Know Ruby: Enumerable grep

Most of us are familiar with the grep program. If you work on a *nix system you’ve almost certainly used it or one of its newer kin. These programs allow for globally searching files with a regular expression and printing the results. It’s so common we use it as a verb.

“Did you grep the code base before making that change?”

Are you familiar with its Ruby counterpart? Ruby plays a bit fast and loose with the name but the underlying concept is similar. Let’s see what grep can do.

What is a pattern?

According to the documentation, grep takes a “pattern” argument and an optional block. If you’re like me that doesn’t mean much. Thankfully the documentation provides a helpful nugget of information. It says that it checks the argument against each element with ===.

You might have heard the triple equals referred to as “case equality”. That’s because === is the check case statements use against their when clauses. Now that we have some idea of what we’re working with let’s break down our options.


Legally speaking, I don’t think you can call a method grep if it doesn’t support regular expressions.

Given a list of first names let’s get all the names that start with a “j”.

> %w[bob amber john jane aaron michael nikki].grep(/\Aj/)
# => ["john", "jane"]

The equivalent select statement would be:

> %w[bob amber john jane aaron michael nikki].select { |n| n[/\Aj/] }
# => ["john", "jane"]

I like the brevity provided by grep in this case.

We’ve confirmed that the grep name isn’t completely misused for this method. What other things can it do?

Exact Match

Finding exact matches couldn’t be simpler.

> [1, 1, 0, 0, 1].grep(1)
# => [1, 1, 1]

Of course, it’s also not too useful. Grouping items by values? Useful. Counting items by values? Useful. Culling a list to a single value? Dubious. How often does that really come up? Either way, there it is.


We can also find elements that match a specific class.

> [1, 1.2, 'a', 2].grep(Integer)
# => [1, 2]

> [1, 1.2, 'a', 2].grep(Numeric)
# => [1, 1.2, 2]

We can even find elements that include a particular module.

> [1, [], {}, 'a'].grep(Enumerable)
# => [[], {}]


Want to find all of the B’s in a list of test scores?

> { rand(100) }.grep(80..89)
#  => [83, 87, 81, 84, 82, 81]

It’s worth noting that under hood === uses include? rather than cover?. That’s usually not a concern but it’s certainly something to be aware of. If you’re not familiar with the differences I would recommend checking out this Stack Overflow question about it.


Of course, defining our own meaning for === is an option. Let’s say we have value objects that represent various grades.

class B < Grade
  def self.===(other_or_grade)
    self == other_or_grade || (80..89).include?(other_or_grade)

> { rand(100) }.grep(B)
# => [87, 86, 88, 83, 88]


Sometimes we’ll have a complicated check and nothing above will help. At that point proc (or lambda) steps in to save the day.

Using === on a proc actually calls it. The value on the other side of the === is passed in as an argument.

> EVEN = ->(n) { n.even? }
# => #<Proc:[email protected](pry):21 (lambda)>

> EVEN === 1
# => false
> EVEN === 2
# => true

That’s just crazy. It should go without saying that under normal circumstances you should never do this. It’s only there for situations like this:

> (1..10).grep(EVEN)
# => [2, 4, 6, 8, 10]

At this point are we better off switching to select? Probably. If we use select we can call even? without the proc.

> (1..10).select(&:even?)
# => [2, 4, 6, 8, 10]

Using the shorthand &:even? syntax is one thing grep can’t do. There’s a reason for that. Remember earlier when we talked about the arguments grep takes? It takes a pattern and a block.

What about the block?

Every element that makes it past the pattern filter gets sent to the block. It’s like doing a select and a map all at once.

> (1..10).select(&:even?).map { |n| n * 2 }
# => [4, 8, 12, 16, 20]


> (1..10).grep(EVEN) { |n| n * 2 }
# => [4, 8, 12, 16, 20]

Instinctually, I thought the grep version would be faster. It’s only looping over the elements once. This is why we test our assumptions. It turns out that sometimes grep is faster than select. Sometimes it’s the other way around. It really depends on what you’re doing. Adding map to the mix, especially with a small number of elements, didn’t change that outcome.


Most of us will reach for select without giving it a second thought. Next time you do take a second and consider grep. It might be just the thing you need.

If you enjoyed this dive into grep, check out other posts in the Know Ruby series.