Searching a Hash with Regular Expressions

Credit: Ben Giddings


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


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

	# => [["ficus", "plant"], ["apple tree", "plant"], ["plesiosaur", "animal"]]
	h.grep(/plant/) # => [["ficus", "plant"], ["apple tree", "plant"]]
	h.grep(/i.*u/) # => [["ficus", "plant"], ["plesiosaur", "animal"]]



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"]



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