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

catherine — Thu, 10 Jan 2019 02:48:38 +0000

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 ☺️

DEV Community: catherine

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

My experience with regex

The cheatsheet

Resources