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catherine
catherine

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Regex Cheatsheet A regex cheatsheet for all those regex haters (and lovers) 馃憖

My experience with regex

I have always stayed far away from regex. In one of my first year computer science labs, there was an exercise that involved regex. I think that was the very first time I was introduced to it. I thought it was cool at the time but it seemed too hard so I've been avoiding it or just googling how to solve a certain regex problem. But I finally took some time to learn it properly 馃帀

After reading some resources and dabbling around, it is safe to say I am not afraid of regex anymore! I found myself using it in many of the coding exercises I've been doing. All it takes is practice! Below is a cheatsheet (with examples) I've compiled of the regex I've learned and the resources I used 馃憖

The cheatsheet

I've included some of the regex I've learned that is not available in Javascript. For these, I commented them out. Remember the "g" modifier if you need it! For my examples, I left modifiers out.

let regex;

/* matching a specific string */
regex = /hello/; // looks for the string between the forward slashes (case-sensitive)... matches "hello", "hello123", "123hello123", "123hello"; doesn't match for "hell0", "Hello" 
regex = /hello/i; // looks for the string between the forward slashes (case-insensitive)... matches "hello", "HelLo", "123HelLO"
regex = /hello/g; // looks for multiple occurrences of string between the forward slashes...

/* wildcards */
regex = /h.llo/; // the "." matches any one character other than a new line character... matches "hello", "hallo" but not "h\nllo"
regex = /h.*llo/; // the "*" matches any character(s) zero or more times... matches "hello", "heeeeeello", "hllo", "hwarwareallo"

/* shorthand character classes */
regex = /\d/;  // matches any digit
regex = /\D/;  // matches any non-digit
regex = /\w/;  // matches any word character (a-z, A-Z, 0-9, _)
regex = /\W/;  // matches any non-word character
regex = /\s/;  // matches any white space character (\r (carriage return),\n (new line), \t (tab), \f (form feed))
regex = /\S/;  // matches any non-white space character

/* specific characters */
regex = /[aeiou]/; // matches any character in square brackets
regex = /[ck]atherine/; // matches catherine or katherine
regex = /[^aeiou]/; // matches anything except the characters in square brackets

/* character ranges */
regex = /[a-z]/; // matches all lowercase letters
regex = /[A-Z]/; // matches all uppercase letters
regex = /[e-l]/; // matches lowercase letters e to l (inclusive)
regex = /[F-P]/; // matches all uppercase letters F to P (inclusive)
regex = /[0-9]/; // matches all digits
regex = /[5-9]/; // matches any digit from 5 to 9 (inclusive)
regex = /[a-zA-Z]/; // matches all lowercase and uppercase letters
regex = /[^a-zA-Z]/; // matches non-letters

/* matching repetitions using quantifiers */
regex = /(hello){4}/; // matches "hellohellohellohello"
regex = /hello{3}/; // matches "hellooo" and "helloooo" but not "helloo"
regex = /\d{3}/; // matches 3 digits ("312", "122", "111", "12312321" but not "12")
regex = /\d{3,7}/; // matches digits that occur between 3 and 7 times (inclusive)
regex = /\d{3,}/; // matches digits that occur at least 3 times

/* matching repetitions using star and plus */
regex = /ab*c/; // matches zero or more repetitions of "b" (matches "abc", "abbbbc", "ac")
regex = /ab+c/; // matches one or more repetitions of "b" (matches "abc", "abbbbc", but not "ac")

/* matching beginning and end items */
regex = /^[A-Z]\w*/; // matches "H", "Hello", but not "hey"
regex = /\w*s$/; // matches "cats", "dogs", "avocados", but not "javascript"

/* matching word boundaries 

positions of word boundaries:
1. before the first character in string (if first character is a word character)
2. after the last character in the string, if the last character is a word character
3. between two characters in string, where one is a word character and the other isn't */
regex = /\bmeow\b/; // matches "hey meow lol", "hey:meow:lol", but not "heymeow lol"

/* groups */
regex = /it is (ice )?cold outside/; // matches "it is ice cold outside" and "it is cold outside"
regex = /it is (?:ice )?cold outside/; // same as above except it is a non-capturing group
regex = /do (cats) like taco \1/; // matches "do cats like taco cats"
regex = /do (cats) like (taco)\? do \2 \1 like you\?/; // matches "do cats like taco? do taco cats like you?"

//branch reset group (available in Perl, PHP, R, Delphi... commented out because this is a js file)
// regex = /(?|(cat)|(dog))\1/; // matches "catcat" and "dogdog"

/* alternative matching */
regex = /i like (tacos|boba|guacamole)\./; // matches "i like tacos.", "i like boba.", and "i like guacamole."

/* forward reference (available in Perl, PHP, Java, Ruby, etc... commented out because this is a js file) */
// regex = /(\2train|(choo))+/; // matches "choo", "choochoo", "chootrain", choochootrain", but not "train"

/* lookaheads */
regex = /z(?=a)/; // positive lookahead... matches the "z" before the "a" in pizza" but not the first "z"
regex = /z(?!a)/; // negative lookahead... matches the first "z" but not the "z" before the "a"

/* lookbehinds */
regex = /(?<=[aeiou])\w/; // positive lookbehind... matches any word character that is preceded by a vowel
regex = /(?<![aeiou])\w/; // negative lookbehind... matches any word character that is not preceded by a vowel

/* functions I find useful */
regex.test("hello"); // returns true if found a match, false otherwise
regex.exec("hello"); // returns result array, null otherwise
"football".replace(/foot/,"basket"); // replaces matches with second argument
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Thank you Sarthak for creating a GitHub gist of my cheatsheet and Xian-an for translating to Chinese 馃憦

Resources

That's it folks! Hope this was helpful 鈽猴笍

Top comments (43)

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frankusky profile image
Frank Sam Hu • Edited

I would like to share my two favorites tools to create, edit, visualize and debug regex:

  • Debuggex.com
    my favorite tool, it can make a diagram of how your regex will work, you can add multiple lines to test if the regex match the strings that you expect and also has a cheatsheet

  • RegExr.com
    Similar to debuggex except it doesnt generate the diagram, but in my opinion their cheatsheet is cleaner and easier to find what you need.

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dgx32123 profile image
Dominik Gorczyca

Regex101 also seems like a great and quick site to know if your regex works!

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simov profile image
simo

Hey @catherinecodes , I think you've missed the greedy vs lazy matching:

// greedy
/".+"/.exec('a "witch" and her "broom" is one') // "witch" and her "broom"
// lazy
/".+?"/.exec('a "witch" and her "broom" is one') // "witch"
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catherinecodes profile image
catherine

Nice! I didn't learn those yet

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simov profile image
simo

You can achieve laziness with negated set too:

/"[^"]+"/.exec('a "witch" and her "broom" is one') // "witch"
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ryan profile image
Ryan • Edited

Just to nitpick, the second wildcard example should be .*, and your group examples will fail because you didn't capture a space (it is ice cold outside and it is聽聽cold outside match but not it is cold outside)

Regex is great! And not at all as hard as it looks. Although I still don't have the hang of lookahead/lookbehinds yet 馃槄

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catherinecodes profile image
catherine

Good catch Ryan! Thank you! I've updated the cheatsheet 鈽猴笍

I still need some more practice as well 馃槄lookaheads and lookbehinds were totally new to me! I didn't know about them during my CS lab 馃槻

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joemaffei profile image
Joe Maffei

You should add a note about lookbehind compatibility. It's not available in all browsers quite yet.

github.com/tc39/proposal-regexp-lo...
tc39.github.io/proposal-regexp-loo...

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mdebeus profile image
mdebeus

I am going to bookmark the Gist version of your very wonderful cheat sheet. I look forward to using it every time I work with regex. The samples appear in a very logical progression that makes it very easy to understand and use. Thank you!

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catherinecodes profile image
catherine

So happy you find it useful!!! 鈽猴笍鈽猴笍鈽猴笍

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madray profile image
Vic Danilov

Very nice and brainstorming regex challenge is RegexGolf - alf.nu/RegexGolf :)

Another view on "regex games" is Regex Crosswords. Very nice idea. regexcrossword.com/

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catherinecodes profile image
catherine

These look fun! Thanks for sharing :)

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guneyozsan profile image
Guney Ozsan • Edited

Regex = Black Magic
Convince me if not! Never sure why something works. I just don't touch it and put the script inside as many as folders possible in case some evil spirit leaks out to our world.

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catherinecodes profile image
catherine

Hide it away!!! 馃槀馃槀馃槀

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gjorgivarelov profile image
gjorgivarelov • Edited

Lookaheads and lookbehinds would be the features closest to "if-then" logic one uses while coding.
Just finished Bonnie Schulkin's course on regular expressions on Udemy. Great course, learned a lot about regex.

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mroeling profile image
Mark Roeling

Another really great resource is regular-expressions.info/

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nikoskip profile image
Nikolas

Nice post! I recommend this site too: regex101.com/

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sarthology profile image
Sarthak Sharma • Edited

Great post, Catherine. 馃憦馃徏馃憦馃徏

Created a gist out of this post. 馃槉

Link to Gist

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catherinecodes profile image
catherine

Thank you Sarthak! I'll link it in the post :)

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smh30 profile image
Stephanie Hope

Totally saving this, as I'm supposed to be learning regex next week in class!

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catherinecodes profile image
catherine

Yay, thank you! Have fun! :)

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technovice6 profile image
technovice6

In the 鈥榳ildcards鈥 example, you鈥檝e written

regex = /h.llo/; // the "" matches any character(s) zero or more times... matches
"hello", "heeeeeello", "hllo", "hwarwareallo"

How does it match "hwarwareallo" ? Does not the 鈥.鈥 indicate only one character?

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konkretnekosteczki profile image
KonkretneKosteczki

If you are adding usefull regex functions at the end of the code, you could also add information on replace function taking a callback as an argument.

const p = 'The quick brown fox jumps over the lazy dog. If the dog reacted, was it really lazy?';
p.replace('dog', (match, matchIndex, originalString) =>{
return match+"test"+matchIndex // match based replacer
})

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sesc profile image
Sebastian Schleussner • Edited

Another recommendation (not free though): Regular Expressions Cookbook

Need a better mental model for async/await?

Check out this classic DEV post on the subject.

猸愶笍馃巰 JavaScript Visualized: Promises & Async/Await

async await