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Metacharacters And Literals
While it may not seem apparent, our grep searches have been using regular expressions all along, albeit very simple ones. The regular expression “bzip” is taken to mean that a match will occur only if the line in the file contains at least four characters and that some- where in the line the characters “b”, “z”, “i”, and “p” are found in that order, with no other characters in between. The characters in the string “bzip” are all literal characters, in that they match themselves. In addition to literals, regular expressions may also in-
clude metacharacters that are used to specify more complex matches. Regular expression metacharacters consist of the following:
^ $ . [ ] { } - ? * + ( ) | \
All other characters are considered literals, though the backslash character is used in a few cases to create meta sequences, as well as allowing the metacharacters to be escaped and treated as literals instead of being interpreted as metacharacters.
Note: As we can see, many of the regular expression metacharacters are also char- acters that have meaning to the shell when expansion is performed. When we pass regular expressions containing metacharacters on the command line, it is vital that they be enclosed in quotes to prevent the shell from attempting to expand them.
