Regular Expressions

Most of the search mechanisms listed in this chapter use regular expressions in one form or another. A regular expression is a mathematical mechanism for specifying the ordering of symbols. Historically, regular expressions originated from discussions of the Theory of Computation, a seriously hard-core branch of mathematics that is far removed from the day-to-day rigors of, say, locating the smutty email to your girlfriend that you misplaced in some gargantuan file system at work.

However, the same principles govern both tasks. To get full value for your Linux dollar, you will need some understanding of regular expressions and how they are used to specify search patterns.

Regular expressions exist to give you a mechanism to specify patterns of characters. The implementation of a regular expression includes three classes of characters:

literals	The literal character you typed in (a, b, c , 1, 2, 3 etc.).
wildcards	Special characters used to represent one or more characters other than themselves. For example, the "" character will match any number of any other characters ("d" matches "date", "day", "dally", and anything else starting with the letter d.) The "." character will match one instance of any other character ("d.te" matches with "date" and "dote").
metacharacters	Metacharacters are characters that have a special meaning. For example, the caret character "^" usually matches the beginning of a line. The "$" character matches the end of a line.

In addition, it is possible to specify groups of characters. For example, the regular expression "[aAbB]*"would match any string of any length that started with the letter a or b, either uppercase or lowercase. (This group is delimited by square braces.) See the grep entry for more information about the implementation of regular expressions.

The commands covered in this section include

`egrep`	Grep with extended regular expressions
`find`	Search the directory tree
`finger`	Display information about a user
`fgrep`	Grep variation for matching fixed strings
`grep`	Search for a pattern in a file
`locate`	Search the locate database for a file
`updatedb`	Update the locate database
`which`	Search the directories of your $PATH for a file