Backslash reform

• The \p and \P properties become intrinsic grammar rules (<prop ...> and <!prop ...>).


• The \L...\E, \U...\E, and \Q...\E sequences become \L[...], \U[...], and \Q[...] (\E is gone).


• Note that \Q[...] will rarely be needed since raw variables interpolate as eq matches, rather than regexes.


• Backreferences (e.g. \1) are gone; $1 can be used instead, because it's no longer interpolated.


• New backslash sequences, \h and \v, match horizontal and vertical whitespace respectively, including Unicode.


• \s now matches any Unicode whitespace character.


• The new backslash sequence \N matches anything except a logical newline; it is the negation of \n.


• A series of other new capital backslash sequences are also the negation of their lower-case counterparts:
  • \H matches anything but horizontal whitespace.
  
  
  • \V matches anything but vertical whitespace.
  
  
  • \T matches anything but a tab.
  
  
  • \R matches anything but a return.
  
  
  • \F matches anything but a formfeed.
  
  
  • \E matches anything but an escape.
  
  
  • \X... matches anything but the specified hex character.
  
  

Regexes are rules

• The Perl 5 qr/pattern/ regex constructor is gone.


• The Perl 6 equivalents are:

      rule { pattern }    # always takes {...} as delimiters
      rx/ pattern /       # can take (almost any) chars as delimiters




Related Reading Perl & XML By Erik T. Ray, Jason McIntosh Table of Contents Index Sample Chapter Read Online--Safari

• If either needs modifiers, they go before the opening delimiter:

      $regex = rule :ewi { my name is (.*) };
      $regex = rx:ewi/ my name is (.*) /;



• The name of the constructor was changed from qr because it's no longer an interpolating quote-like operator.


• As the syntax indicates, it is now more closely analogous to a sub {...} constructor.


• In fact, that analogy will run very deep in Perl 6.


• Just as a raw {...} is now always a closure (which may still execute immediately in certain contexts and be passed as a reference in others)...


• ...so too a raw /.../ is now always a regex (which may still match immediately in certain contexts and be passed as a reference in others).


• Specifically, a /.../ matches immediately in a void or Boolean context, or when it is an explicit argument of a =~.


• Otherwise it's a regex constructor.


• So this:

      $var = /pattern/;

  no longer does the match and sets $var to the result.



• Instead it assigns a regex reference to $var.


• The two cases can always be distinguished using m{...} or rx{...}:

      $var = m{pattern};    # Match regex, assign result
      $var = rx{pattern};   # Assign regex itself



• Note that this means that former magically lazy usages like:

      @list = split /pattern/, $str;

  are now just consequences of the normal semantics.



• It's now also possible to set up a user-defined subroutine that acts like grep:

      sub my_grep($selector, *@list) {
          given $selector {
              when RULE  { ... }
              when CODE  { ... }
              when HASH  { ... }
              # etc.
          }
      }



• Using {...} or /.../ in the scalar context of the first argument causes it to produce a CODE or RULE reference, which the switch statement then selects upon.

Backtracking control

• Backtracking over a single colon causes the regex engine not to retry the preceding atom:

      m:w/ \( <expr> [ , <expr> ]* : \) /

  (i.e. there's no point trying fewer <expr> matches, if there's no closing parenthesis on the horizon)



• Backtracking over a double colon causes the surrounding group to immediately fail:

      m:w/ [ if :: <expr> <block>
           | for :: <list> <block>
           | loop :: <loop_controls>? <block>
           ]
      /

  (i.e. there's no point trying to match a different keyword if one was already found but failed)



• Backtracking over a triple colon causes the current rule to fail outright (no matter where in the rule it occurs):

      rule ident {
            ( [<alpha>|_] \w* ) ::: { fail if %reserved{$1} }
          | " [<alpha>|_] \w* "
      }

      m:w/ get <ident>? /

  (i.e. using an unquoted reserved word as an identifier is not permitted)



• Backtracking over a <commit> assertion causes the entire match to fail outright, no matter how many subrules down it happens:

      rule subname {
          ([<alpha>|_] \w*) <commit> { fail if %reserved{$1} }
      }
      m:w/ sub <subname>? <block> /

  (i.e. using a reserved word as a subroutine name is instantly fatal to the ``surrounding'' match as well)



• A <cut> assertion always matches successfully, and has the side effect of deleting the parts of the string already matched.


• Attempting to backtrack past a <cut> causes the complete match to fail (like backtracking past a <commit>. This is because there's now no preceding text to backtrack into.


• This is useful for throwing away successfully processed input when matching from an input stream or an iterator of arbitrary length.

by Allison Randal, Damian Conway

Editor's note: this document is out of date and remains here for historic interest. See Synopsis 5 for the current design information.