Lookbehind patterns in Regular Expressions

Here is the video version for this topic. You can read the written version which comes after the video section.

From the previous lesson lookahead patterns , we saw how to match a string when it is "followed by" another string, and also when it is "NOT followed by" another string.

What about the characters behind a string? We can also try a check pattern for that. This is where lookbehind patterns come in.

A lookbehind pattern asserts that a match for a pattern, is preceded by another pattern. Let's see an example where we would use lookbehind patterns.

Say we have this string: “He bought 50 items with $500”. We have two numbers here: 50 and 500. Let's say we want to match the number that represents the price he spent. We can use a lookbehind pattern here.

First let's look at the syntax of a lookbehind pattern.

/(?<=X)Y/

In the previous lesson, we saw that the syntax of a lookahead pattern is /Y(?=Z)/. Take note of the ?=. For lookbehind, the syntax is then ?<=.

Coming back to the string: "He bought 50 items with $500". We can match the number that represents the price with this pattern:

/(?<=\$)\d+/

The pattern starts with a lookbehind for "$". We escaped this character \$ because $ is a special character . Then we have one or more digits represented with \d+. So our pattern says "match one or more digits that is preceded with a dollar sign":

As you can see here, 50 is not matched because it is preceded by an empty space " ". But 500 is matched because it is preceded by "$".