POSIX extended regular expressions (PERE) is a way to describe string using wildcards and pattern. The following description doesn't provide a full description of PERE but just intend to help you at the beginning.
the following rules are available:
|a|b||search for a or b|
|ab||search for the symbol a followed by symbol b|
|.||this symbol match any other.|
|a*||symbol a from 0 upto unlimited times.|
|[abc]||search for a or b or c|
|[a-z]||search for symbol between a (included) and z (included).|
|[^a-z]||search for symbol not between a (included) and z (included).|
|^abc||search for line beginning by the word abc.|
|abc$||search for line ending by the word abc.|
You can mix these rules as you need.
Here is some samples: