Searching a Hash with Regular Expressions

Credit: Ben Giddings

Problem

You want to grep a hash: that is, find all keys and/or values in the hash that match a regular expression.

Solution

The fastest way to grep the keys of a hash is to get the keys as an array, and grep that:

h = { "apple tree" => "plant", "ficus" => "plant", "shrew" => "animal", "plesiosaur" => "animal" } h.keys.grep /p/ # => ["apple tree", "plesiosaur"]

The solution for grepping the values of a hash is similar (substitute Hash#values for Hash#keys), unless you need to map the values back to the keys of the hash. If that's what you need, the fastest way is to use Hash#each to get key-value pairs, and match the regular expression against each value.

h.inject([]) { |res, kv| res << kv if kv[1] =~ /p/; res } # => [["ficus", "plant"], ["apple tree", "plant"]]

The code is similar if you need to find key-value pairs where either the key or the value matches a regular expression:

class Hash def grep(pattern) inject([]) do |res, kv| res << kv if kv[0] =~ pattern or kv[1] =~ pattern res end end end h.grep(/pl/) # => [["ficus", "plant"], ["apple tree", "plant"], ["plesiosaur", "animal"]] h.grep(/plant/) # => [["ficus", "plant"], ["apple tree", "plant"]] h.grep(/i.*u/) # => [["ficus", "plant"], ["plesiosaur", "animal"]]

Discussion

Hash defines its own grep method, but it will never give you any results. Hash#grep is inherited from Enumerable#grep, which tries to match the output of each against the given regular expression. Hash#each returns a series of two-item arrays containing key-value pairs, and an array will never match a regular expression. The Hash#grep implementation above is more useful.

Hash#keys.grep and Hash#values.grep are more efficient than matching a regular expression against each key or value in a Hash, but those methods create a new array containing all the keys in the Hash. If memory usage is your primary concern, iterate over each_key or each_value instead:

res = [] h.each_key { |k| res << k if k =~ /p/ } res # => ["apple tree", "plesiosaur"]