A regex cheatsheet for all those regex haters (and lovers) π
Catherine π©π»βπ»
Jan 10
Updated on Jan 24, 2019
γ»4 min read
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 π
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 π
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 π
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 βΊοΈ
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.
Hey @catherinecodes , I think you've missed the greedy vs lazy matching:
Nice! I didn't learn those yet
You can achieve laziness with negated set too:
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
andit isΒ Β cold outside
match but notit 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 π
Good catch Ryan! Thank you! I've updated the cheatsheet βΊοΈ
I still need some more practice as well π lookaheads and lookbehinds were totally new to me! I didn't know about them during my CS lab π²
Very nice and brainstorming regex challenge is RegexGolf - alf.nu/RegexGolf :)
Another view on "regex games" is Regex Crosswords. Very nice idea. regexcrossword.com/
These look fun! Thanks for sharing :)
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!
So happy you find it useful!!! βΊοΈβΊοΈβΊοΈ
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...
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.
Hide it away!!! πππ
Another really great resource is regular-expressions.info/
Hi, useful post!
Another online regex tester: extendsclass.com/regex-tester.html
Thanks for sharing!
Thank u! It's a powerful tool!
My pleasure :)
Awesome post thank you!
Glad you like it!
Another recommendation (not free though): Regular Expressions Cookbook
Love this. I wish more people would take advantage of regex, it's so powerful! I love using regex101.com since it does a great job of explaining your regex as you write it out.
+1 to regexr.com
Great site... but now you have two problems.
Awesome! I also translated one Chinese copy: gist.github.com/cxa/901e1862cd9ddf...
Cool! Thank you for this!
Nice post! I recommend this site too: regex101.com/
Great post, Catherine. ππΌππΌ
Created a gist out of this post. π
Link to Gist
Thank you Sarthak! I'll link it in the post :)
I would add "how to reverse the sides of an equation" to this list. There are a few other handy tricks for coders, but this is the most common I can think of right now
Yes. If I understand you correctly, that would at the same time be a good example case for another detail I was about to suggest: Match-parenthesis references in the replacement.
Something like:
"a = b + c".replace(/^(.+?) *= *(.+)$/, "$2 = $1")
not free, but RegexBuddy is awesome
that alone helped me to love regexs and made it super easy to deal with complex expressions