| 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 These replace the two behaviors of the Perl 5 null pattern // , which is no longer valid syntax for rules. |