TL;DR

Regular expressions are easier than you think. Let's focus on the bare necessities in a quick and dirty reference with examples in JavaScript. If you are new to regular expressions or need a quick review on the basics, this is a great place to start. We'll dive deeper into use cases in part 2.

What is Regex?

Regular expressions is a concise language for matching patterns of text. In JavaScript, a regular expression can be defined like this /expression/. In it's most basic form a regular expression can look like this:

/pizza/.test(entree);

Hint: If you would like to follow along and play with some of these expressions, I've found Regex Pal to be very helpful

In the above example, if the entree variable contained pizza anywhere within the string, it will return true. Some examples that will return true: pizza pizzas spizzas i like to eat pizza.

Don't Fear RegEx

Regular expressions can look intimidating if you haven't had some experience with them. If you can write code you can learn regular expressions. It's just syntax and almost every language supports them. I use regular expressions almost daily to search for files. You know, those files someone is looking for, on that server that so many people have touched. This can be maddening without the right tool.

There are however, some cases where you should just reach for a library. One notorious example is validating email. If a regular expression gets too complex, it can become difficult to debug and maintain. So let someone else do that work if you can.

Okay lets look at the operators first. Remember, this is not a comprehensive list. Just enough to get started. I'll list resources at the end.

Operators

There are a few must know operators. Master these and you will cover the majority of use cases. I'll start with an example, and then explain what's happening.

It's important to note that the following examples are case sensitive. The expression /B/ is different than the expression /b/. This can be changed with a modifier which we'll discuss later.

The Wild Card *:

/cat*/.test("category");
// returns true

The simplest operator, * matches any character or combination of characters for any length. Lets see some examples of matches and mismatches.

const matchRed = /red*/;
matchRed.test("reddit"); // returns true
matchRed.test("credit"); // returns true
matchRed.test("brew"); // returns true
matchRed.test("book"); // returns false

So what's happening here? The wildcard will match any combination of text after the combination re is found. But wait! We wanted to match red not re. How do we remedy this? The * makes the last character an optional one, so really it matches anything. We can add an extra character to the expression using the . operator like this:

/red.*/.test("brew"); //returns false

More on the . below.

The Dot .

const matchAt = /.at/;
matchAt.test("cat"); // returns true

The . operator matches any character one time. In the above example it matches cat. Lets take a look at a some other examples.

const matchAt = /.at/
matchAt.test("rat"); // returns true
matchAt.test("bat"; // returns true
matchAt.test("#at"); // returns true
matchAt.test("at"); // returns false

Okay. What's going on here. rat, bat and #at match because we specified any single character with the . and the letters at after. Just the letters at fail because there is no character in front of the letters.

Word Character \w

const matchAt = /\wat/;
matchAt.test("cat"); // returns true

Almost identical to the . operator, The \w operator matches any word character one time. This means it will match any letter or number (and also underscores _).

const matchAt = /\wat/
matchAt.test("rat"); // returns true
matchAt.test("bat"; // returns true
matchAt.test("#at"; // returns false
matchAt.test("at"); // returns false

Digits \d

const matchNumber = /number\d/;
matchNumber.test("number1"); // returns true

In the expression above, we will match anything that contains the letters number followed by any digit hence the \d operator. Here are a few more examples.

const matchNumber = /number\d/;
matchNumber.test("number9"); // returns true
matchNumber.test("somenumber8plus"); // returns true
matchNumber.test("4"); // returns false

So even if the sequence number8 is in the middle of the string, it will return true.

Word Boundary \b

const matchMoo = /\bmoo\b/;
matchMoo.test("the cow says moo"); // returns true

The \b operator matches boundaries of words. The cow can only say moo. That's it. If we were to test a few more cases:

const matchMoo = /\bmoo\b/;
matchMoo.test("I have a moo cow"); // returns true
matchMoo.test("the cow says moot"); // returns false
matchMoo.test("the cow says smoo"); // returns false

Simple enough. If the word moo and only that word is anywhere in the sentence, it will return true.

Character Classes []

const matchThese = /[1Aa]/;
matchThese.test("pineapple"); // returns true

Character classes are a powerful part of regex. In the above example, any word containing 1, A or a will return true. pretty straight forward right? How about these examples.

/[a-e]/.test("banana"); // returns true
/[A-E]/.test("banana"); // returns false
/[1-5]/.test("The number 3"); // returns true

Inside character classes we can define ranges of numbers, lowercase characters or uppercase characters. You can even combine them.

const matchThis = /[a-eA-E1-5]/;
matchThis.test("banana"); // returns true
matchThis.test("BANANA"); // returns true
matchThis.test("3"); // returns true

Now we have one regular expression to match all words containing letters from a to e and numbers from 1 to 5.

Start ^

/^a/.test("apple"); //returns true

The ^ represents the beginning of a string. Lets look at a couple of cases where this would return false.

/^a/.test("pineapple"); //returns false
/^a/.test("eat apples"); //returns false

These fail because the string does not start with a.

End $

/e$/.test("apple"); // returns true

The $ represents the end of a string, so because apple ends in e, it returns true. Simple enough, but here are some failing cases to be clear.

/e$/.test("eat"); // returns false
/e$/.test("street"); // returns false

Both cases fail simply because they do not end in e.

Modifiers

Modifiers are less about matching a pattern, but help describe how you would like to search. In JavaScript we have 6 total, but the 3 must know modifiers are i, m and g. These modifiers can be used individually, or in any combination and are placed just after the last / like this /expression/m where m is the modifier.

Ignore Case i

/c/i.test("Coffee"); // returns true

This one is pretty self explanatory. Placing the i modifier after the expression allows us to match anything containing a lower case c and an uppercase C.

const matchBat = /bat/i;
matchBat.test("WOMBAT"); // returns true
matchBat.test("Bat"); // returns true

Multiline m

const groceryList = `apples
oranges
peaches
coffee
chocolate`;

/coffee/m.test(groceryList); // returns true

The multiline modifier allows the regular expression to look beyond any new lines. Without it, the above example would return false.

Global g

"apple".replace(/p/g, "b"); // returns abble

A great way to use the JavaScript String.replace is with regular expressions. However, If you want to replace all occurrences of a matched pattern, you need to include to g modifier to search globally. Without it, the above expression will only replace the first p it finds.

"apple".replace(/p/); // returns abple

That's all for now. Stay tuned for Regex Necessities Part 2, where we dive deeper into use cases, putting it all together.

Resources

RegExp Object MDN

RegExp Reference w3schools

Regex Pal