"That hat is red" =~ /hat/; # matches 'hat' in 'That'
Not all characters can be used 'as is' in a match. Some characters,
-called B<metacharacters>, are reserved for use in regex notation.
-The metacharacters are
+called B<metacharacters>, are considered special, and reserved for use
+in regex notation. The metacharacters are
{}[]()^$.|*+?\
-A metacharacter can be matched by putting a backslash before it:
+A metacharacter can be matched literally by putting a backslash before
+it:
"2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
"2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
In the last regex, the forward slash C<'/'> is also backslashed,
because it is used to delimit the regex.
+Most of the metacharacters aren't always special, and other characters
+(such as the ones delimitting the pattern) become special under various
+circumstances. This can be confusing and lead to unexpected results.
+L<S<C<use re 'strict'>>|re/'strict' mode> can notify you of potential
+pitfalls.
+
Non-printable ASCII characters are represented by B<escape sequences>.
Common examples are C<\t> for a tab, C<\n> for a newline, and C<\r>
for a carriage return. Arbitrary bytes are represented by octal
e.g., C<\x1B>:
"1000\t2000" =~ m(0\t2) # matches
- "cat" =~ /\143\x61\x74/ # matches in ASCII, but
+ "cat" =~ /\143\x61\x74/ # matches in ASCII, but
# a weird way to spell cat
Regexes are treated mostly as double-quoted strings, so variable
A B<character class> allows a set of possible characters, rather than
just a single character, to match at a particular point in a regex.
-Character classes are denoted by brackets C<[...]>, with the set of
-characters to be possibly matched inside. Here are some examples:
+There are a number of different types of character classes, but usually
+when people use this term, they are referring to the type described in
+this section, which are technically called "Bracketed character
+classes", because they are denoted by brackets C<[...]>, with the set of
+characters to be possibly matched inside. But we'll drop the "bracketed"
+below to correspond with common usage. Here are some examples of
+(bracketed) character classes:
/cat/; # matches 'cat'
/[bcr]at/; # matches 'bat', 'cat', or 'rat'