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 section presents the following topics:
Filenames versus patterns
List of metacharacters available to each program
Description of metacharacters
Metacharacters used in pattern matching are different from metacharacters used for filename expansion. 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
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
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.
The characters ...