Perl 6 and Parrot Essentials, Second Edition

Built-in Rules

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 These replace the two behaviors of the Perl 5 null pattern //, which is no longer valid syntax for rules.