All presented solution have a time complexity of O(n). For simplicity I use String#include? to check for the word. This could be done instead with a regular expression in the form string=~ regex.

Read the complete file and search in it.

File.read(filename).include?(word)

If your file is very large, this is not an optimal solution, as you would read the complete file into memory and start searching afterwards. Your memory complexity is O(n)

Read the file line by line and search in each line

File.open(filename) do |f|  f.any? do |line|    line.include?(word)  endend

If your file is very large, but you know your lines are upperbounded by a constant value, you now have a memory complexity of O(1).

Read chunks of the file and search in it

File.open(filename) do |f|  tmp= f.read(1024)  next true if tmp.include?(word)  until f.eof?    tmp= tmp[(-1*word.size)..-1] + f.read(1024)    next true if tmp.include?(word)  end  next falseend

In this variant, we are reading equaly sized chunks from the file. So no matter what the conditions of the file are, our memory complexity is O(1)

Something like this might help:

def word_exists_in_file   f = File.open("your_file.txt") #opens the file for reading   f.each do line      print line      if line.match /your_word_to_match/         return true      end   end   falseend

File.readlines(file).each {|l| l.grep(/#{exp_search}/).each {|r| puts file + ' : ' + r}}