Regular Expression Words-of-Caution

Regular expressions describe patterns for which you are searching. A regular expression usually defines the pattern for which you are searching using wildcards. Since a regular expression defines a pattern you are searching for, the terms "regular expression" and "pattern matching" are often used interchangably.

Let's get down to a couple of words-of-caution immediately:

• Regular expressions are different from file matching patterns used by the shell. Regular expressions are used by both the shell and many other programs, including those covered in this chapter. The file matching done by the shell and programs such as find is different from the regular expressions covered in this chapter.

• Use single quotes around regular expressions. The meta-characters used in this chapter must be quoted in order to be passed to the shell as an argument. You will, therefore, see most regular expressions in this chapter quoted.