If you are familiar with Perl, you already know the syntax. If you are familiar with Unix, you should know that there are subtle differences between Perl's regular expressions and Unix' regular expressions. In its simplest form, a regular expression is just a word or phrase to search for. This could be used to find subjects on oxygen, boxy houses or oxymorons. The above expression could be used to find top ten lists.
Some characters have a special meaning to the searcher.
These characters are called metacharacters . Although they may seem confusing at first, they add a great deal of flexibility and convenience to the searcher. It matches exactly one character, regardless of what the character is. For example, suppose we wished to search for references to pi. The backslash can be used to indicate that the character immediately to its right is to be taken literally.