax and some regex resources for your use below!

📄 Table of Contents

• "Balderdash" Basics (of Regex)
• "Flapdoodle" Flags
• "Gibberish" Characters
• "Bafflegab" Special Characters
• "Rigmarole" Ranges
• "Jargon" Quantifiers
• "Gobbledygook" Groups
• "Malarkey" Anchors
• "Mumbo Jumbo" Regex Resources
• Contributing

This repo contains a powerpoint presentation that can be viewed online here.

The lab file in this repo contains real-world practice problems and links to gamified resources for the DSC event. You are welcome to try your hand at some of them.

The Redoku folder of this repo contains the app "Redoku," a simple React Native application created for this event that allows you to hone your Regex skills through sudoku-like puzzles. This was heavily based on redoku, an awesome website with the same name. Thank you to @padolsey for granting permission to use the name "Redoku!" Download it below!

iOS	Android

Notes

• Regex has different flavors depending on the language you are using. Different engines support different features and some patterns have different meanings. While this resource attempts to cover as much as possible, there may be slight differences.
• UltimateRegexResource uses Javascript as the default regex engine. If there are differences between languages I attempt to note them. For a full review of what regex patterns are legal in each language, check out this awesome gist.
• Anywhere used below, character represents either a letter, digit, or symbol.

✒️ "Balderdash" Basics (of Regex)

• Regular expressions start and end with "delimiters." For example, Javascript regex literals generally have "slash" characters /, and Python regex usually begins with r" and ends with ". (While Python doesn't necessarily have Regex literals perse, Regex is written more easily using raw strings to avoid worrying about string escapes).
• Patterns return the first case-sensitive match they find by default.

Therefore: given the sample string I scream, you scream, we all SCREAM for ice cream, /scream/ matches the first instance of "scream."

This behavior can be modified with flags.

🚩 "Flapdoodle" Flags

Syntax	Flag	Behavior	Example
g	global	Returns additional matches	/foo/g
i	insensitive	Allows case-insensitive matches	/foo/i
x	verbose	Ignore whitespace & allow comments	/foo/x
u	unicode	Expressions are treated as Unicode (UTF-16)	/foo/u
s	singleline	Treats entire string as one line (allows . to match newline)	/foo/s
m	multiline	Start & end anchors now trigger on each line	/foo/m
n	nth match	Matches text returned by nth group	/foo/n

Regex includes several flags that are appended to the end of the expression to change behavior. Using the string I scream, you scream, we all SCREAM for ice cream, the updated regex /scream/gi will now return scream scream SCREAM.

✏️ "Gibberish" Characters

Syntax	Character	Matches	Example String	Example Expression	Example Match
.	any	Literally any character (except line break)	a-c1-3	a.c	a-c
\w	word	ASCII character (Or Unicode character in Python & C#)	a-c1-3	\w-\w	a-c
\d	digit	Digit 0-9 (Or Unicode digit in Python & C#)	a-c1-3	\d-\d	1-3
\s	whitespace	Space, tab, vertical tab, newline, carriage return (Or Unicode seperator in Python, C#, & JS)	a b	a\sb	a b
\W	NOT word	Anything \w does not match	a-c1-3	\W-\W	1-3
\D	NOT digit	Anything \d does not match	a-c1-3	\D-\D	a-c
\S	NOT whitespace	Anything \s does not match	a-c1-3	\S-\S	a-c

🖋️ "Bafflegab" Special Characters

Syntax	Special Character	Matches	Example String	Example Expression	Example Match
\	escape	The following when preceding them: [{()}].*+?$^/\	)$[]*{	\[\]	[]

Syntax	Substitute	Behavior
\n	newline	Insert a newline character
\t	tab	Insert a tab character
\r	carriage return	Insert a carriage return character
\f	form-feed	Insert a form feed character

🖌️ "Rigmarole" Ranges

Syntax	Range	Matches	Example String	Example Expression	Example Match
[pog]	word list	Either p, o, or g	awesomePOSSUM123	[awesum]+	awes
[^pog]	NOT word list	Any character except p, o, or g	awesomePOSSUM123	[^awesum]+	o
[a-z]	word range	Any character between a and z, inclusive	awesomePOSSUM123	[a-z]+	awesome
[^a-z]	NOT word range	Any character not between a and z, inclusive	awesomePOSSUM123	[^a-z]+	123
[0-9]	digit range	Any character between 0 and 9, inclusive	awesomePOSSUM123	[0-9]+	123
[^0-9]	NOT digit range	Any character not between 0 and 9, inclusive	awesomePOSSUM123	[^0-9]+	awesomePOSSUM
[a-zA-Z]	word range	Any character not between a and z, inclusive	awesomePOSSUM123	[a-zA-Z]+	awesomePOSSUM
[a-zA-Z]	word range	Any character not between a and z, inclusive	awesomePOSSUM123	[a-zA-Z]+	awesomePOSSUM

There are also a few (mostly) semantically identical patterns in Golang and PHP. These do not appear to be supported in JS or Python:

Syntax	Range	Matches	Example String	Example Expression	Example Match
[[:alpha:]]	alpha class	Any character between a and z, inclusive, not case sensitive	Woodchuck could chuck 33 wood logs.	[[:alpha:]]+	Woodchuck
[[:digit:]]	digit class	Any digit 0-9	Woodchuck could chuck 33 wood logs.	[[:digit:]]+	33
[[:alnum:]]	alphanumeric class	Any character between a and z, inclusive, not case sensitive, and any digit 0-9	Woodchuck could chuck 33 wood logs.	[[:alnum:]]+	Woodchuck
[[:punct:]]	punctuation class	Any of ?!.,:;	Woodchuck could chuck 33 wood logs.	[[:punct:]]+	.

In some flavors of regex, the above are also called "Character Classes."

🖊️ "Jargon" Quantifiers

Syntax	Quantifier	Matches	Example String	Example Expression	Example Match
?	optional	0 or 1 of the preceding expression	ccc	c?	c
{X}	X	X of the preceding expression	ccc	c{2}	cc
{X,}	X+	X or more of the preceding expression	ccc	c{2,}	ccc
{X,Y}	range	Between X and Y of the preceding expression	ccc	c{1,3}	ccc

Beyond standard quantifiers, there are a few additional modifiers: greedy, lazy, and possessive.

Syntax	Quantifier	Matches	Example String	Example Expression	Example Match
*	0+ greedy	0 or more of the preceding expression, using as many chars as possible	abccc	c*	ccc
+	1+ greedy	1 or more of the preceding expression, using as many chars as possible	abccc	c+	ccc
*?	0+ lazy	0 or more of the preceding expression, using as few chars as possible	abccc	c*?	c
+?	1+ lazy	1 or more of the preceding expression, using as few chars as possible	abccc	c+?	c
*+	0+ possessive	0 or more of the preceding expression, using as many chars as possible, without backtracking (Not supported in JS or PY)	abccc	c*+	ccc
++	1+ possessive	1 or more of the preceding expression, using as many chars as possible, without backtracking (Not supported in JS or PY)	abccc	c++	ccc

Put simply, greedy quantifiers match as much as possible, lazy as little as possible and possessive as much as possible without backtracking.

What this means in practice is that possessive quantifiers will always return either the same match as greedy quantifiers or if backtracking is required they will return no match. Therefore, posessive quantifiers should be used when you know backtracking is not necessary, allowing increased performance.

🖍️ "Gobbledygook" Groups

Groups allow you to pull out specific parts of a match. For example, given the string Peter Piper picked a peck of pickled peppers and the regex literal [peck]+ of (\w+) , an additional "capturing group" group 1 is returned.

By default, the whole match begins at group 0, and then every group after is n where n is 1 + the previous capturing group.

Syntax	Group	Matches	Example String	Example Expression	Example Match
|	alternate	Either the preceding or following expression	truly rural	truly|rural	truly
(...)	isolate	Everything enclosed; treats as separate capture group	truly rural	truly (rural)	truly, rural
(?:...)	include	Everything enclosed; enables using quantifiers on part of regex	truly ruralrural	truly (?:rural)+	truly ruralrural
(?|...)	combine	Everything enclosed; treats all matches as same group	truly rural	(?|(rural)|(truly))	truly
(?>...)	atomic	Longest possible string without backtracking	truly rural	(?>rur)	rur
(?#...)	comment	Everything enclosed; treats as comment and ignores	truly #rural	truly (?#rural)	truly

⚓ "Malarkey" Anchors

Syntax	Anchor	Matches	Example String	Example Expression	Example Match
^	start	Start of string	she sells seashells	^\w+	she
$	end	End of string	she sells seashells	\w+$	seashells
\b	word boundary	Between a character matched and not matched by \w	she sells seashells	s\b	s
\B	NOT word boundary	Between two characters matched by \w	she sells seashells	\w+$	seashells

There are additional anchors available that are unaffected by multiline mode m.

Syntax	Anchor	Matches	Example String	Example Expression	Example Match
\A	multi-start	Start of string	she sees cheese	\A\w+	she
\Z	multi-end	End of string	she sees cheese	\w+\Z	cheese
\Z	absolute end	Absolute end of string, ignoring trailing newlines	she sees cheese	\w+\Z	cheese

📌 "Mumbo Jumbo" Regex Resources

• Regex101, an incredible testing utility for all flavors of Regex
• RegexOne, a great way to learn Regex through brief lessons
• Regexr, another way to test your expressions
• Rubular, a Ruby-based regex tester w/ quick reference
• Regex.Info, a plain but detailed guide to regex
• RexEgg, the self-proclaimed "world's most tyrannisaurical regex tutorial"
• CodeAcademy Regex Tutorial, a 1-hour course w/ certification
• SitePoint Learn Regex, a great tutorial of the fundamental concepts
• Regex Basics, as stated, with no deep knowledge required
• @ziishaned's Learn Regex, a repo that contains more info on Regex to learn it "the easy way"
• RegexHub, a collection of commonly used Regex
• "Greedy" vs "Lazy", a SO post that acts as a deep dive into their differences
• Difference between [] and () in Regex, a SO post that hopefully helps
• MIT Regex Printable PDF Cheatsheet, for those who need the physical copy
• Stanford Regex Printable PDF Cheatsheet, for those who prefer a pinker physical copy
• When/How Not To Use Regex, an article by the founder of SO that discusses what you'd expect from the title
• Awesome Regex Resources, a comprehensive list of Regex books, articles, and utilities far larger than this
• Regex Cheat Sheet, a visual Regex cheat sheet with samples and examples

👥 "Codswallop" Contributing

1. Fork UltimateRegexResource here
2. Create a branch with your improvements (git checkout -b improvement/fooBar)
3. Commit your changes (git commit -am 'Add some fooBar')
4. Push to the branch (git push origin improvement/fooBar)
5. Create a new Pull Request

Meta

Created by @GoldinGuy for the FAU Google DSC Regex Event.