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

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 🎉

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 👀

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

Thank you Sarthak for creating a GitHub gist of my cheatsheet and Xian-an for translating to Chinese 👏

Resources

• The "Regular Expressions" challenges that is part of the "Javascript Algorithms and Data Structures Certification" on FreeCodeCamp
• MDN Regular Expression Docs
• RegexOne
• Regex101 for testing (you can also use the Chrome Developer Console)
• HackerRank regex challenges for practice

That's it folks! Hope this was helpful ☺️

Comments

[Frank Sam Hu]

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.

[Dominik Gorczyca]

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

[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"

[catherine]

Nice! I didn't learn those yet

[simo]

You can achieve laziness with negated set too:

/"[^"]+"/.exec('a "witch" and her "broom" is one') // "witch"

[Ryan]

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  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 😅

[catherine]

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

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

[Guney Ozsan]

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.

[catherine]

Hide it away!!! 😂😂😂

[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!

[catherine]

So happy you find it useful!!! ☺️☺️☺️

[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...

[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/

[catherine]

These look fun! Thanks for sharing :)

[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
})

[gjorgivarelov]

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.

[Mark Roeling]

Another really great resource is regular-expressions.info/