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Chapter 6. Pattern Matching

A number of Unix text-processing utilities let you search for, and in some cases change, text patterns rather than fixed strings. These utilities include the editing programs ed, ex, vi, and sed, the awk programming language, and the commands grep and egrep. Text patterns (formally called regular expressions) contain normal characters mixed with special characters (called metacharacters).

This chapter presents the following topics:

For more information on regular expressions, see Mastering Regular Expressions, listed in the Bibliography.

6.1. Filenames Versus Patterns

Metacharacters used in pattern matching are different from metacharacters used for filename expansion (see Chapter 4, and Chapter 5). When you issue a command on the command line, special characters are seen first by the shell, then by the program; therefore, unquoted metacharacters are interpreted by the shell for filename expansion. The command:

$ grep [A-Z]* chap[12]

could, for example, be transformed by the shell into:

$ grep Array.c Bug.c Comp.c chap1 chap2

and would then try to find the pattern Array.c in files Bug.c, Comp.c, chap1, and chap2. To bypass the shell and pass the special characters to grep, use quotes:

$ grep "[A-Z]*" chap[12]

Double quotes suffice in most cases, but single quotes are the safest bet.

Note also that in pattern matching, ? matches zero or one instance of a regular expression; in filename expansion, ? matches a single character.

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