A number of named rules are provided by default, including a complete set of POSIX-style classes, and Unicode property classes. The list isn't fully defined yet, but Table 7-7 shows a few you're likely to see.
Table 7-7. Built-in rules
Rule
Meaning
<alpha>
Match a Unicode alphabetic character.
<digit>
Match a Unicode digit.
<sp>
Match a single-space character (the same as \s ).
<ws>
Match any whitespace (the same as \s+ ).
<null>
Match the null string.
<prior>
Match the same thing as the previous match.
<before . . . >
Zero-width lookahead . Assert that you're before a pattern.
<after . . . >
Zero-width lookbehind. Assert that you're after a pattern.
<prop . . . >
Match any character with the named property.
<replace( . . . )>
Replace everything matched so far in the rule or subrule with the given string (under consideration).
The <null> rule matches a zero-width string (so it's always true) and <prior> matches whatever the most recent successful rule matched. These replace the two behaviors of the Perl 5 null pattern // , which is no longer valid syntax for rules.