grep is a familiar Unix command that is also a method defined in the Enumerable module. The Enumerable module is included in the Array and Hash classes, which makes the grep method accessible.
Here is how to use grep() to select all elements from this array [“mat”, “lat”, “cat”, “cool”, “dude”, “fat”] with the letters “at”:
["mat", "lat", "cat", "cool", "dude", "fat"].grep(/at/)
The select() method can also be used to arrive at the same result, but the syntax is more complicated:
["mat", "lat", "cat", "cool", "dude", "fat"].select{|word| word.include?("at")}
A block can also be passed to the grep method. Suppose you have a people = [“Matt P.”, “Cat P.”, “Lat S.”, “Joe L.”] array and would like to find every person with “P.” as a last name and replace it with Powers. The desired result is [“Matt Powers”, “Cat Powers”].
p = ["Matt P.", "Cat P.", "Lat S.", "Joe L."]
p.grep(/P./) {|name| name.sub(".", "owers") }
Passing a block to the grep method is very similar to using the map() method, but slightly more concise:
p.grep(/P./).map {|name| name.sub(".", "owers") }
Ruby/Rails Programming 