Before we examine more examples, its important to point out that UNIX provides an extensive set of special characters that enables us to define very powerful pattern matches.

These special characters used with regular expressions shouldn’t be confused with the shell metacharacters used for filename expansions or other specific shell interpretations. To be more specific, notice the interpretation of:

The ? (question mark) used to repeat the previous expression one or more times.The | (pipe sign) used to create a logical ORing.The . (period) to match any character that is printable; which includes spaces and tabs.

Notice how we have special characters that allows us to span an expression! We’ll see some powerful examples shortly.

The key is to simply remember how you’re these special characters.