Lookaround patterns in Regular Expressions

Moving on to a more advanced concept in regular expressions: lookarounds 👀

Let's say we had a string example like this:

He has 20 dollars and is 10 years old

Say you wanted to write a pattern that would match this person's age, what would it be?

You might think:

/\d+/g

First we have \d, a digit meta character which matches digits. Then we have the ONE or MORE special character . This pattern means "one or more digits":

From the matches, we see "20" and "10". But our target is only "10" 🤔. You could then expand the regular expression to this:

Now we have "10 years old" as a match. But, this includes other characters we're not interested in: " years old". So how can we specifically target the number that "is followed by" the characters " years old" without matching " years old"? Lookaround patterns to the rescue.

In the same vain, say we had a string like:

He bought 50 items with $500

How do we match the number that is preceded by "$" without matching "$"? Lookaround patterns.

In Regex, you can create lookaround patterns. What this means is that you specify a pattern, let's call this pattern A, then you have a lookaround pattern, let's call this pattern B. The idea is that the regex engine will look for a match for pattern A, then it will look around this match to see if it can find a match for pattern B. If it finds a match for pattern B, then the match for pattern A is valid. But if it does not find a match for pattern B, then the match for pattern A is not a valid match.

Don't worry if this does not make sense. We'll look at examples to simplify this.

Lookarounds can be divided into two:

• Lookahead (that is, in front of the match) and
• Lookbehind (that is, before the match)

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Let's start with lookahead in the next lesson.