Cheatsheet for the Regex Cheatsheet, Part IV: Assertions

Intro

I was recently doing a code challenge for a job interview that required me to strip out all nonalphabetic characters. "Ah! I should use Regular Expressions for this!" I thought in triumph, impressed that I even knew what regular expressions were. That fleeting moment of glory faded once I decided to brush up on regular expressions and landed on the encouragingly-named Regular Expressions Cheatsheet. I had no idea how to use it!

So, for people like me, here is a Cheatsheet for the Regular Expressions Cheatsheet, Part IV: Assertions

What's an Assertion?

We use assertions when we want to assert a specific sequence and set of conditions for a match. It's really better explained with examples, so read on!

Anatomy of a regular expression

1. Forward slashes go on either end like so: /something/
2. Add g for "global" at the end to find every instance, like so: /something/g
3. Add m to "multi line" to the beginning/end of each line, not just the beginning/end of each string, like /something/g or /something/gm

Assertions

?= Lookahead assertion

• ?= is used in /lion (?=roared)/ to find the following: The lion roared
• Example on regex101.com
• Example in Javascript:

let sentence = "The lion roared";
let regex = /lion (?=roared)/;
let found = sentence.match(regex);
console.log(found); // [ 'lion ', index: 4, input: 'The lion roared', groups: undefined ]

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?! Negative lookahead

• ?! is used in /lion (?!yawned)/ to find the following: The lion roared
• Example on regex101.com
• Example in Javascript:

let sentence = "The lion roared";
let regex = /lion (?!yawned)/;
let found = sentence.match(regex);
console.log(found); // [ 'lion ', index: 4, input: 'The lion roared', groups: undefined ]

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?<= Lookbehind assertion

• ?<= is used in /(?<=lion) roared/ to find the following: The lion roared
• Example on regex101.com
• Example in Javascript:

let sentence = "The lion roared";
let regex = /(?<=lion) roared/;
let found = sentence.match(regex);
console.log(found); // [ ' roared', index: 8, input: 'The lion roared', groups: undefined ]

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?<! Negative lookbehind

• ?<! is used in /(?<!asparagus) roared/ to find the following: The lion roared
• Example on regex101.com
• Example in Javascript:

let sentence = "The lion roared";
let regex = /(?<!asparagus) roared/;
let found = sentence.match(regex);
console.log(found); // [ ' roared', index: 8, input: 'The lion roared', groups: undefined ]

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Regex Assertions that are not supported in Javascript

?> Once-only subexpression

?() Condition [if then]

?()| Condition [if then else]

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