Search Multiple Words / String Pattern Using grep Command

From http://www.cyberciti.biz/faq/searching-multiple-words-string-using-grep/

How do I search multiple strings or words using the grep command? For example I'd like to search word1, word2, word3 and so on within /path/to/file. How do I force grep to search multiple words?

The grep command supports regular expression pattern. To search multiple words, use following syntax:
grep 'word1\|word2\|word3' /path/to/file

In this example, search warning, error, and critical words in a text log file called /var/log/messages, enter:
$ grep 'warning\|error\|critical' /var/log/messages

To just match words, add -w swith:
$ grep -w 'warning\|error\|critical' /var/log/messages