Lesson 6

Sets, Ranges and Alternation

Introduction

Everything we've learnt so far requires you to know the exact characters that you want to match (or at least the type of character).

Regular expressions are more powerful than that, though.

Sets ranges, and alternation allow you to perform more complex logic in your expression.

Let's take a look...

Sets

Enclosing one-or-more characters inside square brackets means 'match any of these characters'. This is called a 'set'.

For example, the set [abc] will matches either, a, b, or c.

This expression selects either f or d followed by ish:

Notice how wish is not selected, because it does not begin with f or d.

Excluding characters from sets

You can also exclude characters by prefixing the set with a ^.

The expression [^abc] means everything except for the characters a, b, or c.

Here's the opposite of the previous expression. It matches any character except for f or d, followed by the text ish.

Notice how this only matches wish now, and no longer matches fish or dish:

Ranges

Specifying individual characters is fine when you only need to match a couple of them, but it's not so good when you have something more complex.

For example, to match any number from 0 to 9 using that method, you'd have to write [0123456789].. that's not ideal.

That's where 'ranges' come in, letting you specify a range of characters to match.

Using ranges

Ranges are also enclosed in square brackets.

Instead of individual characters though, you specify the start and end of the range:

• [0-3] matches the numbers 0, 1, 2, or 3
• [e-h] matches the characters e, f, g, or h
• [A-Z] matches the uppercase characters A to Z
• etc.

This expression matches any (lowercase) character from a-z, followed by the literal text ish:

Excluding ranges

The ^ character can also be used to 'negate' a range, the same way as we did with individual characters above.

This expression selects any character except for the ones a-f, followed by ish. See how it no longer selects 'dish'. because it starts with a character in this range:

Combining ranges in sets

Just as you can combine multiple characters in a set ([abc] matches the individual characters a, b or c), you can also specify multiple ranges in a set.

The expression below matches either a character from a-d or a character from p-z.

You'll commonly see this used in the set [A-Za-z], which captures any upper or lowercase letters (when the /i modifier isn't being used to specify case-insensitivity, of course).

Using Quantifiers

You can also use quantifiers with sets and ranges. They are added after the square brackets.

Remember the + quantifier from Lesson 5? It means 'one-or-more times'.

Here we use it to match any number (0-9) one or more times (+), followed by a % symbol:

Alternation

The | symbol in a regular expression acts like an 'or'.

This expression finds the word 'creativity' or the word 'intelligence':

This is called 'alternation'.

You can also use alternation as part of a bigger expression, by enclosing the options in brackets. You can have as many options as you like, as long as you separate them with the | symbol.

This expression finds either the word Tell, Teach, or Involve, followed by the word me:

These brackets are called a 'capturing group', and they're really useful for other reasons too.

We'll look at capturing groups in more detail in the next lesson.

Mini-Game

The combination of sets, ranges and alternation allows us to do some powerful things.

You're going to need all of them for this game, I'm afraid.. it's tricky one!