Expressions Are Strings and Wildcards

When using the programs in this book, such as grep and vi , you provide a regular expression that the program evaluates . The command will search for the pattern you supply. The pattern could be as simple as a string or it could wildcards. The wildcards used by many programs are called meta- characters .

Table 24-1 shows a list of meta-characters and the program(s) to which they apply. Only the programs covered in this book ( awk , grep , sed , and vi ) are shown in Table 24-1. These meta-characters may be used with other programs, such as ed and egrep , as well, which are not covered in the book. Table 24-1 describes the meta-characters and their use.

Table 24-1. Meta-Characters and Programs to Which They Apply

Meta-Character	awk	grep	sed	vi	Use
.	Yes	Yes	Yes	Yes	Match any single character.
*	Yes	Yes	Yes	Yes	Match any number of the single character that precedes * .
[ ... ]	Yes	Yes	Yes	Yes	Match any one of the characters in the set [ ... ] .
$	Yes	Yes	Yes	Yes	Matches the end of the line.
^	Yes	Yes	Yes	Yes	Matches the beginning of the line.
\	Yes	Yes	Yes	Yes	Escape the special character that follows \ .
\{ n,m \}	Yes	Yes	No	No	Match a range of occurrences of a single character between n and m .
+	Yes	No	No	No	Match one or more occurrences of the preceding regular expression.
?	Yes	No	No	No	Match zero or one occurrence of the preceding regular expression.
	Yes	No	No	No	The preceding or following regular expression can be matched.
()	Yes	No	No	No	Groups regular expressions in a typical parenthetical fashion.
\{ \}	No	No	No	Yes	Match a word's beginning or end.

You may want to refer to this table when regular expressions are used for one of the commands in the table.