Everything you need to know about /REGEX/g

And about 20% more that you don't

RegEx

That weird thing that you have to look up on stackoverflow once a year, but it's really useful when you do

This talk will:

• Start very theoretical

• End very practical

What is "language"?

Grammar

Syntax

When you come to donuts, have a donut.

Some rules that apply to this sentence:

• In a complex sentence, a dependent clause must follow or precede an independent clause

• (you) have a donut: an independent clause must have a subject, verb, and an optional object

Jan walks.

Jan walks the dog.

A combination of symbols that represents a grammar rule is an

 expression

IC = SV*O

Note: a regular language is a language that has a well defined and strict grammar

Many spoken languages are not regular, because humans.

So an expression that defines the grammar of a regular language is a 

Regular Expression

Pivot

What is a grammar, in the context of a programming language?

programming languages as​ Languages

Sets of rules (grammars) which defined what keywords, expressions, characters can appear in what order, and what the meaning is of those things.

For example:

An expression is anything that can be on the left of an equals sign.

const a = b + 3

State Machines

Any computer program can be modeled as a "state machine", shown as a graph with edges and nodes

ab

abc+

an 'a', followed by a 'b', followed by one or more 'c's

Pivot

Epiphany: Hey, we can use this for pattern matching!

When you define a regular expression for a string, you are (behind the scenes) defining a grammar for a new language that the string is a part of 

/Hello World/

is a regular expression for the grammar of a language in which "Hello World" is the only valid entry

So, when

"Hello World"

matches the regex

/Hello World/

what that really means is that it is a valid part of the new Hello World Language

Parts of a Regular Expression

/expression pattern/flags

/Hello World/i

match exactly "Hello World" case insensitive

/Hello/m

Match exactly "Hello" over multiple lines

Some useful regex rules

ab       an 'a' followed by a 'b'

a|b      an 'a' or a 'b'

a*       any number of 'a's (including 0)

a+       at least one 'a'

a?                 an optional single 'a'

\?                  an escaped '?'

Some useful regex rules

.        any character

[a-z]    any character in the range of a->z

[A-Z]    "   "                 capital A->Z

[0-9]    "   "                         0->9

\d                 any single-digit

\w                any single "word" character (alpha numeric)

Some useful regex rules

^        Beginning of a string or line

\b       Word boundary

()       Group, Capture Group

$        End of a string or line

Let's build one

Match some dates in this format

12/25/2017

http://regexr.com/

Let's build one

Match emails of a certain domain

mdjasper@ldschurch.org

http://regexr.com/

Note: The actual rule to validate spec email addresses is ... longer

Let's build one

Find text between "${" and "}"

Hello, ${name}

http://regexr.com/

Using

Regular Expressions

In JavaScript

Primitive Data Type

Regular expressions in javascript are primitive data types.

You can save them to variables or call their methods directly

Methods that use RegEx

Regex Methods

RegEx.test(string) //boolean

RegEx.exec(string) //data

const isFound = /Hello World/.test("Hello World"); 
//true

const foundData = /Hello/.exec("Hello World");

//Array
//  "Hello"
//  index: 0
//  input: "Hello World"

Methods that use RegEx

String Methods

String.match(RegEx) //array

String.search(RegEx) //index

String.replace(RegEx) //string

String.split(RegEx) //array

"Hello World".match(/World/); //Data

"Hello World".search(/World/); //6

"Hello World".replace(/World/, "Everybody!"); //Hello Everybody!

"Hello World".split(/\s/); //["Hello", "World"]

Live code a thing